brake problems. and i cant figure it out.

[BIGDOME10]

Last week I blew the right rear wheel cylinder during an emergency braking situation. I Limped the car home and replaced the cylinder. When I put it all back together my brake pedal when depressed goes straight to the floor but when pumped gains and holds pressure for as long as i hold the pedal down, but if i leave my foot off of the brake pedal for more than a second or two all of the pressure is gone. Also when i pump the brakes if the car is idling the engine bugs down and dies. I went and had the brakes bled at a local shop and the problem is still there. I just replaced the master cylinder today with no other results.

i have no clue what is going on with my car. Any leads will help.

[dlb]

sounds like there is a small leak somewhere. i would follow all of the brake lines and look for a wet spot, which would indicate a hole in the line. also check the rubber flex hoses for the same thing. if you really stomped on the brakes, it's likely that something marginal gave, just like the wheel cylinder did.

as for the engine bogging and dying if you pump the brakes at idle, there must be a vacuum leak between the master cylinder and the intake manifold. check the big vacuum hose that runs between them.

[ARCHINSTL]

What he said.
It would REALLY be worthwhile to replace the rubber lines; after all, yours are going on 28 years old...
Cheap insurance.
Tom M.

[keith]

The new wheel cylinder might be defective or you might need to adjust those rear brakes. The quick way to adjust them is to work the parking brake a few times. It could also be an issue with the starwheel or the bar that goes just under it.

[Petros]

IF there is no chance that air is trapped in the system, than likely the other wheel cylinder is leaky. If the pedal holds after pumping it, that means the master is likely good and either there is a leaky wheel cylinder or caliper, or there is still air trapped in the system. When the master cyl is bad the pedal bleeds down after you pump it up and hold it.

IT is pretty rare to have one wheel cylinder go bad, and not have the other one not far behind. All mechanics would replace the wheel cylinders or calipers in pairs.

[splatterdog]

As long as you got the back brakes together and adjusted correctly,it sounds a little like a typical weak master cylinder. What happens is on a rapid/firm apply and hold, the flared seal lips shown crudely( ]\___/[ ) get forced in to the bore harder from fluid pushing back against them. Relax that pressure, sometimes in the slightest, and down goes the pedal because of loss of outward hydraulic pressure against the seal lip.

Did the shop verify rear brakes before bleeding? When driving does it pull when braking? The stumble is from the vacuum leak known as your power booster. You may have some running issues that are exagerated by the rapid pumping of the pedal. It causes larger amounts of engine vacuum to be used vs normal pedal pressure. That's another issue though. I have never seen a booster cause a low pedal(unless someone messed with an adjustable pushrod). Only hard/unassisted pedal or constant vacuum leak(hissing).

[keith]

There is a problem with the design of the rear brakes on Japanese cars, all of them. The shoes hinge at the bottom and the star wheel adjuster is just below the wheel cylinder. In older American cars, the star wheel adjuster was at the bottom and was the pivot point. As the shoes wore down and the star wheel was adjusted outward, it pushed the top of the shoes back in place, so the wheel cylinder seals did not travel outward. They stayed within a very narrow place inside the wheel cylinder.

With the Japanese design, the shoes are constantly moving out at the top, never at the bottom. When there is a certain amount of wear, the star adjuster moves out a notch, holding the shoes further apart at the top. The seals inside the wheel cylinder are constantly moving outward.

I can see one of two scenarios that can explain what is going on. Either the star wheel is not adjusted properly, it is all the way in and every time you take your foot off the brake pedal, the springs pull the shoes all the way in so that when you hit the brakes again, you have to pump the shoes back out before any brake pressure builds up. There is no air in the system.

The other scenario is that the combined wear of the shoes and the drums has taken the wheel cylinder seals past their intended travel limit. I have seen this happen before. There may actually be some material left on the shoe, enough to pass inspection, but the wear on the drum, which is harder to measure it too much and the seals travel out too far and leak out some fluid and let in some air. To correct this, you need to replace both the shoes and the drums. If this is the case, you should see some brake fluid on the affected tire. BTW, this could also be the reason the previous wheel cylinder failed.

I tend to believe that the first scenario is your case. The shoes do not float like disk pads do so they can be pulled to far in after each application of the brakes if they are not adjusted properly.

[Tercel__jon]

Hmmm seeing as how I have the same problem (but with completely new brakes out back) Ill check my adjustments too. I added a new master cylinder and thought it might be the culprit as well. We shall see.