Master Cylinder and Proportioning

[Wulfman]

OK - I went back and read Ray's Disc Brake conversion info and he says to use the 1980 Chevy C30 1 Ton Master Cylinder and if you are installing disc's on all 4 wheels, you do not need a proportioning valve. He also states (at the time I printed his lit) that he has not tested any proportioning valves, but a proportioning valve is only required when you are mixing disc with drum.

So I've had all my parts for a while and am putting it all together and one of the auto part's stores I went to said the MC I have will not work and I will need a proportioning valve or a different MC or the rear wheels will lock up.

So what's the scoop here. Now I'm confused and I want to run the lines and finish the brake and fuel lines so I can start on the dropping the engine back in.

[Charles Talbert]

OK - I went back and read Ray's Disc Brake conversion info and he says to use the 1980 Chevy C30 1 Ton Master Cylinder and if you are installing disc's on all 4 wheels, you do not need a proportioning valve. He also states (at the time I printed his lit) that he has not tested any proportioning valves, but a proportioning valve is only required when you are mixing disc with drum.

So I've had all my parts for a while and am putting it all together and one of the auto part's stores I went to said the MC I have will not work and I will need a proportioning valve or a different MC or the rear wheels will lock up.

So what's the scoop here. Now I'm confused and I want to run the lines and finish the brake and fuel lines so I can start on the dropping the engine back in.

We have installed this system many times without a proportioning valve, always worked just great. The only time I would say a proportioning valve is absolutely necessary has been on some of our dual circuit systems that have incorporated a booster for each circuit. In this case, a valve is needed in the rear axle circuit unless the truck is carrying a load in the bed all the time, (such as brush fire trucks hauling a tank full of water and a pump/hose reel unit.) A no load condition on the rear axle will allow a boosted system to lock the rear wheels unless you cut back on fluid pressure in the rear circuit. With a non-boosted system, we have never seen it as an issue since the rear drum system is far less effective than the new front disc system.

My advice to you, if you are not doing a boosted system, install your system without the valve, when you are ready for a road test, do so carefully using common sense principles. If you determine that a proportioning valve is needed, it is no big deal to cut a line in the rear circuit along the frame rail and install an adjustable valve. With that in place you will be able to tune in the exact pressure needed to bring about the system function that you desire.

[scottyg]

Charles,

Sorry to hi-jack the thread, but would the dual MC work fine as is on the stock drum system? Or, not necessary until I go the disc route?

Thanks,

Scott

[Charles Talbert]

Charles,

Sorry to hi-jack the thread, but would the dual MC work fine as is on the stock drum system? Or, not necessary until I go the disc route?

Thanks,

Scott

The Chevy M/C mentioned here is designed without a residual pressure check valve specifically for a disc system. A M/C that has a residual check valve is better in a drum system.

If you intend on converting to disc in the future, and wish to convert to a dual line system now, you can use the Chevy cylinder with the drum system. The difference will be that there will be no residual pressure reserve to hold a modest amount of fluid in the lines. This is necessary as the shoes in the drum system are drawn back via spring pressure that forces fluid to return to the reservoir. It will function just fine as long as you stay on top of shoe adjustment to keep the shoe to drum clearance as close as possible. If shoe to drum clearance is allowed to get a little loose, you will find that you will have to pump the brake pedal once to fill the lines with fluid before pressure starts to build. Since there is no spring return of the pads in a disc system, the fluid is not forced back when brakes are released, thus no need for a residual pressure check valve.

[scottyg]

Thanks for the quick reply.

Scott

[W_A_Watson_II]

On my M with Disc/Disc I had to add the Proportioning Valve. With out it the rear brakes locked up way to early.

On the PW with Disc/Drum I'm installing one, as well as the Residual Valve.

[Wulfman]

Disc/Disc

William - Did you use 1/4" or 3/16" line and what part number of residual valve did you use? That's what I was told would happen - the rear's would lock before the fronts and if plumbing the valve is the fix, that seems easy enough. Any pix of your install on the valve?

Thanks.

[W_A_Watson_II]

I ran 1/4" lines. I picked up the Residual Valve from Summit, don't remember the part number. The Residual Valve is for Drums, to control the rear wheel premature lock up you want the Proportioning Valve.

Here is a picture of my Proportioning Valve.