Jim Strathearn

Got a question about an 87 that belongs to a friend of mine...

The left front brake will not bleed unless the engine is running and there is assist from the brake booster.

Car pulls to the right when braking. (DUH!)

When you can get enough brake pressure to apply the front left brake, the caliper will not release unless you open the bleeder.

Calipers, rotors, disc pads and hoses for each front brake have been replaced and the symptoms are still there.

Any ideas? Could this be the ABS unit?

TIA!!!

EastCoastHD

I had a similar problem with a prior GM vehicle. Turned out to be the brake hose. They get very hard over time and restrict flow.

Is the car equiped with the original rubber brake hoses? If so I would suspect that first.

CAJUN C4

I wonder if that could also not let brake fluid back in the M/C? My friend has an 84' and the right front brake won't release.
I wonder if that's the problem?? 25 year old hoses?

JrRifleCoach

As previously listed, check the mechanicals first.

The ABS uses 3 channels, LF RF & Rear. It could be a fault of the ABS. You might try pulling the ABS fuse and going for a ride. Worse case senario, you'll lock 'em up when panic stopping. But the brakes will still work.

Jim Strathearn

Thanks for the replies. Both hoses were changed as well as both calipers. We'll try pulling the ABS fuse.

Jim Strathearn

Pulling the ABS fuse made no difference. Seems like something is plugged up. Does the ABS unit get clogged?

c4cruiser

It's possible that there is a restriction in the hard line from the ABS unit to where the LF hose attached to the hard line. IF the fluid in the system was old, there could have been some debris come loose during a flush.

But having the engine running shouldn't make that much of a difference if the lines are clear; I bleed my 87's brakes with the engine off and there is no problem with getting enough pressure.

The ABS modulator valve cannot raise pressure to the brake lines and the ABS should not play a part in bleeding that line with the engine running or not. Unfortunately there are no on-board diagnostics (paper clip method) for the ABS system, you have to use an ABS diagnostic tester. It could be that one of the two relays is not working properly. The ABS does a self-test when the engine is started and the car moves forward thru 3-4 MPH and is an audible buzzing sound. Do you hear that??

If you have access to a pressure bleeder, remove the hose at the hard line connection and let the pressure bleeder run a fair amount of fluid through that line. If the fluid comes out clear and the problem still exists, I would suspect a bad modulator valve in the ABS. That valve is not serviceable so the repair is a replacement of the ABS pump.

pcolt94

Definitely could be the ABS unit. But you have to first determine if the flex line is defective. If the line is good its almost a sure bet the ABS is bad. Does not mater if you pull the fuse or not the valves don’t care if they are stuck.

We just had another bad ABS unit last week.
http://forums.corvetteforum.com/c4-t...releasing.html

Jim Strathearn

Can we just bypass the ABS module altogether?

c4cruiser

Pulling the ABS fuse effectively bypasses the ABS system. And unless the ABS activates, normal braking effort doesn't use the ABS.

Do a flush of that line first to see if the problem clears up. You may have to pass 3-4 ounces of fluid through under pressure to flush out any gunk.

Did you buy new calipers or rebuilds? Sometimes rebuilds will not be really clean or the pistons may be causing the problem by not sitting in the bores properly or a rough surface in the bore or the piston wall. Actually after thinking about that, I'll bet the problem is in the caliper.

Might be worth pulling that caliper and taking the piston out and flush out the fluid and clean everything up then inspect it carefully. You should be able to get new seals to re-assemble the caliper. Start there before playing with the ABS too much.