I inherited the brake system on my truck. It is an underfloor power assist master cylinder with front disk, rear drum. How do I tell if it needs a separate proportioning valve. It didn't come with one. This truck has never been driven since restoration was started, so don't know if what is there is correct. Wish I could identify who made the parts. It is a GM style master, maybe early Corvette.

 

I have the same type setup with a corvette master cylinder and yes you will need a GM style proportioning valve but I don't know the part number mine came with the kit I got from speedway motors. Maybe they can help you find the right one

 

I will send you pic of mine on Monday maybe that will help.

 

FYI for future truck or car builders...anytime you use front discs and rear drums, you will need a proportioning valve. I thought I remember reading you will also need another type of valve, maybe like a backflow preventer, ONLY with Master Cylinders under the floor though. Might have been found on chtopping.com ??????

 

I'm working on a frame mount disc-disc set up from CPP. Emailed them a couple days ago asking about proport valve & residual check. They said to check the #'s on the prop valve - I might not need one, I but could add a residual if I needed to. I hope someone can clarify that. This is disc - disc, I also see the prop valve is ported 3/16 & the rear is for 1/4 lines - can I reduce to 3/16 to the rear?

 

I should have said manifold valve before the other valve.

 

Thanks guys. That is what I assumed.

 

To the best of my knowledge, drum brakes need a residual check valve at the master cylinder typically rated at 10 psi. This is regardless where the master cylinder is mounted. It does two things, keep a little pressure on the cup seals, expanding them inside the wheel cylinders to reduce potential leaks, and prevent backflow to the master cylinder on low frame mounted master cylinders. OEM drum brake master cylinders have it built into the outlet port going to the drum brakes. Be aware, these aftermarket kits might not have it, so an additional external residual check valve is needed. On disk brakes, from what I have read with a frame mounted master cylinder, a 2 psi residual check valve is needed to prevent backflow to the master cylinders. Like others mentioned, don't forget a proportioning valve as well on drum/disk combinations.

 

In other words - disc disc, 2lbs. X front & back.

 

Here are some pic of my set up hope it helps

 

rken302, What brakes are you using?

 

First off not all systems require a proportioning valve regardless of the type of brakes used, e.g. drum or disc, and the the other valve you folks are trying to describe is a "Residual Pressure Valve" (RPV) not a backflow preventer valve. Again there are several factors that come into play on whether or not a RPV is needed. The job of the RPV, just like their name implies, is to hold a minimal amount of pressure, typically 2 or 10 psi, this does several things one of which has been already mentioned but it also holds the friction material in close proximity to either the drum or disc to minimize pedal movement when applying the brakes in additional to other beneficial factors.

That being said most trucks will benefit from an adjustable proportioning installed so that pressure to the rear can be reduced. Most of the systems installed are cobbed together systems stolen from some other vehicle and therefor are not engineered for the weight, distribution, size and several other factors that go into an engineered system for a specific vehicle. For example the "Corvette" system mentioned above is from a front and rear disc brake system with a low center of gravity, performance oriented suspension and a more ideal front to rear weight ratio, none of which is in a typical old truck regardless of what you have done to it. Therefor it will take tweaking to optimize it potential. Brakes are not something to screw with, you can talk to three different people and get four different answers.

Please, for your own safety and others, do proper and serious research on what you need to have your brakes operate properly.

 

The front are a power disk conversion kit with 11 inch rotors I got from speedway motors or POL.inc don't remember witch of the top my head. The rears are 10 by 21/2 inch drum brakes that came with the new 9 inch rear