01-12-2008, 02:10 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.