brake cylinder troubles

[ozzmonaut]

So I have a curious issue with my 48 Special Deluxe. About a year and a half or so ago, I had a wheel cylinder go bad, and decided to change all of them. After the install was done, I could not get the system to bleed properly, or so I thought. I decided to replace the MC because it was old and likely no better than the wheel cylinders. My thinking was that it was damaged when the pedal went the floor when the wheel cyl failed.  Replacing the MC seemed to do the trick. Everything bled and the pedal held tight and my brakes felt like on a new car.

 

Due to a mistake of my own, a wheel cylinder was recently damaged. I replaced both rears even though they were not very old. I am now having the same issue bleeding the system. I believe the air is out. I can pump the pedal a few times and everything feels up to snuff. But once I let off of the pedal for a minute, it goes right back to the floor on the next try. This is exactly what happened last time when I had to replace the MC. 

 

Is it possible that everythime the pedal goes to the floor, it instantly kills the MC. This can't be possible... right? I would imagine  not, but here we are unable to get the system right with the exact same symptoms as before. Thanks for any help you can give me.

[Merle Coggins]

Did you readjust the brake shoes after replacing the wheel cylinders? It sounds like there may be too much clearance. You can pump them out with a couple of pumps, but then when you let them fully return you need to pump them back out to get good contact again.  

[ozzmonaut]

Yeah. I adjusted each one out until it locked the drum from turning, then adjusted it back in until it just barely let loose. I will try adjusting them back out to where the drums are locked, just to see if that is the issue. Maybe i am missing the "sweet spot" between locked and loose. I arced the shoes pretty much perfectly when I installed them. It makes it very difficult to find that spot because there is such a slight turn of the minor adjustment nuts between full shoe-to-drum contact and zero contact. But at least locking them down will help eliminate the MC as the culprit. Thanks for the input.

[Racer-X-]

Is there a residual pressure valve in the rear line?  Has it failed?

 

If that valve fails, or if for some reason it's missing, the springs will pull the shoes back too far and push the fluid back to the master cylinder.  Then it takes a few pumps to get the shoes back out in contact with the drums.

 

And excellent that you arced the shoes.  So many folks don't have the tools to do that, and don't know how important it is.

[ozzmonaut]

The good news is that I had underestimated how much fluid had been lost. Enough to allow air into the front lines... that's how much.

[desoto1939]

yes when you do a rear brake replacement with new cylinders youwill also need to bleed the front brakes to insure there is no air inthe lines.  Also remember that there is also some pedal play or about 1/2 or a little more prior to the piston pushing the fluid refer to you repair manual.

 

 

Rich Hartung

Desoto1939@aol.com

[ozzmonaut]

Yeah, the single-circuit MC conundrum kind of slipped my mind. I'm just glad that parts are so readily available for this car. I have another vehicle that there are not really aftermarket parts available for. Especially brakes. Basically everyone just hordes original MCs and wheel cylinders and keeps resleeving, honing, and resealing them. 

But for this car I can pop online and pretty much get anything I need for fairly cheap. It sure makes it easy.

[Tom Skinner]

Typically I will go around and Bleed 2-3 Times until everything is clear fluid - no bubbles - no troubles.

[ozzmonaut]

Once I realized that air had made it to the front lines, everything bled out and pumped up great.