Manual Disc/Drum Headache

[Gyratingyak]

I have been fighting an issue with my disc/drum set up since I finished it. I replaced everything front to back. MC, front disc kit and proportioning valve for a 71 a body with front disc from Dr.Diff. Ran all new lines made of Cupro Nickel 3/16" line and added new brake hardware, hoses and shoes on the rear, the drums didn't have a ridge so I left them as is. The car will stop amazing but then the pedal feels as if it sucks air, then when it sits after use the pedal sinks to the floor once and then comes back up. I have bench bled the master multiple times and ran probably a gallon of fluid through the system. Along with going through every fitting to check for leaks, I added a 10lb residual pressure valve to the rear circuit and called Cass to see what his thoughts were, he believed it to be a bad MC and sent me another one. I bench bled it an then installed, re-bled the system once again and was having the same issues. It is always on the rear circuit, I never get air from the front. I'm about at my whits end. I'm unsure as to what I should do. Does anyone have any ideas? I have been fighting this issue for months and get to it whenever I can, I even took it to two different mechanics after I came to a roadblock and they said I hadn't bled it enough but the problem persisted afterward. I have zero fluid leaks from the brakes, no wet spots or stains from brake fluid. I hope one of you guys can give me a few pointers, because I'm at my wits end. The only thing I haven't done is change the distribution block, but I asked Cass and he said it wouldn't effect the system in such a way. Do I just have more bleeding to do or is something amiss? My next step was making a weekend of it and going through all the flares in the system and re-bleeding. But I would prefer to spend my time with the old lady if I could avoid it and cruise around the Coronet instead of wrenching non stop on the same issue. Thanks in advance for any help, I'm scratching my head on this one.

 

[Kern Dog]

A disc-drum system needs a proportioning valve, not a distribution block. Maybe you have the wording mixed up?
The "distribution block" term is used to describe the junction block for a drum/drum system. It has no proportioning to it, it merely connects all the lines and includes a shuttle valve to warn in the event of a pressure loss.
This may not be the problem but it surely isn't the correct part for a disc-drum car.

 

[Gyratingyak]

A disc-drum system needs a proportioning valve, not a distribution block. Maybe you have the wording mixed up?
The "distribution block" term is used to describe the junction block for a drum/drum system. It has no proportioning to it, it merely connects all the lines and includes a shuttle valve to warn in the event of a pressure loss.
This may not be the problem but it surely isn't the correct part for a disc-drum car.

Sorry, wrong terminology. I edited the post to fix the issue. I do have the Proportioning valve for a disc/drum car.

 

[cryplydog]

If there are any leaks- anywhere there would be fluid leaking. I say it's the M/Cyl. did you use a different Rod between pedal and cylinder. is it stopping in between some how, and would it make any difference?

 

[Gyratingyak]

If there are any leaks- anywhere there would be fluid leaking. I say it's the M/Cyl. did you use a different Rod between pedal and cylinder. is it stopping in between some how, and would it make any difference?

I cannot find any leaks from the brake system. I have checked multiple times and cannot find anything. I used the adjustable brake pedal pushrod supplied by dr.diff. Could a bad flare let it suck air but not leak?

 

[cryplydog]

The inside pressure is far greater during breaking than it is lifting the pedal.

 

[Gyratingyak]

The inside pressure is far greater during breaking than it is lifting the pedal.

Could the proportioning valve be bad and cause the issue? I'm about to just redo all the lines on the car on the rear circuit at this point due to a possible bad flare. I just want to cruise around again without fear of destroying my car due to bad brakes.

 

[cryplydog]

Who's prop valve?

 

[Gyratingyak]

Who's prop valve?

I believe its from inline tube, but I purchased it from Dr.Diff

 

[Ceedawg]

What procedure do you use to bleed?

 

[Gyratingyak]

What procedure do you use to bleed?

I've done the tried and true two people where one pumps the pedal, the self bleeder with the tube and bottle with brake fluid in it and I bought a Mity Vac and used it.

 

[vance.dykes]

it is possible that the wheel cylinders are sucking air and you not see any leak. get a wheel cylinder hone and take the wheel cylinder apart and see if rust pitting and or a cut inner cup might be nicked or cut.

 

[dsd1967]

Check your rear drums to make sure the self adjusters are working and also if your residual valve is working properly. The residual vtalve is supposed to hold pressure to the rear drums cylinders and there are springs fighting back that want to retract the shoes. If the residual valve is allowing internal leak back then the shoes are backing away from the inner drum surface. If the residual valve is slightly working, it will allow you to pump the brakes and it will momentarily hold position while you pump again and get a solid pedal. One way to check if this is happening is to go pump your brakes and then open the master cylinder to see if the reservoir slowly fills back up.

 

[Ceedawg]

I've done the tried and true two people where one pumps the pedal, the self bleeder with the tube and bottle with brake fluid in it and I bought a Mity Vac and used it.

For every pump-bleed you have to do one back and one front to completely drain-purge the MC, remember there’s two systems in that MC. I use 3 people, one pumping and 2 bleeding.
One other thing, on all drum brake MC there’s a residual valve in both ports, disc MC, only one for the drum side. So after bench bleeding the disc side could and will just drain or get air back in. If you have external residual valves it will have both ports open. Bleed the MC by cracking the lines at the port first then proceed to the shoes-disc (I do this with any MC regardless)
I’m guessing this last part i described is where your problem is

 

[493 Mike]

I have gotten two bad master cylinders in a row.
Mike

 

[biomedtechguy]

I have had problems with the Right Stuff front wheel drum to disc brake conversion kit I got from Cass aka Dr. Diff over 5 years ago.
The latest attempt to fix what starts out as a firm, high pedal on my manual brakes was replacing the rear brake line junction block/rear end breather with the integrated rubber hose.
Pedal started out better than ever.
By the end of the week of Cruisin the Coast, the pedal was soft again and would approach the floor.
I'm giving up and replacing it with a Wilwood 4 wheel disc brake system and master cylinder of course.
My inner fender has suffered bad paint damage from brake fluid coming out through the caps on the master cylinder that came with the kit. Cass sent me thicker round caps a few years ago but that didn't help much at all.

 

[66chargerpat]

I had a rear wheel cylinder that suck in air when it cooled off. It was new.Change it out and it never happen again.

 

[Billccm]

Here was my headache:

https://www.forbbodiesonly.com/mopa...-front-disc-conversion-68-road-runner.127670/

Look for a subtle issue with a flare on your brake lines. VERY SUBTLE issue.

 

[Darter6]

Remember the old pressure bleeders ? We called them bleeder balls. Maybe something like that would help.Possible someone with a old school shop that has been around my have one. Just a thought.
I agree that parts these days are crap. I had a power brake kit from Jegs and had to replace the Master after 6 months. The front brakes were not releasing,Pulled the master apart everything looked good, cleaned,reassembled,same story. I'm going thru this with a rear disc brake caliper now.I hate replacing,got to when I can't find the problem.

 

[1 Wild R/T]

Simple test.... Foot off the brake pedal for a few minutes..... It this point by your description normally the pedal would be low correct?.... Try this, Set the parking brake, then apply the brakes... Now foot off the brake pedal for a few minutes....How is the pedal?