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Fading brakes

I am well aware that when brakes have fading problems, you’re supposed to bleed them, which I have to kingdom come. But the following problem still persists:

When you depress the pedal to the point where the wheels lock up as they should, then slightly relieve the pressure, then press again, the pedal goes down the same amount as the first push before the pressure builds to lock the wheels again. If this push-release process is repeated 4-5 times, the pedal goes to the floor, rendering the brakes useless until the pedal comes back to the top.

There are no leaks that would be causing this, and the last 2 times I bled them, there was no change in pedal response. I have a theory, but I’m hoping someone can dissuade me from doing something to the master cylinder.

Bleeding is not always the answer to brake fading. If the master cylinder is failing no amount of bleeding will cure that.

If bleeding is done, then bleed the master cylinder first followed by bleeding the right rear wheel first and then in turn the LR, RF, and the LF.

If the truck has drum brakes on the rear these should be inspected as a worn or out of adjustment rear brake shoes can mimic a failing master cylinder.

What you describe is not ‘brake fade’. Brake fade is when the brakes don’t stop as well after extended use. What you describe sounds like a bad (leaking) master cylinder, or a problem elsewhere in the system (maybe ABS?). What year and miles do you have?

Texases is correct of course. Brake fading used to be common back in the 1930s and 1940s. It’s caused because drum brakes don’t dump heat as effectively as modern disk brakes. If you get them hot enough – say by driving down a good sized mountain with a lot of braking, the brake fluid will boil. Brake fluid vapor – unlike brake fluid liquid – is highly compressible. … causing the brake pedal to go straight to the floor.

Modern brakes – even drum brakes probably – are better at shedding heat. And modern brake fluids are better also. And modern brake systems are less likely to get a lot of water dissolved into the brake fluid. Water in the brake fluid lowers the boiling point.

Texases (and I) doubt that you are really boiling the brake fluid. But the process you describe of repeated braking really is how you would boil it if you really wanted to. So, … just maybe …

My impression is that most modern braking systems are split such that you should have pedal and two working brakes even if one brake has failed. I’m not sure what happens if you actually manage to boil the fluid in one circuit.

I also think it’s the master cylinder.
In the future change the brake fluid every 3 years.

It’s a '99 Sport with 133k, and ABS is not an issue. I bypassed it once I discovered the pump was jammed and was plugging my rear brake line. However, the issue was still there before I did this, meaning it’s either the front brakes or the cylinder, if that narrows it down any.

I’m fairly certain that it’s not a boiling issue; it happens whether the truck is sitting and running, moving, or has been off for a week.

I just bought the truck 3 weeks ago, so I have no idea when the last fluid change was, but I’ve put about 2 quarts of new fluid into the system just replacing the lines and bleeding.
As for your (collective) diagnosis, that’s what I fear, because I think they run about $50-70, and I barely have the finances to get it on the road as it is.

It does indeed have drums in the rear, but as noted in my response to texases (below), the issue occurred even when the rear brakes were non-functional. This doesn’t rule that out, and I’d like to know what to look for in case that is the source in the future, but right now, my fears of master cylinder failure are being realized.