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Dear Car Talk: I just had my rear brake cylinder replaced because it had a leak. I did the brake bleed process, and the brake pedal will get hard but then slowly go back down to the floor.
What can cause the brakes to not bleed properly? — Peter
Lack of experience, mostly. First, you say you replaced a rear cylinder. But you have two rear cylinders, Peter. So make sure you bleed the other one, too, to be sure there’s no air in that line.
When you try to bleed it, you’ll inevitably snap off the bleeder, which is what happens on old, rusty brake cylinders. Then you can replace that cylinder, too.
Next, adjust the rear brakes. If the shoes are too far away from the drums, that could explain a pedal that sinks. Then, check for a leak. That’s easy enough to do. And if you correctly tightened the brake line to the new wheel cylinder, no fluid should be escaping there.
Finally, if you’ve correctly bled all the lines, adjusted the brakes, and confirmed there’s no external leak, then the only remaining explanation is an internal leak. That means you need a master cylinder.
A brake master cylinder is essentially a pump. And when they get old, the seals around the pump’s piston (the plunger) break down and allow brake fluid to sneak around it. So instead of pressurizing the fluid and sending it out to the brakes, the fluid is sneaking around the piston and remaining inside the master cylinder.
When that happens, guess what you notice? The brake pedal starts out firm, and then if you keep your foot on it, it slowly sinks to the floor.
Hopefully you actually had a wheel cylinder leak in the first place and a sinking pedal isn’t what led you to get a new one. But in any case, now it’s likely that you need a new master cylinder. And if you’re looking for good news, Peter, at least you may have bled the brakes correctly.
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