Dr. DIFF cobra brake/ master cylinder question

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SR77

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I'll provide as much info as I can, so this might be a little long. I'll just break it up into sections.

VEHICLE: 72 Dodge Demon 340, purchased it back in 2013 with I believe a right stuff manual front disc brake kit. Decided to install the Dr. DIFF setup for better braking and liked the looks. The brakes on the car have always sucked since I got it. First manual brake car so I wrote it off as that.
Didn't really have issues with the install but we cannot get the air bleed from the brakes.

ISSUE: Didn't really have issues with the install. At first we could not get the fronts bled. We traced that back to the matser cylinder. It's a Mancini retro fit, the two bowl master cylinder with adapter plate. I am not sure which bore the master cylinder is, 1.032" or the 1.125".
We seen the brake lines where reversed on the master. The master was installed on the car years ago and has been ran like that since. Hence the crappy braking.
We pulled the lines, reflared them since they only had a single flare not double, and reinstalled and tried bleeding again.
Our issue moved to the rear brakes now.
We tried bleeding the porpotioning valve to the rear and worked our way back.
We are still getting air and we are starting to think the master cylinder may be to blame.

QUESTION: with having the 13" front disc brakes and rear drum, what master cylinder is recommended. This is still a manual brake car.
We have no drips, anywhere on the brake lines, proportioning valve, wheel climbers, or calipers. We can't figure out where we are getting air in the system.
 
how are you bleeding it? and you're saying you still get bubbles out at the wheel cylinders? if there are no leaks at the fittings, i'd suspect the wheel cylinders. might be pulling air in past the seals

edit: it's not losing fluid is it?
 
kursplat said:
how are you bleeding it? and you're saying you still get bubbles out at the wheel cylinders? if there are no leaks at the fittings, i'd suspect the wheel cylinders. might be pulling air in past the seals

edit: it's not losing fluid is it?
Click to expand...

Two man system, we checked the wheel cylinders and did not see any sign of leaking. Not losing any fluid either.
 
Silly question the bleeders are above the brake lines ? Not a bend in a line higher ? Seen where air will sit in there ?
 
well cheap is replace the wheel cylinders. after that it could be the MC leaking internally. have you pulled up the carpet under the brake peddle to make sure nothing is dripping down the fire wall?
 
Biff said:
Silly question the bleeders are above the brake lines ? Not a bend in a line higher ? Seen where air will sit in there ?
Click to expand...

Bleeders are on top, no brake fluid in the carpet or seen leaking on the interior
 
Did you take the rear drums off, open the 2 dust covers on each wheel cylinder and look for seepage?

Since you already spotting previous owner had swapped the lines going into the master cylinder incorrectly....AND it only had single flare lines to it... you need to start from scratch.

Trust nothing was installed correctly.
-Verify every part in the brake system
-note was is stock
-what is aftermarket
-what looks 50 years original
 
autoxcuda said:
Did you take the rear drums off, open the 2 dust covers on each wheel cylinder and look for seepage?

Since you already spotting previous owner had swapped the lines going into the master cylinder incorrectly....AND it only had single flare lines to it... you need to start from
Trust nothing was installed correctly.
-Verify every part in the brake system
-note was is stock
-what is aftermarket
-what looks 50 years original
Click to expand...

It had an aftermarket small disc setup, I believe a Right Stuff kit.
Proportion valve looks original.
The drums came off and we pulled the dust covers off and seen no seepage.
 
SR77 said:
It had an aftermarket small disc setup, I believe a Right Stuff kit.
Proportion valve looks original.
The drums came off and we pulled the dust covers off and seen no seepage.
Click to expand...

Were any other hard lines replaced other than the ones to the master cylinder? Were the axle flex lines changed?
 
Wait is the master cyl off a drum brake car ,I believe there is a difference?
 
No hard lines where replaced, just new hose to the calipers. The master cylinder is a 5 year old classic performance 1.031 bore purchased from Summit. Not a lot of info on Summits page as far as disc or drum or by doing a web search.
 
Oh boy do you know anyone,or a shop with a pressure bleeder, sometimes ,rare but sometimes a two man bleed system will not get the air out ? Older chevs when you tripped the proportioning valve the only way to trip it back was to use a pressure bleeder .
 
Biff said:
Oh boy do you know anyone,or a shop with a pressure bleeder, sometimes ,rare but sometimes a two man bleed system will not get the air out ? Older chevs when you tripped the proportioning valve the only way to trip it back was to use a pressure bleeder .
Click to expand...

I do have a shop down the street and free towing. I'm tempted.
I think we narrowed it down to a bad master cylinder only because we had air in the front then when we switched the brake lines to the correct ports we had air in the back.
 
I run the DoctorDiff 13” cobra style brake kit on my Duster. I also run DoctorDiff’s 15/16” master cylinder with it, works great. I personally wouldn’t run anything larger than a 1” bore master with them, but that’s just me. I’ve run that master cylinder with both drums and now with 11.7” rear disks.

The proper procedure for bleeding the brake is to go from the passenger rear to the drivers rear, then the passengers front and finally the driver front brake. That should reset the factory prop valve.
 
SR77 said:
We used the existing master and we tried bleeding it while it was on the car.
Click to expand...
You can quickly bench bleed on the car, if necessary, by plugging the two ports on MC and hand pushing the brake pedal. When you can barely move the pedal, the MC is hydraulically locked and virtually 100% sure there’s no air in the MC. Then remove the plugs one at a time and reattach the brake lines.
 
bc3j said:
You can quickly bench bleed on the car, if necessary, by plugging the two ports on MC and hand pushing the brake pedal. When you can barely move the pedal, the MC is hydraulically locked and virtually 100% sure there’s no air in the MC. Then remove the plugs one at a time and reattach the brake lines.
Click to expand...

That’s not necessarily true. With the master cylinder in the car you shouldn’t actually be able to mechanically bottom out the master cylinder with the pedal and stock brake rod. And if you plug both ports there’s no where for the air to go if there is any, you need the ports routed back into the master cylinder bowls. Otherwise if you do anything you’ll force fluid or air past the seals. And that’s not what you want to do.

Most of the time there’s enough travel to fully bleed the master cylinder on the car, but that’s the reason there’s a “bench bleeding” procedure to begin with. On the bench you can fully bottom the master cylinder out, whereas in the car that shouldn’t actually happen.
 
Give it a try one time. Air escapes through the holes in the bottom of the master. If you look inside during the process, the air bubbles are visible as they escape into the master. The system is closed when plugged and once all the air vents through the holes the system locks. The re-builders include plugs for that purpose and recommend that procedure. Been doing it exclusively that way with excellent results. Never had air in all the MCs I’ve changed.
 
autoxcuda said:
Did you take the rear drums off, open the 2 dust covers on each wheel cylinder and look for seepage?

Since you already spotting previous owner had swapped the lines going into the master cylinder incorrectly....AND it only had single flare lines to it... you need to start from scratch.

Trust nothing was installed correctly.
-Verify every part in the brake system
-note was is stock
-what is aftermarket
-what looks 50 years original
Click to expand...

Question , if the bowls on a new 15/16'' master cyl. are the same size , does it make a differance which way the lines are hooked up ?
 
Not the expert, but i think it has to do with the order the brakes are applied(?) I'm guessing by looking at a cut away pic of a master cylinder.
 
bc3j said:
Give it a try one time. Air escapes through the holes in the bottom of the master. If you look inside during the process, the air bubbles are visible as they escape into the master. The system is closed when plugged and once all the air vents through the holes the system locks. The re-builders include plugs for that purpose and recommend that procedure. Been doing it exclusively that way with excellent results. Never had air in all the MCs I’ve changed.
Click to expand...

I have tried it, it doesn't always work. If all the air goes back into the reservoir it's ok. But that method doesn't push fluid through all of the passages out to the ports like what happens when you're actually using the master cylinder (they're plugged remember?). Not to mention, the crappy little plastic plugs that are supplied for that purpose don't have much strength to them, the plastic threads are not all that durable. And of course some of them are made better and fit better than others. Push too hard on the piston and it can blow the stupid little plugs out of the ports and across your bench, making a big mess. Had that happen to you yet?

This video shows both methods. The second method works better, and EVERY time. Even in this video the guy doing the better method clearly isn't super familiar with it, he's pumping the piston too fast and creating bubbles in the reservoirs with the return fluid. Or maybe he's just rushing because he's doing it for a video.



Do what you like, the method of connecting the ports back into the reservoirs is the better method if you do it correctly. If you really are having problems with the master cylinder trapping air, that method guarantees you'll clear the MC if the master cylinder is good and working properly.

Really, a lot of the time you can just fill the thing up, gravity bleed it a little and just bleed the brakes, no bench bleeding necessary. But not always, and when you really need to bleed the master cylinder because you're having issues the second method in the video is the sure bet.
 
Last edited:
Good points. Thanks. This is what I was referring to about the process. Several ways available to bench bleed. This is just the one that I’m most comfortable with and have been 100% successful.
 
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