|05-03-2009 06:15 AM
thans triage,you just solved all my problems too.......great site!!!!What a great idea this was ,Jon
|01-12-2008 03:10 PM
cont. from above...
The same goes for a binding or cocked brake pad. The pad would be flexed during apply and result in the same thing. It is not common for these vehicles to experience either of these problems, but you still should check. We checked our vehicle and found nothing wrong.
The next thing to check for is the possibility of air in the system. Air at both calipers could account for the pedal travel we were experiencing. If it is air, the air would have to be lower than the line lock. Remember, the pedal was high and hard before the line lock was removed. To check for this, simply bleed each front wheel. It should only take two to three pedal cycles to remove any air. It is highly unlikely that air is the problem unless the vehicle had been serviced recently. I had my students perform this step just for practice and the result was no change in pedal height.
This leaves us with only one thing left on our list. Perhaps the calipers are not self adjusting like they are designed to do. One of the biggest advantages disc brakes have over drum brakes is they are truly self adjusting. The pistons will self adjust due to lining wear which maintains good pedal height. The caliper can experience internal conditions that will change the timing of the self adjustment.
The two most common conditions that will cause a change in self adjustment timing are a corroded piston sealing surface and corrosion in the square-cut seal groove. Either of these conditions can cause the square-cut seal to grip the piston tighter.
The result of this is a change in self adjustment timing. Instead of the adjustment taking place when it is supposed to, it is delayed. This delay causes a larger than normal gap between the pads and rotor resulting in a low pedal. There is no definitive test for this condition. You need to approach this condition using a process of elimination. Check for everything else on your list to figure out whether this is the problem. On this vehicle, there is a slight problem using this approach. It involves the fact that this vehicle, and all those listed at the beginning of this article, use what are known as low-drag calipers.
A little background is in order to properly understand the impact low-drag calipers have on the system and the problem being diagnosed. In 1980, General Motors became the first manufacturer to use low-drag calipers on its vehicles. Shortly there after, Ford, Jeep and Nissan also used low-drag calipers on some of their vehicles.
The driving force behind the development of low-drag calipers was the energy crisis. Low-drag calipers were designed to reduce what are known as parasitic losses. A parasitic loss is something that increases the rolling resistance of the vehicle. The slight friction created by the pads rubbing against the rotor is considered a parasitic loss. Engineers found a small change in the square-cut seal groove would cause the piston to be pulled back twice as far as a conventional caliper. Figure 3 shows this change.
The low-drag caliper uses a 30-degree bevel instead of the conventional caliperís 15 degrees. Figure 4 shows the result of this Ė twice the seal flex. If you flex the seal twice as far, it will pull the piston back in twice as far and you have a low-drag caliper. I imagine the engineers in the caliper department that came up with this simple solution were considered heroes. This design change resulted in an increase in fuel economy on these vehicles. It also resulted in something else Ė an entirely new type of master cylinder.
What was discovered was a conventional master cylinder would not work on these vehicles. The volume of fluid needed to take the gap up created by the low-drag calipers exceeded a conventional master cylinderís capacity. The engineers in the master cylinder department were given the assignment of coming up with a master cylinder that would provide a large volume of low pressure fluid on initial brake application, and then turn into a conventional master cylinder. No easy task. My guess is they werenít so happy with the guys in the caliper department!
The result of their efforts is known as a quick-take-up master cylinder. It gets its name from what it does. It quickly takes up the gap created by the low-drag calipers to prevent a loss of pedal height.
There are two main differences between a quick-take-up master cylinder and a conventional master cylinder. These differences are shown in Figure 5. The first is known as a step bore. Notice the flange end of the quick-take-up master cylinder has a larger bore. This will provide the large volume of fluid needed to take up the gap created by the low drag calipers.
The second difference is the placement of a valve between the primary reservoir and primary cylinder bore. This valve is known as the quick-take-up valve. It will control the flow of fluid in and out of the large step bore. Letís review how it works.
* The GM quick-take-up valve consists of the following components (See Figure 6):
* Main valve body;
* Lower valve body;
* A one-way cup seal; and
* A spring-loaded check valve.
GM uses two different types of check valves in its quick-take-up master cylinders. The first is shown in Figure 6. It is a spring-loaded ball. The ball sits against a seat in the lower valve body. The valve does not form a perfect seal because of the small square notch shown in Figure 7. The purpose of this notch will be explained shortly.
The second, and I believe, based on my experience, more common type, is shown in Figure 8. I call this type the "puck-style" quick-take-up valve because of its design. Instead of using a check ball and square notch, they use a small "puck" with a slot in the base of it. The edge of the puck is forced against the lower valve body to form a seal. The slot prevents the puck from forming a perfect seal.
On initial pedal apply, the fluid in the large step bore is pushed forward. The pressure created by this causes the cup seal around the quick-take-up valve to seal preventing the fluid from going around it. The check ball (or puck) in the quick-take-up valve will prevent the fluid from getting to the reservoir through the discharge port.
A small amount of fluid will pass through the square notch or puck slot at the base of the check valve (Figure 9). Since the fluid cannot get to the reservoir, it has to move forward into the smaller bore.
The primary cup seal is meant to form a seal when it has pressure in front of it, not behind it. The fluid from the step bore passes over the primary cup seal into the smaller diameter pressure chamber of the primary circuit (Figure 10). The result of pushing a large volume of fluid into a smaller area is the buildup of low pressure in the hydraulic system. The large volume of low-pressure fluid acts on the caliper pistons to take up the gap created by the low-drag calipers. This all occurs in the first 1/4 to 1/2 inch of pedal travel.
Once the gap is taken up, the pressure in the system rises sharply. When it reaches around 100 psi, it will overcome the check ball (or puck) spring in the quick-take-up valve causing the quick-take-up valve to open (See Figure 11). The opening of the quick-take-up valve discharges the step bore portion of the primary bore allowing the primary cup seal to form a high-pressure seal. The master cylinder now functions as a conventional master cylinder. The high pressure created causes the disc and drum brakes to function and stop the vehicle.
On pedal release, the secondary portion of the master cylinder acts like any other. Replenishing takes place through the replenishing port and over the secondary piston primary cup seal. The replenishing in the primary portion of the master cylinder has an additional step. The fluid has to get around the quick-take-up valve in order to get to the replenishing port. The one-way cup seal on the quick-take-up-valve allows this to happen. Fluid travels around the cup seal, through the replenishing port and over the primary cup seal during the replenishing cycle (See Figure 12).
Venting in the secondary circuit takes place in the conventional method through the secondary vent port. Primary venting has two steps as a result of the quick-take-up valve. The fluid path for primary circuit venting is through the conventional vent port and then either the square notch in the base of the ball-style quick-take-up valve or the slot in the puck-style valve (See Figure 13).
As long as the quick-take-up valve functions properly, the pedal will be at the right height. If the quick-take-up valve is not operating at 100 percent, it will have an impact on pedal height. The first thing to understand about these valves is they donít experience total failure. Remember, their job is to make sure the fluid in the large step bore has to go to the calipers. What will happen if the check ball or puck stop sealing like they are supposed to? A loss of pedal height will result. This problem occurs more commonly on the puck-style than the ball-style valves.
What happens is the sealing surface of the puck corrodes over time due to the moisture in the brake fluid (See Figure 14). This corrosion prevents the puck from sealing at 100 percent efficiency. Any reduction in sealing efficiency will be seen in additional pedal travel. The amount of extra pedal travel will be proportional to the loss in sealing efficiency. On average, my experience has shown a loss of one to three inches.
The question now becomes how to diagnose this condition? The answer depends on whom you talk to. Some people would have you check the function of the quick-take-up valve by performing what is commonly referred to as the "geyser test." A piece of cellophane is placed over the reservoir and the fluid discharge from the quick-take-up valve is observed during brake apply. The theory is a bypassing quick-take-up valve will cause an instant geyser.
In my opinion, there are a couple of flaws with this theory. The first deals with the square notch or puck groove. These "secondary" vent ports allow some fluid to flow into the reservoir during normal operation. It is hard to determine what is normal and what is additional, especially since this process takes place in a split second. The other flaw in the theory is the fact that the test expects total failure, which I personally have never seen.
The partial failures I see in the field are impossible to detect with the geyser test. The first step in diagnosing the condition has already been performed Ė the line lock test. Hereís the question you have to ask yourself: If the quick-take-up valve is bypassing and line locks are installed on the front brake hoses, what will the pedal feel like?
Answer: High and hard.
The only reason you need the quick-take-up valve is due to the low-drag calipers. With the line locks installed, the low-drag calipers are out of the system and there is no need for a quick-take-up valve. You could take the quick-take-up valve out of the master cylinder and the pedal would still be high and hard. The next step is to perform the front brake inspection for the conditions listed earlier. GM front disc brakes are far from perfect, but they donít experience too many conditions that would cause additional pedal travel. Most of the time your inspection will not yield anything related to the loss of pedal height. This is your first clue. It is related to a bypassing quick-take-up valve.
To my knowledge there is no documented test for a bypassing quick-take-up valve other than the geyser test. You would have to deduce that the quick-take-up valve must be the cause based on the process of elimination you conducted through the line lock test and front brake inspection. I have been working on a quick test for this condition (no pun intended). I have been using it in the field for about a year with very positive results. To test for a bypassing quick-take-up valve, refer to the following steps.
Perform the line lock test and determine the problem is coming from the front brakes.
Inspect the front brakes for possible causes of the loss of pedal height. If the inspection doesnít uncover the source of the problem, continue with Step 3. If problems are found, repair them and retest.
Drain all but 1/2 inch of fluid from the master cylinder reservoir.
Insert a dry wall screw into the discharge port of the quick-take-up valve (See Figure 15). The valve body is plastic and it will only take about 1/4 turn of the screw to make a seal. The dry wall screw has a very fine, sharp point which makes it perfect for the purpose. If done properly, the test will not damage the quick-take-up valve. Set the reservoir cap back on the master cylinder.
Get in the vehicle without applying the brake pedal. Start the vehicle. Apply and hold the brake pedal while noting if the pedal height has improved. If the pedal height improves dramatically, the cause is a bypassing quick-take-up valve. DO NOT pump the brake pedal. Apply only once and hold.
If it is necessary to retest the vehicle remove the dry wall screw to relieve the trapped pressure. Reinstall the screw and repeat the test.
You might have already figured out the purpose of the dry wall screw. If the quick-take-up valve is bypassing, the dry wall screw will prevent the fluid from getting to the reservoir. The test forces the quick-take-up valve to work. The fluid has to go the calipers.
There is an important point to understand about the test. It involves the part about only applying the pedal once. The process of plugging the discharge port also eliminates the secondary vent port and the normal fluid bypass that is supposed to occur. Instead of some of the fluid being allowed back into the reservoir, all of it has to go to the calipers. The pedal will feel better than it would if the quick-take-valve were working properly. This has to be factored into the diagnosis.
The other thing you must understand is that if you were to pump the pedal with the screw installed, each additional pump would increase the amount of trapped pressure. This would result in the pedal getting higher and higher and the brakes would lock up. That is the reason for only one apply and the need to release the pressure if you want to perform the test again.
The quick-take-up valve is not serviceable. If the test resulted in an improved pedal, the master cylinder will have to be replaced. The quick-take-up master cylinder requires special steps during both bench bleeding and system bleeding. In general, when stroke bench bleeding these master cylinders, it is necessary to wait 15 seconds between strokes. This allows the large step bore to refill.
The same holds true when foot bleeding these vehicles. Use the single stroke method with a 15-second pause between strokes. Failure to follow these steps can lead to difficulty in getting the pedal back. There are other methods available that allow faster bleeding of these systems, but it is beyond the scope of this article to cover them.
I have spent a great deal of time on this issue and have found that the information I am providing you is accurate when applied correctly. My experience is you will find the majority of pedal height complaints on the vehicles listed in the beginning of this article to be caused by either the RWAL valve (if equipped) or the quick-take-up valve.
The vehicle in my class was a combination of a bypassing dump valve, slightly out of adjustment rear brakes and a faulty quick-take-up valve. When we delivered the vehicle to the customer, he could not believe the difference in pedal height and feel. If you apply the information presented here on these vehicles when faced with low brake pedals, it will save you considerable time and effort.
|01-12-2008 02:59 PM
If the air is bled out and there is too much pedal travel then something is wrong. S10's (like many post 83 GM vehicles) use a step-bore (aka quick take-up) master cylinder to go along with the drag reducing calipers. See this very informative article.
http://www.babcox.com/editorial/bf/bf60133.htm (quoted below in case the link goes dead)
The larger bore on the manual brake master cylinder is smaller on the manual brake S10 MC then the power brake S10 MC. They both have the same working bore of 15/16" (actually 24mm I think). You might try the power brake MC as it will give you a firmer higher pedal but under heavy application will act just like your manual brake master cylinder.
Follow a Diagnostic Process When Curing a Low Brake Pedal, Bill Williams, Brake & Front End, June 2001
There are certain constants in this world and dealing with low brake pedals is one of them. Incorrect diagnosis of low brake pedals accounts for a great deal of wasted manpower and replacement of unnecessary parts. The list of possible causes of a low brake pedal differs from one vehicle to another. Unfortunately, one of the common mistakes made by technicians when diagnosing this condition is treating all vehicles the same. This is a big mistake. It would be like a doctor performing the same physical on a male and female patient. I think you get the idea.
In this article, we are going to take a close look at one group of vehicles that tends to experience low brake pedals more than most others. The vehicles in question are General Motors trucks built from 1980 to 1999. This includes the C-, K-, M-, R-, V-, G-, S- and T-series platforms. I think you recognize by this list the chances of you being impacted by these vehicles on a regular basis are pretty high. The better the handle you have on what can cause a low brake pedal on these vehicles, the more effective you will be in diagnosing the problem. For the sake of space we are not going to cover all aspects of diagnosing a low brake pedal condition. I would suggest you dig out your October 1999 issue of Brake and Front End and do a quick review. (The article can also be found on www.brakeandfrontend.com.) This article goes into specifics about the group of vehicles listed above.
Regardless of your opinion of GM trucks, you have to admit, when operating properly they do have a nice high, firm brake pedal. If the system is operating properly, the brake pedal in these vehicles should only go down between one to 1-1/2 inches. Another way to describe a good brake pedal in one of these vehicles is "it will be considerably higher than the gas pedal when the system is working properly." Now the question of what to do when it isnít where it should be?
These vehicles will commonly have pedal heights that range from a couple of inches of travel to what most people describe as "it goes to the floor." I encountered one such vehicle during a hands-on training class I was conducting. The vehicle in question was a 1989 Chevy 4x4. The customerís complaint was excessive pedal travel. As it turned out, this proved to be an understatement. The truck had just over six inches of pedal travel!
There is a very effective diagnostic process you can apply to most any brake problem you encounter. It involves five steps. These steps are:
* Think about the system;
* Think about how it operates;
* Think about the problem;
* Generate a list of possible causes; and
* Diagnose and inspect in a logical manner.
We are going to run this vehicle through this process. This is exactly what I did in the class I was conducting. I made the students apply each of the steps to come up with the actual causes of the low brake pedal.
Step 1: Think about the system
Steps 1 and 2 are the most important steps in the process, yet are often ignored when performing brake diagnostics. These steps will set the foundation for the remainder of the process. They will allow you to generate a complete and accurate list of possible causes. If we apply Step 1 to the vehicle in question, we get the following:
* The truck is equipped with a front-to-rear split hydraulic system;
* It has floating disc brakes on the front and duo-servo drum brakes on the rear;
* It is equipped with a combination valve that houses a metering valve, proportioning valve and pressure differential switch;
* It is equipped with rear-wheel ABS (RWAL);
* It has only one brake warning light which also serves as the ABS warning light;
* Three things will turn the red light on Ė parking brake, hydraulic failure and an ABS problem; and
* It has low-drag calipers and a quick-take-up master cylinder.
Step 2: Think about how it operates
Using the information from Step 1 will allow us to get an accurate picture of how the system operates. If Step 1 was not complete, or had inaccurate information listed, it will have an impact on how accurate Step 2 will be. It generates a snowball effect that will carry throughout the rest of the process. Here are the main points of how the system operates:
* It will have roughly a 75/25 braking balance from front to rear;
* The rear brakes self adjust when backing up;
* The metering valve will hold the front brakes off until the rear brakes start to apply;
* The proportioning valve will work to prevent rear brake lockup during panic braking;
* The pressure differential switch will turn the red light on if there is a hydraulic failure;
* The RWAL system will prevent rear lockup if the rear wheels lose grip with the road due to road conditions (snow, ice); and
* The low-drag calipers and quick-take-up master cylinder will work together to improve the vehicleís fuel economy.
Step 3: Think about the problem
The problem is excessive pedal travel. In order to generate an accurate list of the possibilities, we need to relate the problem to the system in question. We have to look at the components in the system that can contribute to the problem being diagnosed.
Step 4: Generate a list of possible causes
The problem being diagnosed is excessive pedal travel. There are different degrees of excessive pedal travel. The one we are dealing with would represent a severe case of excessive pedal travel. Six-plus inches of travel will usually have more than one cause. This is why performing Steps 1 and 2 accurately are so important. If Steps 1 or 2 are not done properly, you will not be able to generate a complete list of possible causes. The list of possible causes for our vehicle would include the following:
* Air in the hydraulic system;
* Improper rear brake adjustment;
* Front brake problems creating excessive clearance between the pads and rotors;
* Bypassing dump valve in RWAL valve; and
* Bypassing quick-take-up valve in master cylinder.
Notice I did not include a leak or bypassing primary cup seal in the master cylinder. The reason for this involves the pedal feel. The pedal is not bottoming out during the stroke. It simply is traveling too far. A hydraulic leak or cup seal problem would give a different pedal feel. The problems listed could all contribute to the pedal problem we are diagnosing. Determining which ones are causes is the next step.
When generating your list, it is important to make sure that the list is complete and factual. Not having a compete understanding of the system being diagnosed and how it operates can result in "short listing" the vehicle. This is where not everything that could cause the problem is looked at. Making sure you use fact and not fiction is another important consideration. Our industry is full of myth and speculation. It is easy for this myth and speculation to affect the accuracy of your list.
When you have had enough practice and gained the necessary knowledge, you will not only be generating complete lists, you will also learn to prioritize your lists. This involves listing the causes in either the most common to least common causes or from the easiest to check to the hardest to check. When you get to this point you will be able to diagnose the root cause(s) of your problems in the shortest time possible.
Step 5: Diagnose and inspect in a logical manner
Step 5 will be fairly straightforward if Steps 1 through 4 were done correctly. Letís prioritize our list first. The new order of the list would be:
* Bypassing dump valve in RWAL valve;
* Bypassing quick-take-up valve in master cylinder;
* Improper rear brake adjustment;
* Front brake problems creating excessive clearance between the pads and rotors; and
* Air in the hydraulic system.
The order of this list is based on a combination of the techniques listed earlier. They are listed in both the most common cause and easiest to check. This re-organizing allows us to accomplish Step 5 in the most logical sequence.
The diagnosis of a bypassing dump valve is simple and straightforward. The steps are covered in detail in last monthís issue (May, BRAKE & FRONT END, page 22). This should be the first thing you check for in any vehicle equipped with rear-wheel ABS that is experiencing a low brake pedal. We performed the check on the vehicle in question and found the dump valve to be bypassing. The question then became should we stop our diagnosis or check to see if there is anything else contributing to the problem?
The answer comes from an understanding of what effect the valve can have on the pedal height. The amount of pedal travel caused by this condition will be based on the following:
* Diameter of the master cylinder piston;
* Diameter of the accumulator piston in the RWAL valve;
* Accumulator piston travel; and
* Brake pedal lever ratio.
In general, a failed dump valve will add between two and four inches of pedal travel depending on the vehicle in question. There is a simple way to determine if there are other factors contributing to the low brake pedal. It involves taking the RWAL valve out of the picture so its influence on the brake pedal is eliminated temporarily. To do this, follow the steps below.
Remove the accumulator cap screw from the RWAL valve body. On the torpedo-style valves with the cylindrical bodies, this will require a 1-1/4-inch wrench or socket. The block-style valves use a 7/8-inch cap screw. Remove the cap screw and spring.
This step involves finding or making a spacer to hold the accumulator piston in place. Figure 1 shows a brake line fitting and two 1/4 M20 nuts being used to prevent the accumulator piston from moving. It is held in place by the cap screw. The spacer for the torpedo-style valves will have to be about 1-1/8 inches. The block style will require a spacer of about 1-3/4 inches.
With the spacer installed, start the vehicle and check the brake pedal height. I had my class perform this step and the result was a better "low" pedal. We gained about three to 3-1/2 inches by eliminating the dump valve. This still left us with 2-1/2 to three inches of pedal travel.
The steps above allowed us to determine that we needed to continue our diagnosis. It also took the failed dump valve out of the picture so it was not "clouding" our findings. It is important to note that this test step is NOT meant to be used as a fix for the failed dump valve. It is only to be used as a diagnostic technique.
The next most logical step to perform was a line-lock test. The line-lock test would allow us to determine where the remaining pedal travel was originating. The students installed line locks at each brake hose. We confirmed a good test by applying the brake pedal with the vehicle running and spinning each of the wheels. The result was a high, hard pedal. Based on this result, we could determine that the system was good from the line locks up, right? The answer to this question is yes and no. I will explain this in a minute.
To determine how much impact the rear brakes were having on the problem, we simply took the rear line lock off. By taking off the rear line lock we added the rear brakes back into the system. Properly adjusted rear brakes should add between 1/8 and 1/4 inch of pedal travel. When we performed this step we got just under 1/2 inch of pedal travel. We could deduce that we might have a minor adjustment problem based on this. During inspection it was found the left rear adjuster was not working (See Figure 2) resulting in the extra 1/4 inch of pedal travel. This still left us with more than 2-1/2 inches of extra pedal travel.
The next thing that was done was to add the right front wheel back into the picture. With the pedal still depressed, the right front line lock was removed. The pedal dropped about 1-1/2 inches. Next, we removed the left front line lock. This step resulted in just over one inch of additional pedal drop. Based on the results of this test, we could conclude the rest of the problem is in the front brakes, right? Again, the answer to this question could be yes and it could be no.
To determine which it is we have to generate a list of what could cause a loss of pedal height in a disc brake system. We would apply our same 5-step process, but only to the disc brakes. The list we would produce would include:
* Excessive wheel bearing play;
* Seized slider mechanism;
* Binding or cocked brake pads;
* Air at one or both front calipers; and
* Self adjustment is not being allowed in the caliper(s).
The list is organized in the most logical order. It is listed from the easiest to check to the hardest to check. Excessive wheel-bearing play usually wonít show up unless the vehicle is moving, but should be checked. The wheel bearings on the vehicle were all adjusted properly and showed no excessive play.
The next two items on our list are easy to check. You can start by having someone apply and release the brakes several times while watching the caliperís movement. If the slides are seized, it causes the inboard pad to flex the rotor. This would cause the caliper piston to have to move too far. It takes only a little extra movement of a caliper piston to have a dramatic effect on the brake pedal.
|01-12-2008 02:23 PM
i have all the air bled out, it is the pedal travel im trying to reduce.
|01-12-2008 01:23 PM
You shouldn't need it. That is just a band-aid. A rebuilt master would be about the same money and easier to install. If bleeding is the problem power bleeding should do the trick. Here is a start to a wiki article on how I made my $20 power bleeder. I'll finish it up as I get more time. Back out to work on my trucks...
|01-12-2008 08:02 AM
im gonna be putting a 2lb. residual valve in the line before i go any further now.
Originally Posted by Triaged
The bigger (1-1/8" vs 15/16" for the S10) master cylinder won't make the brakes drag. It will make your pedal harder to push and firmer. However my guess is that you either have a bad master cylinder or still too much air in the lines.
|01-12-2008 12:50 AM
The bigger (1-1/8" vs 15/16" for the S10) master cylinder won't make the brakes drag. It will make your pedal harder to push and firmer. However my guess is that you either have a bad master cylinder or still too much air in the lines.
|01-06-2008 09:42 PM
master cylinder interchange
i have an s10 with rear disc brakes with steel braided lines and a manual brake master cylinder. i have bled all of the air out brakes but the peddle is still spongey the master cylinder is bench bled but is a used one. if i would install one from a 73-87 fullsize chevy pickup for example, would i have too much drag on the front brakes?