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Old 01-12-2008, 01:59 PM
Triaged Triaged is offline
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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. (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 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.

Last edited by Triaged; 01-12-2008 at 02:10 PM.
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