Brake pedal fade

If you are not losing brake fluid, the master cylinder is leaking internally. That often starts out as an intermittent problem. If you are losing brake fluid, for an intermittent problem the best suspect is the rear wheel cylinders. You may still have a GM-only wheel cylinder design where it is held to the backing plate with an o-ring with two fingers. The hour glass-shaped hole in the backing plate rusts out, then the wheel cylinder is free to spin. In severe cases the pistons will pop out and all the brake fluid will be lost.

If you have a real slow leak, typically seepage from a wheel cylinder, you will reach the point where the master cylinder will be drawing air. That will cause a low pedal. That can be intermittent at first too. Not all vehicles have a low-fluid switch as part of the red "Brake" warning light circuit, but if you do, that light would have been on by now if low fluid was the cause of the problem.

No fluid loss or leaks. I suspect master cylinder, but I am wondering why temperature difference makes it fail. I was thinking putting ice on master to see if it fixes it temporarily. Also I have never changed master with ABS attached did not find instructions on youtube did read it could be done without special equipment, but no actual 1 2 3. Looked at it did not see whats holding ABS to master.

If you have the hydraulic controls bolted right to the master cylinder, that system is very inexpensive and pretty reliable and effective. Two hollow bolts go through the controller and thread into the ports of the master cylinder.

To make bleeding easier, bench-bleed the new master cylinder, install it, then slowly push the brake pedal down about 1/3 of the way. Never push a brake pedal over half way on any master cylinder more than about a year old, to avoid damaging it on the crud and corrosion that build up in the lower halves of the bores, but more importantly, on GM vehicles you can trip a valve by going over half way to the floor. That takes a lot of work to reset it.

Release the pedal quickly, then slowly push it part-way down again. Pushing it slowly lets any air bubbles float back up while brake fluid is going down. Releasing the pedal quickly washes any air bubbles up into the reservoir along with the brake fluid that's rushing back. You should not have to bleed at the wheels.

When you say trip a valve you mean in the ABS unit?

ABS light is on so its not working anyway?

I did see two bleeding screws on the abs unit near top front close to each other.

Nope. That valve is in all master cylinders for GM front-wheel-drive cars. I don't know if it is used when the car has ABS and I'm not aware of any other car brand that uses it.

It's function is to block two ports when there are unequal pressures, as in a leak or improper bleeding procedures. In the case of a leak, that circuit will not build any pressure but the other circuit will. If the brake pedal is pushed over about half way to the floor, that valve will trip and block the port with the leak AND the diagonally-opposite port. That means if the right front has a leak, the valve will block fluid flow to the right front and the left rear wheels. The advantage is you won't lose all the brake fluid. The disadvantage is the only symptom may be the red warning light. The car will still stop in a straight line.

After resetting the pistons in the calipers to install new pads, the brake pedal has to be stroked to run those pistons back out. One will get there first and that circuit will start to build pressure before the other side does. That will trip the valve too if the pedal is pushed over half way down. There are people who believe you can avoid this by bleeding in a certain sequence since bleeding can cause this too, but that is not the case. Failure to build pressure in either circuit can trip the valve. Each circuit has one front wheel and one rear wheel in it.

The only way I have ever found to reset that valve is to loosen the cap on the reservoir, open one bleeder screw for one of the wheels where no brake fluid will come out, then give that bleeder screw a quick short blast of compressed air. After that, let it gravity-bleed. You don't want to blow in so much air that it fills the line all the way to the master cylinder. Doing that just makes a lot more work to bleed it out again. You only need enough air to push that valve a fraction of an inch. After that, the air should bleed out in 10 to 15 seconds. Close the bleeder, "irritate" the brake pedal a little by hand to wash any remaining air bubbles into the caliper, then open the bleeder for a few seconds to burp it out.

That actually sounds like a good idea! But I also have been trying to find pictorial or video instructions for changing master with ABS and no luck so far.

I assume I have ABS Vl but the site I posted did not show a pic for it. I also heard people have rebulit master on car without detaching ABS but the kits I found were more expensive than used part.

Ok this site says you have to perform a scan tool process to relieve spring pressure in order to seperate master from ABS. Anyway I also heard some say eliminate ABS and put proportioning valve in line to rears.I recall I had an old dodge van that had bad back lines and cylinders so I just blocked them off! Ran on front brakes only! Couldnt really notice the difference!

I think I may have figured out why it fails sometimes seems always on hot days after driving and parking. The engine compartment heat is staying longer and raising temp of master and fluid causing viscosity loss and maybe cylinder size change. Hot today so will try opening hood after driving and see if it stays working. Haha

What I ended up doing was to ad a capful of lacquer thinner to reservoir what this did after a couple days is make the seals expand enough to maintain contact with cylinder walls its been working ever since until I get time to replace master. Hehe

Oh my. Tell me you're kidding! I've never considered what lacquer thinner is made from, but if any petroleum product gets into the brake fluid, the repair will be more expensive than the car is worth. Rather than retype a really long answer, here is a copy of a short version I've posted before. If I need to, I'll find the long and boring version with a lot more information. Remember, this was written to someone else, so just take from it what you can:

If the fluid is contaminated with petroleum product the car is probably not worth repair. The first clue is the rubber bladder seal under the reservoir cap will be blown up and mushy. All the rubber parts in the hydraulic system will be grown the same way. The lip seals inside the master cylinder will grow past the fluid return ports and block them. That keeps the trapped fluid under pressure and unable to return to the reservoir, and that keeps the brakes partially applied. The excessive heat buildup from driving the car like that makes the brake fluid get real hot and expand. Since the fluid is trapped, when it expands, it applies the brakes harder. Eventually the car will barely move and one or two wheels will get real hot, both symptoms you're describing now.

The only proper fix is to remove everything with rubber parts, flush and dry the steel lines, then install new parts. That includes replacing the new calipers and wheel cylinders, all rubber flex hoses, the combination valve, and the master cylinder. If any part, such as the new calipers, are left on the car, the contamination will leach out of the seals and recontaminate the new fluid.

Every week we read about someone who accidentally put power steering fluid, engine oil, or transmission fluid in the master cylinder but there are other ways to contaminate the fluid. One problem that was more common years ago was repacking front wheel bearings, then wiping hands on a rag, then reseating the bladder seal into the cap when refilling the master cylinder. The grease residue on fingertips is enough to contaminate the system.

Another problem showed up many years ago at a mass merchandiser. A new mechanic wiped out a funnel used for engine oil, then used it to fill a brake pressure bleeder ball. The residue in the funnel contaminated five gallons of brake fluid and many cars.

Experienced brake mechanics will even wash their hands with soap and water before touching any seal or other rubber parts they're working on. Also, they will never use penetrating oil to free up rusted fittings in the brake system.

Brake fluid can be contaminated with water too but that won't cause the rubber parts to swell. That will simply lower the boiling point from well over 400 degrees to closer to 212 degrees, and it will promote corrosion of metal parts but that's all. Moisture contamination comes from leaving the master cylinder cap off or the cap off the bottle of new fluid. Brake fluid sucks humidity out of the air, and it will absorb some over time in the car's hydraulic system.

Lets hope you don't find contaminated fluid. It's fairly common to find rust buildup inside crimp fittings and brackets on flex hoses restricting them so fluid can be pumped to a caliper but it takes a long time to release. Often the additional clue is the brake pedal is higher and harder than normal when you press it.

Still working hopefully the small amount and its high tendency to evaporation will keep the system from degrading.

I, m going to inspect cap seal for bloating too.

I mean its a 20 yr old vehicle with a lousy computer interface and brake, engine, abs lights on I paid 150 for.