Brake Power Booster? Master Cylinder? Brake Pedal?

MJONES98806

MEMBER
1998 OLDSMOBILE ACHIEVA
6 CYL
FWD
AUTOMATIC
150,000 MILES

I recently replaced the left rear wheel cylinder on my 1998 Oldsmobile Achieva SL and, like an idiot, tried to bleed the brakes without replacing the drum on the outside of the rear wheel assembly. I had also taken the drum off of the right side of the vehicle so I could compare the two sides and ensure that I was putting the left side together correctly. The right rear wheel cylinder burst during the bleeding process. Once again, I replaced the wheel cylinder, but on the opposite side. This time I went to bleed the brakes and noticed that there was little to no pressure building under the pedal. When I turned the car on, the brake lights stayed on and the pedal had to be pulled up and held in this position in order to turn the brake lights off. When the car is off I can build enough pressure in the brake system to bleed the brakes by pulling the brake pedal up and pushing it to the floor. I know that pressure is being maintained in the brake system when the car is off because once the bleeder is loosened, the pedal then sinks all the way to the floor.
So with the car on, I have a useless brake system I can't keep pressure in and the brake pedal sits at half mast with the brake lights on unless I hold it in a fully returned position with my foot. With it off, I can build pressure in the brake system and bleed the brakes, but I have to force the pedal to fully return in order to do so. This was not a problem before I started messing with the rear brake shoe/drum assembly in order to fix this leaky wheel cylinder which has just turned into so many different problems.

1 Reply

CARADIODOC

MECHANIC
33,916 POSTS

I've done a lot of brake work but never ran into this problem. It sounds like the valve in the power booster is leaking. That would cause engine vacuum to pull the pedal down when it is started.

I had a half-hour long conversation a few years ago with a fellow who works for a power booster rebuilder about a different model GM front-wheel-drive car. He told me they have two diaphragms with a valve between them. When one diaphragm leaks, which was common, the pedal would be harder to push than normal but to most people other than the owners, it would seem fine. That was the case with the car I was working on. He also said if you get a booster from the salvage yard, it is real important to not lay it down while hauling it home. It must stay in the upright position to prevent that valve from falling out of place. Rebuilt boosters from the parts stores come with a plastic cone inserted on the master cylinder end to hold that valve in place until the booster is installed on the car.