The Stovebolt Site Forums The Shop Area Driveline Brake Pedal Firms Up

The dual master cylinder on my '48 has been having a problem. While driving around and occasionally using the brakes, the pedal will firm up and, eventually, the brake shoes will drag. After sitting all night, it goes back to normal.

I've studied the following illustration for a dual MC and propose one reason consistent with the symptoms. The pushrod is too long and will not let the "rear seals" go behind (or to the left) of the "fluid intake and return ports." The pushrod is adjusted to be as short as possible. It has no added valves in the lines.

What do you think?

Dual MC (My favorite MC diagram.)

Since it isn't a stock system there's no way for anyone to know what the "right" length of the pushrod should be. Did the system ever work properly?

Wally, just a little fodder for thought.

Your thought process on the plunger not fully retracting is a good possibility. Can you take and post pictures of your setup as well as provide info (if known) on vendor and parts used in your system???

Some dual reservoir/circuit master cylinders come already internally equipped with residual valves. If your master cylinder has functioned normally for some time, such residual valves may have no bearing on the problem. If the problem has been on-going since you installed and started using the brakes regularly, they could be part of the problem???

That being said, maybe there is a way for you to remove the existing pushrod, shorten it (while leaving whatever length adjustment ability exists to make up most if not all of the removed length) to create a rod that actually allows for said length adjustment???

See attached screenshot of one of the common aftermarket kits that shows the actuating rod.

In my mind you could unscrew that rod from the spherical joint connector, cut say 1/4" off the "threaded" length of the rod, use a die and extend the locking nut threads farther out on the shortened rod by the same amount, then put it all back together with the rod screwed deeper into the spherical end to reduce the overall length. When that is done, you could then adjust the length of threaded rod to where it needs to be (in bare contact but not forcing the pistons forward) and lock down the nut to maintain rod length??? Just thinking out loud here.

We need more info to brainstorm this theory of yours.

Dan

What shape are your brake hoses in?

Couple of things to consider. If all drum system, you need a dual master that is about 1 in. Any larger and you get a hard pedal. Second, no new dual master comes with pressure residual valves. The original single pot master had one to keep a little pressure on the wheel cylinder cups and to keep the springs from fully retracting. Original dual masters from back in the day, and rebuilt masters from back in the day had these little duck billed pressure residual valves. One each for each port in a drum brake application, one in the rear port for a disc/drum set up.

If you look into the ports on an original master, you will see that the seat is made of brass. They were removable. The valve and spring were installed behind the brass seat. With todays new masters, the seat is cast iron machined directly into the casting. No way to retrofit a stock type duck bill residual valve. Rebuilders like Cardone stopped installing the residual valves a number of years ago but the Wagner rebuild kits still have them so if you had an original master with a removable brass seat, you could rebuild the master and install the valves.

For today's masters with the cast iron direct machine seats, you need to install a 10lb in line residual valve to compensate. Wilwood is one source for those valves.

So it may pay you to pull the master and see what you have size wise for the bore of your master. If its wrong, or the unit is all junked up, now is the time to replace it. Sort out the pressure residual valve issue, then the rod adjustment issue. From that point you at least know the major parts are correct.

I don't know if this helpful, but this is a common problem on Jeep JKs (front disks). The resolution is replacing the rubber brake lines going from the steel brake line to the caliper. The theory is that the rubber line is somewhat internally collapsed, works good enough when you push the fluid via the brake pedal. But the fluid can not recede enough by itself because of the reduced internal diameter of the rubber brake line. Anyway that is the theory, in practice it solved the problem on my Jeep and many others who had the same problem. So this may be something to explore if you can't find any other causes.
don

Thanks, Guys. A lot of good points. I think I'll first drive it around the 'hood and get the brakes dragging a bit. I'll then jack up the front and then the back to see if front and rear are both dragging or just one of them. (But there is football today.)