I have an 87 C4 that has the single master cylinder brake reservoir. A few weeks ago the brake pedal went to the floor a couple of times while driving. The brakes could be pumped up easily and stopped the car, and the condition is very intermittent. I took it to the dealer who had it 3 out of the past 4 weeks and they finally thought that they had tracked it to the mater cylinder which was replaced. The brakes are still not right and I have inconsistent pedal. The WEIRD thing is that it keeps popping the seal on the rubber master cylinder cap. The edge of the cap will be up like it has burped the reservoir. I have never seen that on any brake system, and I didn't see any threads in the forum that matches this condition. The dealer is stumped and I am not sure what could cause that to happen. Any thoughts would be greatly appreciated.

 

Did the dealer bleed the ABS? Could be that the lines to the ABS are clogged or the ABS pump is not working.

When you start the car and move forward thru 2-4 MPH, do you hear a buzzing sound from behind the driver's seat? That sound is the ABS doing a self-test.

Try removing some fluid from the M/C reservoir to where it's at the add fluid line (the lower of the two lines of the reservoir).

 

They did check the ABS, and yes the pump runs when first backing up. I do get an ABS warning light very infrequently, but they said they checked for codes and didn't find any. Thanks!

 

Hi

Having to pump the pedal to get it up where it should be is usually a sign of air in the brake fluid, however warped disc rotors or worn wheel bearings can also have the same effect.

Do you feel the brake pedal pulsate? when appling the brakes?
that is a sign of warped discs.

Next will be raising a front wheel from the ground, and check for any bearing play by placing a hand at the top and bottom of the tire. Now apply force to top and bottom and look for any play, do the same side to side.

I do know the ABS system has a certain bleeding procedure, someone may post the correct procedure.

I would recommend you bleed the brakes via gravity(it may save your booster, that extra travel usually tears the old diaphragm in the brake booster).

open the reservoir cap and add new fluid, now open the brake bleeder screws a couple of turns on the rear callipers. Check the same ammount of brake fluid comes out each side, and keep topping up the reservoir. Now tighten the bleeders and do the same with the front callipers, when done the pedal should be firm no spongyness.

If you are still having issues check your brake hoses, they should not be moist (sign of leak) they must be dry and even not have any swolen bits. Old brake hoses can swell with pressure giving a spongy feel.

If you still have an issue bring it to a brake shop than can pressure bleed the brakes and they should know the correct way with abs brakes.

The brakes should have a firm pedal with only a very small amount of travel, as i said air or warped disc rotors/ wheel hub bearings or air in the ABS pump can cause what you are experiencing.
Should be easilly be fixed by a qualified knowlegable mechanic, a worn master cylinder gives a slowly dropping pedal feel while constand pedal pressure is applied. (fluid passing the piston seals) yes you can get air in the master cylinder, you need to bleed a new master cylinder.

Do that by filling the reservoir with brake fluid and have hoses attached to where the brake lines would attach, then put the master cylinder in a vice and depress/release the master cylinder piston with a rod or large screwdriver till no more air comes out. Then fit to the car.

Good luck

 

If the hoses are original I'd believe it's time for hoses and a very close inspection of all of the hard lines throughout the vehicle. A plus for you I guess is that all of the bleeder screws are free. I believe I'd replace all of them also while doing brake hoses.