I searched the forum and didn't find anything exactly like what I have so here goes.

I have an 87 IROC with a 5.7 liter and 4 wheel disc brakes. I just replaced the front pads and rotors, the rear pads and rotors still have a lot of meat on them ( they look new except for the rust and dirt and stuff ) The guy who owned the car before me apparantly stripped the bleed screw out of the driver's side rear tire and just never blead that side. I replaced the screw and blead all four wheels but I still have the same problem.

With the car off I can pump up the brakes and they will get rock hard right to the top of the pedal and they won't leak down or slowly sink to the floor. If I turn the car on the vacuum booster kicks on and the pedal immediately drops to the floor with almost no resistance.

I can not under any circumstances lock up the brakes. Sitting at idle the pedal must be to the floor to stop the car from rolling forward (at about 750 - 1000 rpm idle) I cleaned the wheels a few days ago and there is brake dust on the rims again which tells me the brakes are working some what all around. I'm not losing any brake fluid and don't have to periodicly top off the master cylinder.

I need help on this one guys I'm kind of stumped. Could this be my Master Cylinder? Do you think there could be air in the lines after I bled them? Could the calipers have frozen mostly Open?

Thanks for any insight you guys can give!

 

You say the pedal pumps up with the engine off? Sounds like you need to continue bleeding the system. The pedal should be hard and right to the top on the first pump with the engine off.

 

Have you been able to resolve this issue? Let us know!

 

It sounds like your master cylinder is leaking internally - One sure way to test it is to get some plugs that fit each output of the master cylinder and push the pedal - If the pedal moves/sinks, you've got a MC problem.

 

Another way to check the MC is with the engine running, apply 30-40 pounds on the brake, hold firm and then let off "just a little"...and if your MC is bad, it will generally fall under your foot....just hold force constant and you can chase it to the floor

 

I am having the exact same problem with my 88 IROC. We have bled the heck out of the brakes and taken apart the proportioning valve and cleaned it out and still no stopping power. I have a new master cylinder and have not taken it back to see if I had received a bad part from the parts store. Hope fully something will work out soon and as soon as I figure it out I will let you all know. It's been driving me nuts for a few weeks now. lol

 

Depending on the quality of your replacement brake pads, you may need to "burnish" them. Some brands don't have the targeted friction until this is done. This occurs naturally when you drive the car (typically after about 200 stops), but you can accelerate the process by making repeated brake stops a few blocks apart at around 0.2gs (a slightly aggresive stop....maybe 20 pounds brake pedal force on our cars).

The booster is not the issue as it's job is only to reduce the brake pedal force. So, if you get the drop with the engine on, you're just at a higher equivalent pressure.