The Stovebolt Site Forums The Shop Area Driveline Disc/Drum vs Drum/Drum master cylinder

What would happen if a disc/drum cylinder was used in a drum/drum system?

You would need a proportioning valve.

Like This

https://www.jegs.com/i/JEGS/555/631290/10002/-1

Chris

Tommy,
My understanding is that if you would be sending heavy brake fluid pressure to the wheel cylinder, which a disk brake caliper could handle fine after a certain degree of brake pressure.
The proportioning valve is supposed the govern the pressure going to the wheel cylinders brake side. Without you could comprise the line and get a possible leak...

Steve

The only real difference is the size of the reservoir chambers. A disc brake reservoir is larger because more fluid is needed to operate the large diameter pistons of the calipers. The portion of the MC that operates the calipers will also need a much lower pressure residual pressure valve, 1 or 2 PSI instead of the 10 PSI or so that the drum brake wheel cylinders need to keep the cylinder cups expanded and avoid air leaks.
Jerry

What are you planning to do?

I have a Camaro rear end and Camaro disc brakes up front. So, I used a Camaro m/c. I'm pleased with the brakes because they work well because everything matches.

I'm updating the master cylinder on my truck, I have drum brakes. I some times think about putting disc brakes on the front and was thinking I would just put the disc/drum master on now . Howe ver I will due the drum/drum master.

Not sure how you would handle the pressure differential between disc vs drum.
Generally, drum brakes require about 400 to 500 psi, discs 800 to 900 psi. So the disc side of the master cylinder runs at about twice the drum side. Some modern disc run 1500psi to the discs.

Proportioning valve manages the pressure front to back in disc/drum.

If disc/drum master was used in drum/drum .........front drums would lock up prematurely and be super sensitive.

I put disks up front on my 47 and a new dual reservoir master cylinder. Not power. I've got to push the pedal pretty hard and far down but it stops just fine. Opp said I didn't need a proportioning valve. Is there a way to check if I'm getting proper braking front and rear?

Sorry to jack your thread Tommy, seemed relevant tho.

Find a limited traction road-ice, mud, etc., have a buddy stand on the side and watch which wheel locks up first when brakes are applied hard.

Ed

Just take your truck to a local empty parking lot and stand on the brakes from about 30 mph. You should have even braking if everything is right. Likely you are going to have rear wheel lock up due to the weight bias to the front. The " simple " solution is to add an adjustable proportioning valve back by the rear axle. Then do some more brake testing while adjusting the valve till you get hardly any lock up but still have adequate stopping power. https://www.speedwaymotors.com/Speedway-Purple-Adjustable-Brake-Proportioning-Valve,23498.html I use this same valve, it does what it suppose to do, limits the pressure to the rear brakes.

GM and Toyota have a factory proportioning valve hooked to the rear axle so when the rear end sinks down due to a load, the pressure to the rear brakes is increased.

Ed

Thanks Guys

I have stock drum/drum brakes on my 64GMC 1/2 ton, the PO put on a dual master cylinder and I recently installed a full size vacuum booster to make it power brakes. This system is very adequate/safe for any old truck running around empty, which is my situation. I dont think I would consider the expense vs benefit to be worthwhile to put front disc on unless you are hauling loads or trailers/campers.

Update, I put the modern dual master cylinder on and everything works fine drum/drum cylinder. Only one problem, the mount that I purchased from CPP included a push rod for the master cylinder that was to short. I ordered another that was longer and cut to fit, just a small delay. You would think CPP would know the short rod doesn't work, I'm sure I haven't been the only person to have that problem.