Brakes: Brake check valve

[ColoradoHawk]

I know my 63 GT Hawk with disc brakes is NOT supposed to have a check valve in the master cylinder. If it accidentally did, what would be the result/symptoms?
When I rebuilt my entire brake system roughly a year ago I got all the parts from Studebaker International and double checked my order and I did order the correct, non check valve master cylinder. The reason I ask, is I often feel after coming to a stop and then wanting to move the car a little, like at a line to a stop sign, I can feel that the brakes are still engaged and the engine straining against it.

[bensherb]

If you use a drum brake single reservoir master cylinder with a check valve in it on a car with disk brakes in front the front disk brakes will lock up and not release the first time you step on the pedal. Dragging, slow to release front brakes can be caused by front brake hoses disintegrating internally and blocking fluid return. If all 4 brakes are slow to release it could be caused by a partial obstruction of the bypass port (fluid return port) in the master cylinder, a bad spring in the master cylinder not pushing the piston all the way back to completely clear the ports in the master or not enough "toe board" clearance (a badly adjusted master cylinder push rod) not allowing the piston to fully return.
Keep in mind some minor dragging of disk brakes after their release is normal, but it shouldn't be noticeable.

[jclary]

I think we have a problem with definition here. (Proper choice of words/terms) In a braking system, there should be NO true check valve. Perhaps a "proportioning" valve, or a "NO-Roll" valve, but never a true "CHECK VALVE." As a check valve only allows flow in one direction. Bensherb (post #2) provides some good advice on where to investigate. I have never had a disc brake equipped Studebaker, but have heard of problems with the pistons binding. I have also had my drum brakes bind due to improper adjustment (my fault), or corrosion on the supposedly "dry" side of the wheel cylinder pistons restricting release movement. (MY neglect.)

A handy tool for checking problems with binding brakes or a seizing wheel bearing, is an infrared thermometer. They were once very expensive, but now can be had for only a few bucks. One of those can be used to check and compare wheel temperatures. I had a rear disc brake to bind on a late model vehicle so bad that I had to replace the entire disc and brake caliper assembly. Hopefully, yours will not get to that point.

[52 Ragtop]

Residual check vales are used when the master cylinder is below the calipers or wheels cylinders. This prevents the brake springs from pushing fluid back into the M/C, and causing a low pedal on first application. They pump up on the second application of the brakes. Usually, a 2 # check valve is used with disc brakes, and a 10 # with drum brakes. Proportioning vales limit the pressure to either the fronts or rears. Generally, with a dual M/C, the proportioning valve is in the rear lines, so the pressure can be reduced to the rear brakes. "Most" master cylinders, do have the residual check valves built into them. IF you use a stock NON disc brake M/C on the disc brake cars, you will go about 3 miles before the brakes completely lock up. (Don't ask how I know) <G>Also, with power brakes (Hy-DRO-Vac style) I'm pretty sure there is a check valve built into those units also. That is why when you order a new one from SI or any vendor, they as drum or disc brakes. Very important that you get the correct one.

Jim

[bensherb]

Yes, residual pressure valve or residual line pressure valve is the correct nomenclature.

Wow, you made it 3 miles! I didn't make it out of the driveway.

I've never used any residual pressure valve in any disk brake system; I've never found a need for it. They were not typically used back when I was changing front drums to disks, and I haven't changed drums for anything but four wheel disks since 1986. It's just too easy to adapt disks meant for the front to the rear. Today there's enough cars with rear disks it's even easier, and you don't need to build a parking brake assembly.

[52 Ragtop]

The residual check valves keep either 2# (disc) or 10 #(drum)from bleeding back to the M/C. You would probably experience a lower pedal on or brakes without them. They do make a difference.

Jim

[Hawkowner]

I have just finished installing a disc brake kit on my 62 GT.I have the stock master cylinder with the residual check valve removed and a 2# front residual valve and the 10# rear valve installed . I get about a 1/2 pedal on the first app and full pedal on the second pump. I had more pedal when I had the rear brakes adjusted tighter but when I tried to back up the Self actuation caused the brakes to lock. Thinking of going to a dual Master any suggestions on what one to use keeping the drum rears?

Hawkowner
North Port FL

[63 R2 Hawk]

The symptoms you describe on your '63 Hawk sound more like a sticky vacuum servo in the Hydrovac. Have you tried adding oil ( neatsfoot or whatever is recommended by the rebuilder) to the vac servo? Could also be caused by cold weather gelling the servo oil and causing the piston to stick and not fully return.

[Mark Plucenik]

Great timing for me on this thread as I am ready to rebuild in M/C on my 64 GT w/ front disc brakes. My question is are all of the cast M/C housings the same? Reason for my question is I have 4 housings on my bench right now. I am going to glass bead and hone, check for ware and choose the best of the 4. I know for a fact that 2 of these housings came off of disc brake cars and all 4 appear to be the same. Is there any difference ? and is it just the rebuild kit that is different?
As always , thanks for the help. Mark