The Stovebolt Site Forums The Shop Area Driveline Brake residual valve

I am upgrading my front brakes to disc on my 54. I also have a dual reservoir MC. I installed a 2 psi residual valve on the front brakes. Should the 2 psi pressure held on the line be enough to hold the calipers/pads tight against the rotors? I can rotate them but it takes some real effort.

You should check with the supplier to determine if the Master cylinder already has
built in residual valves. Many do and you don't want to duplicate the effort.
Fred

I confirmed the MC does not have a built in residual valve before adding it.

What's the diameter of the caliper pistons? For instance, a 2.5" diameter piston with 2 PSI against it will create nearly 10 pounds of clamping pressure on the brake pads. That will definitely cause some drag, and possibly overheat the rotors. Does the drag on the rotors go away if you crack a bleeder screw open?
Jerry

For disk front / drum rear setup, you need a combination valve. Some call it a "proportioning valve". Like this:
Combination valve

I do have a proportioning valve and a 10 psi residual valve on the rear brake line.

I do have 2.5" pistons. Is my best option to remove the residual valve?

I've been running disk brakes ( '70 - '79 Camaro/Firebird ) since 2000 with out residual valves. I have a combination valve from a early '80 s Chevy car, and a adjustable proportioning valve on the rear brake lines. The rear adjustable is pretty much mandatory due to the lack of weight on the rear axle during braking.

I have never seen the need for the residual valves, why do you want the brakes to drag? The reason I always hear is, they keep the fluid in the line and not overfilling the master cylinder since it's frame mounted. So how does the fluid return, you ever try to push a caliper back in? Gravity in a little brake line nots going to do it. How did the truck stop for the first 50 years with the valves in the lines?

A simple test, go out and step on the brake pedal like you normally due to stop the truck, measure how far down it goes.
do it a couple ties with 5 to 10 minutes between each test.

Remove the valves and bleed out the system.

Repeat the above test and see if the calipers are retracting farther. Wait and hour so the fluid has plenty of time to retract and measure the pedal distant again.

One of the reasons I need a residual valve is the MC sits below the floor and I have to pump the breaks when I first get in the truck because the gravity drains the fluid back toward the MC. That said, I have figured out at least part of my issue with the tight calipers. I was thinking logically. The brake line outlet pointing toward the front of the vehicle would be the brake line for the front brakes. Wrong. I have my front and rear brakes lines reverse on the MC although I don't understand why that could be an issue. Stay tuned. Once I straighten that out I will see if I can get the system working correctly.

I still don't believe the fluid will pull the pistons back, the master would have to be way below the caliper and the lines would need to be much bigger. It's more likely the rear brakes are not adjusted up correctly. With a master cylinders on the firewall, just how much fluid pressure do you think there is pushing through a 3/16" steel line on a caliper seal? It barely bleeds out with the line open.

Originally, the drums all worked correctly, why would disk brakes be any different?

I'm just asking, not trying to offend anyone, I just keep hear the same things over and over on multiple web sites with out any proof its correct.

Bet if you actually measure the fluid hight in the master and front caliper, bet theres hardly any difference, more likely the master is above the caliper slightly.

On the cars that I work on, the front brake line is attached to the port nearest the pedal plunger, which is the back port on a firewall mount. I installed my under floor ones the same way. The plunger points to the front of the truck, and I attach my front line to that front port.

Because, while wheel cylinders aren't typically designed this way(I can't think of any), some calipers are designed to roll the piston back in to the body.