Over the weekend I installed a new master cylinder and rear wheel cylinders on my 87 Dakota, trying to remedy a slowly sinking pedal while applying light to moderate pressure, like at a stop light. There was no loss of fluid, so I figured it was the MC. It was time to replace the shoes, so I did it all at once. Bled the lines and got ready for ‘problem solved’, but that didn’t happen. After driving around today doing what I do, the problem was still there. New MC so I began to suspect a leak. Sat in the driveway with the engine running and applied enough pressure to the brake pedal to get it to sink, removed foot and repeated about 10 to 15 times, turned off engine and got out and looked for leaks on the concrete. There were drips under th MC area. Lifted the hood and found fluid dripping down the booster housing. Damn! Bad new master cylinder, I thought (not a rebuild). The leak was apparently coming from the rear of the new MC.
Then I came across this:
MASTER BRAKE CYLINDER PROBLEMS
Master cylinder problems include external leaks, internal leaks and valve problems. Brake fluid leaking from the rear of the master cylinder bore is usually bad news because it means the seals are leaking. But sometimes a leak at the vacuum booster input seal and/or pushrod misalignment can cause a fluid leak, so a closer inspection is needed to determine the cause.
Brake Master Cylinder
This seems to indicate that the brake vacuum booster could be causing the leak, but I don’t see how and that is why I am posting this. Is anyone familiar with this? Can a bad brake booster cause a good MC cylinder to leak? If so, how? I don’t want to throw a new MC at the truck if that is not the problem.
PS - The MC and wheel cylinders that I replaced were original.