A dual master cylinder by itself does nothing for safety. A pressure differential valve is needed for the safety aspect and that is in a combo valve. Without that, a failure will result in a pedal that goes to the floor and braking ability that is not even remotely safe.

For now, solve the problem and think about upgrades later.


Lawrenceville, GA

I agree with the others, replace the wheel cylinders. I wouldn't bother rebuilding then as new ones are so cheep. Easy to tell if they are leaking, just peel back the rubber cups and you will soon see if they are wet in there.

I don't agree with that. It is two separate systems. A pressure differential valve only alerts you if there is a problem.



My point was if he has to replace the M/C he may as well upgrade it now.

"The Master Cylinder"

Enjoying life at the beach in SOCAL

That's not correct, the pedal will not go all the way to the floor although it will be lower than normal. You will still have half your brakes. The two systems are in no way connected (fluid wise) to each other.

The only way it could happen the way you describe is if you lost fluid in one system AND the seal between the two pistons failed.

BOB

Well I pulled out the two rear bleeder valves and didnt notice anything out of the ordinary. One of them seemed to have a bit more of a point than the other one, but both seemed to have a solid wear pattern around the point that seals them closed, so I dont think they are the culprits. I also looked into the wheel cylinders where the bleeders go in with my little mirror and couldnt see any noticeable crud that would be preventing them from closing.

I went ahead and pulled all the drums off and noticed a slight discoloration on some of the rubber boots, like they were wet at some point, but didnt see any drips on the wheel cylinders or traces of fluid on the backing plates. One of the front drums however had a nice rust pattern on the inside braking surface which looks exactly like something a leaky wheel cylinder would have left behind.

I found that the metal line leading from the master cylinder to the 4-way block on the frame (not sure the correct term for a fitting that has a place for 4 brake lines to thread into) had about a 6 inch piece of rubber hose incorporated into the design. Not sure why this would be. If the master cylinder is bolted to the firewall, and the 4-way block is attached to the frame, I dont see any reason to have flexible hose between the two. I already removed it and replaced it with a metal line.

Im gonna go ahead and buy 4 wheel cylinders, and although I dont see any outside damage to the rubber lines, they do look old enough that they could be replaced, so I will add them to the list as well. When I tried to separate the two front flex hoses from the metal lines at the frame, both metal lines wanted to shear off, so I will be replacing some of the metal lines too. I will make sure to keep you all updated on the repairs. Assuming I can get the parts in the next few days I should have them installed by Wednesday night. Thanks for all the suggestions and we will soon have an answer!

Well, this is my first hand experience, so it is correct. The pedal did, in fact, go to the floor and the truck kept a rollin' after a leak developed at a poor flare in the rear brake circuit. I, too, thought it was suppose to be safer. It is not. Fortunately, my experience took place in my driveway.
That experience made me research the brakes more, because the dual master cylinder by itself was no more safe.


Lawrenceville, GA

Charlie thats why I didnt go ahead and put a dual circuit on my 35 P/up. I could not find anywhere someone who sold a nice bolt on system that worked. Most Duals are 1" Not 1 1/6" as per the original. I kept reading guys saying "yeah it works but the pedal is closer to the floor" that got me thinking , well if it fails the pedal needs to go further to hit the second curcuit, if the pedal is already closer to floor well you just gonna hit the floor and get nothing!

I just bought a new orginal USA made M/Cyl and some speed bleeders and its as it should be. Now if only my new lining would turn up .. MACS !!!!



"Came too close to dying to stop living now!"

Charlie I don't want to argue with you because obviously it was your "first hand experience" so all I'm saying is you should have had front brakes even with the loss of your rear brakes. My feelings are either your other brakes were out of adjustment or something else was wrong. Maybe the seal between the reservoirs was NFG. The M/C should have provided brakes on the front brakes.

The pressure differential valve would not have any bearing on it in any event. As someone already mentioned all it does is tell you there is a problem. The Front and Rear Systems are NOT tied together.

BOB

One of the problems with a dual master cylinder on early cars is that the amount of travel may not be sufficient to engage the safety feature. This has been a problem when the TBirders started to do conversions. The answer, so I have read, was to relocate the pushrod so there was a bit more travel.

More to home, I have dual master cylinder on two of my cars, a 48 Ford Conv and a 54 victoria.

I experienced fluid loss on both cars. The 48 had an aftermarket kit from a well known brake house and the 54 I did a Granada conversion. Both had combo valves and were set up correctly.

The leak on both cars resulted in complete brake failure. I surmise that the pedal travel situation found in the TBirds was the problem.

Be aware that your conversions may have the same situation.

54 Victoria 312;  48 Ford Conv 302, 56 Bird 312
Forever Ford
Midland Park, NJ

This just goes to show how much misinformation is out there. Everything I read about the differential valve indicated that it not only activated the switch, but also cut off flow through the failed circuit to prevent further fluid loss.

If Paul has experienced the same sort of failure with the valve, in two instances, then that must not be true. I had not yet heard about the pedal travel issue. That one should be pretty easy to determine if specs for the dual master cylinder which is being used can be found. Just compare the spec to the linear travel of your pushrod and adjust as necessary.


Lawrenceville, GA