Technical Proportion valve

Working on building brakes on 39 Chevy rod. Front disc rear drums do I need a proportion valve?

 

You may or may not; hard to predict. It's cheap and easy to install during a build and will be there if you need it.

 

I use a CPP distribution block that has one built into it. On my Ford, it is wide open. I never had to utilize it.

 

While not definitive; I have experience with at least six car ranging from a T bucket to a '60 wagon that the valves were left wide open after some testing.

Your case may be different.

As previously stated, a valve is easier to install now than retrofitting after a need is found.

 

If you don't have a disc/ drum distribution valve the rear brakes will most likely lock before the front. If you plumb the brakes directly into the master cylinder an adjustable valve added to the rear line can adjust to correct that

 

I needed a proportioning valve on my wagon when I changed from drum/drum to disk/drum. Without the proportioning valve the back brake locked up as soon as I touched the brake pedal.

now I just plumb one in whenever I do custom brakes.

also, you can use it to shut off the rear brakes for big Smokey burnouts

 

Keep in mind that most, if not all adjustable proportioning valves are still operational "wide open" and not bypassed, normally around 1200 psi.

 

Thanks for the reply’s think I’ll go ahead and plumb one in.

 

I used a Wilwood MC and their Prop valve. I followed their instructions on how to set and it’s perfect.
If your rear end came out a car with power brakes and your NOT going to use power****ist it best you install smaller diameter wheel cylinders. 2 brake suppliers told me this. I used a Ford Explorer with F-150 11” drums and did the change.

 

https://www.jalopyjournal.com/forum...g+valve&o=relevance&c[title_only]=1&c[node]=5

 

Interesting that they would want you to have less rear braking.

 

No..it was done for pedal pressure. Into the 80’s most F-150’s had power brakes and there WC’s were larger diameter than manual brakes. Actually the same for MC’s. Wilwood recommends 7/8” for manual brakes. 1” or 1-1/8” for power.

 

Ummm, smaller output means more pedal pressure.

 

I'll have to disagree with your "suppliers" information, as this is sadly nothing new. Wrong/miss-information from well known brake suppliers has surfaced many times over the years here and on other forums. I don't blame the supplier as much as the "techs" that often are very wet behind the ears on brake system component design/operation.

I have no knowledge of any factory US brake system that used different rear drum wheel cylinder sizes for power and non-power operation on the same vehicle, including the mid-late '70s F-150, which also used the same 1" master cylinder size size for both power and non-power, very common on many vehicles during the '70s transition from manual to power****ist.

There is no logic down-sizing a rear wheel cylinder for manual operation. A smaller wheel cylinder simply reduces braking effort, as 57 Fargo stated above. Why would you want that with no power****ist?