Brake pedal slowly goes down when stopping.

[JLS1]

Got a '93 astro. Today when I braked for a light, I was able to stop, but the longer I waited at the light, the more the pedal would slowly sink to the floor. At the next light, same thing, good stopping brakes, but the longer I kept pressing on the pedal, the more it would slowly sink to the floor. I have two known issues with the braking system: The fluid in the M/C is black and nasty; and a recent check by a mechanic said my rear cylinders were "blown", however I haven't lost hardly any fluid from the resivouir. So I know I am looking at rpl. the rear cylinders and a fluid flush at the minimum. The pads, rotors, shoes and drums are OK.
Does this sound like a bad master cylinder, or a problem with the vaccum booster unit? How can you tell the difference? Thanks for any help! : )

[drew300]

My 1st thought was the master. But the rears are leaking. That's a good place to start. Dirt in the master is bad. It may need replacing too, if the rears don't fix it. (flush for sure)
Do you have a vacuum booster? The newer vans don't.
To check a vacuum booster, with the engine running, push the brake pedal. Turn off the engine, the pedal should go down a bit more. And if you pump the pedal with the engine off, you may hear the air hissing into the booster, and the pedal will stiffen. But that's assumming you don't have any fluid leaks, so you have to fix the rears first.
BTW: I flush out an once or so of brake fluid from each slave every 2 years or so. It helps prevent some of these problems.

[rksnc]

Yea need to repair the wheel cylinders for sure. But still kinda sounds like bad Master if you are not having a lost of fluid.
If the booster was bad you will have a very hard pedal. It would take great effort to stop van. With engine not running pump the pedal 3 or 4 times. While holding the pedal down, start engine, when it starts the pedal will fall some more. If that happens booster is okay.

[JLS1]

Originally Posted by rksnc

Yea need to repair the wheel cylinders for sure. But still kinda sounds like bad Master if you are not having a lost of fluid.
If the booster was bad you will have a very hard pedal. It would take great effort to stop van. With engine not running pump the pedal 3 or 4 times. While holding the pedal down, start engine, when it starts the pedal will fall some more. If that happens booster is okay.

Thanks!! Today I RPL'd the rear cyls, went ahead and got a rebuilt master (only $29 at Advance Auto). I bench bled the master before installing, then did a complete fluid flush, all the way around.
I still have the same problem. ARRGH! I did like you said, engine off, pump pedal and hold, start engine and the pedal falls some more. So far so good. However, if I hold the pedal with engine running, it slowly, slowy creeps to the floor. The funny thing is, if the engine is off, I have a nice, full pedal, and it doesn't creep down at all. It's only when the engine is running that I can push it to the floor.
It's a 2WD van, and has anti-lock braking. Could the ABS raz-matazz be causing this? She's an old cargo van, and I use her as a work van; got alot of tools and parts in back, and I live in the mountains; gotta brake up and down hills. It's the down hill braking at redlights that makes it scary. I run out of brake pedal before the light turns green sometimes. I appreciate any help anybody could give. Thanks.

[JLS1]

Oops. Sorry everybody. After numerous test drives, brake pumping, backing up, etc. it seems the brakes are holding now. Must have had a little air left in the system. Thanks to all and God Bless Automotive Forums!