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Feel free to post CONSTRUCTIVE criticism

There seems to always be people with questions/problems regarding the braking system on their rig. Most of this is due to changing out the factory components and then discovering the brakes aren't working properly. This is an attempt to gather information and troubleshooting tips to help the community. I'm certainly no expert so if anything needs to be added/corrected please let me know.

Section 1 - Major Components of the Braking System
Master Cylinder - basically a tube that houses two pistons which push fluid through the brake lines to make the calipers squeeze the pads against the rotors. There are two brake lines coming off the MC. One is for the rear brakes and one for the front. The line for the rear runs to the combination valve and from the combination valve to the rear axle where a T fitting splits it into two lines, one for each rear brake. The line for the front brakes runs to the combination valve and is split into two lines coming off the combination valve. One of these lines goes to each front brake. The two pistons in the MC are separated by brake fluid. The first piston pushes brake fluid, which in turn pressurizes the first line coming off the MC and the fluid also pushes against the second piston. The second piston pushes brake fluid for the other line coming off the MC. This will be relevant later in the troubleshooting tips.

Brake Booster - most of our vehicles came factory equipped with a vacuum style brake booster. This type uses vacuum from the engine to help push the rod acting on the MC to push the pistons. Diesel engine equipped vehicles can't use a vacuum style booster so they use hydroboost. In this case the booster uses hydraulic pressure from the power steering pump to help push this rod. With either style the important thing to remember is that the booster is an aide, or helper. If you could push the pedal hard enough you could generate the same force without the booster. The MC itself does all the work of moving the fluid, the booster just helps you generate more force by increasing the force on the pistons in the MC.

Combination Valve - the combination valve serves several purposes, hence the name combination valve. The portion of the valve for the rear lines contains a proportioning valve. This limits the amount of pressure put to the rear brakes to help keep them from locking up before the fronts. The portion of the valve for the front brakes may or may not contain a metering valve. The metering valve holds off pressure on the front brakes until a certain threshold is reached at which point it opens up and allows pressure to build on the front brakes. This was used on vehicles that had factory rear drum brakes. The metering function would hold off pressure on the fronts to allow some pressure to build in the rear drums and overcome the force of the springs holding the pads away from the drums. In a front and rear disc brake setup this metering function is not needed. Newer vehicles with all disc brakes don't have the metering function in the combination valve. The combination valve also serves as the trip for the brake warning light. The sensor is mounted in the center of the combination valve. The two halves of the combination valve are connected by a small piston with an indentation in the middle. If the pressure is equal on both sides of the unit (front and rear brake lines both have pressure) then this piston doesn't move. If pressure drops substantially on one side of the unit then this piston may move horizontally and the sensor will be tripped since it will force the plunger in the sensor upward because the plunger is no longer resting in that indentation.

Calipers - In a disc brake system these are what squeeze the pads against the rotors. One important thing to remember is that when bleeding brakes using the bleeder on the calipers the bleeder screw must be at the topmost position. Air rises and you will never get all of the air out of the caliper if the bleeder is at the bottom. This may mean that you have to remove the caliper in order to get the correct orientation for bleeding. If you have converted your rear drum brakes to disc and have the caliper clocked up high then you will likely need to remove the caliper for bleeding.

Section 2 - Braking Theory/Operation
Let's look at some common questions
1. Will larger calipers give me better/stronger braking? Yes/No/Maybe - A larger piston on the caliper can give you stonger braking, but multiple smaller pistons in the caliper will give you more force than one big piston. This is why performance braking solutions utilize calipers with 4 or more smaller pistons. Newer vehicles often have calipers with two pistons. Older vehicles often had one large piston in the calipers (think of a Dana 60 front caliper). Somoene feel free to chime in here with the actual math involved in calculating this. It can also be thought of in terms of force vs. area that force works against. 1500 PSI working against two square inches will generate less braking power than 1500 PSI working against 4 square inches. Multiple smaller pistons when added together can have the same or greater surface area than one large piston, and multiple pistons spaced out will give more even pressure applied to the pad.
2. Will a larger Master Cylinder Give Me Better/Stronger Braking? No not necessarily - A larger MC will move more fluid, but will not generate more pressure. Quite often going to a larger MC will result in LESS fluid pressure. Moving more fluid can help with pedal travel somewhat by taking less pedal movement to make the pads contact the rotors firmly. In a perfect system as soon as you press the pedal all of that force would be transferred through the system and directly to the pads. In reality there is a small amount of "give" to the system. The calipers will release slightly when you take your foot off the pedal, as brake fluid gets older and absorbs moisture it gets spongy, etc. All of this factors in to the small amount of "take up" that occurs before you start getting real pressure applied to the pads when you push the pedal.
3. Will switching to a Master Cylinder made for a disc/disc setup improve performance on a vehicle that I did a rear disc conversion on? Maybe - Master Cylinders for disc/disc setups are generally designed to push more fluid to the rear brakes. This is why those MC's have a larger reservoir for the rear brake portion.
4. Will adding an adjustable proportioning valve help correct braking problems? Maybe, depends on the problem - A proportioning valve can only limit the amount of pressure applied to the brake line. Most commonly you would put on on the rear brake line to turn down the pressure put on the rear brakes to keep them from locking up before the fronts do in a hard stop. Keep in mind the factory combination valve already has proportioning built in, so adding an adjustable one to the system would allow you to turn it down further, but will not help increase rear braking. If you want to truly fine turn your rear braking bias you may need to disable the proportioning function of the factory combination valve and then add an adjustable one to the line. This way you only have the one valve affecting the line pressure. Keep in mind that a proportioning valve can only limit pressure, not increase it. If you have weak rear brakes, a proportioning valve won't help. Using an adjustable proportioning valve has the advantage of tailoring the braking bias to your own needs/preferences. You could bias more braking to the back for towing, less for normal street driving, or equal it out for off-roading.

Section 3 - Troubleshooting
I have had better luck bleeding the brakes and troubleshooting some problems by keeping the engine off so the booster isn't working during this process
Check the reservoir to be sure it is full of fluid first!
1. The pedal isn't very firm and/or can be pressed to the floorboard:
A. If you pump the pedal a few times does it get firmer and stays up off the floorboard? Yes - You have air in the system somewhere. You can verify this by keeping the cover off the reservoir and then pumping the pedal. When you release the pedal do you see a stream of fluid spouting back into the reservoir? If you do that could be the compressed air pushing fluid back into the reservoir.
B. When you press the pedal does it seem to press easily until it feels like it hits a certain spot and then it feels more normal? Yes - you may have air in either the front or rear brakes. What you are feeling is likely one of the pistons in the MC hitting the other piston. If the system has no air then the fluid between the two pistons keeps them from hitting. This design also allows for at least partial braking if there is a failure in either the front or rear. Once the piston hits the other piston it still pushes fluid to the good pressurized system.
2. During a hard stop the rear brakes lock up and the fronts do not - Too much brake pressure to the rear brakes. Use a proportioning valve to limit the pressure to the rear. On the street having the back lock up before the front can cause the rear of the vehicle to swing around.

385 Posts

I enjoyed your blurb. I have posted a question about brakes on my Blazer and no one seem's to have an answer? That's hard to believe on Pirate. I was looking to install 3/4 ton Chevy front rotors/calipers on an ford 9" rear. I was hoping to hear from someone That has done this swap.
Thanks Dan
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