I found this thread and it describes my same problem…well now I can’t find it ugh
brakes started lightly engaging without me touching the pedal, but the more I dove, the more they engage, to the point that I have to stop. I let it cool down a half hour, then I can drive off but only drive a short distance before I have to stop again. Finally got it home. I thought the problem was the brake master cylinder, so I replaced it. It was not the problem, after several miles, the problem came back, limped it home and came here to search for some answers. I read on another post with the same problem, they said they found the problem to be fluid behind the diaphragm of the booster, but not what they did to fix it!? Replace it or can the fluid get cleaned out, someone posted replacing the booster and master cylinder together as a unit, I’m not even sure I can get them together at any of the parts stores here in San Antonio Texas. I’m hopeful to be able to clean out the booster? Any thoughts or suggestions or input would be appreciated
2006 Silverado z71 5.3 320k miles mostly all mine

 

If you’re still needing an answer here…

First, is that MC reservoir completely full of fluid? Meaning, is it full all the way to the cap? If so, remove enough fluid to drop the level down to or just below the MAX mark on the side of the reservoir. There has to be room in the reservoir for the fluid to expand with temperature - that may be the reason you’re still having this problem.

Now, If any brake fluid has been pooling inside the brake booster, I’d recommend replacing it. Brake fluid is corrosive - even if you clean out the fluid, the corrosion that’s already started will continue and eventually the booster will require replacement anyway.

I’m not sure where a complete master cylinder & booster would be found as an assembly, but if the existing MC is recently new, replacing only the booster isn’t going to harm anything. What you need to make sure of is the booster linkage rod is properly adjusted.

As to the cause of this issue, there’s a known problem on some of these trucks with the ABS control module getting gunked up and preventing full release of the brakes. I’m guessing the fluid in the system when you replaced the MC was quite contaminated - if that’s true, you can try flushing the system 4 or 5 times to clean it out.

Pressure bleeding is the best way to do this, but manual bleeding works too. If you can activate the ABS as well (scan tool or by driving it), that will help.

I’d recommend verifying the ABS module is the cause by driving the truck close to home until you feel the brakes starting to grab, then get it back to your driveway and the wheels (at least 2 of them) off the ground ASAP. If it’s the rear wheels, chock the front wheels and shift the transmission to neutral KOEO.

Attempt to turn the rear wheels to get a feel for the resistance, then, starting at the MC, break loose the line fittings one at a time while working towards the wheels, checking the wheel to see if it frees up after each one. When you can turn the wheel after loosening one of the fittings, the source of the restriction/blockage is isolated. So, if it’s a fitting off the ABS module, the module is the likely problem.

This can be much easier/faster if you have a helper.

 

don't forget to check for brake fluid contamination! and i would never try to clean out a booster. i believe many auto parts stores sell a reman master cylinder/booster combo, good luck to you

 

Well after asking a bunch of mechanics on my problem, I finally resolved the issue. Seems that my brake master cylinder (vacuum type ) was leaking into the brake booster. Apparently the fluid builds up inside and gets hot, causing the booster to get stuck in the engaged position. After it cooled, it would release(I was having this problem in the hot summer), and I could drive it a few miles before I had to pull over and let it cool off again. So I replaced the booster and bled the system, problem is solved. 340k miles at time of repair. It still smokes a little oil and leaks oil from the main seal, but that’s a project for another day.