[Rick_L]

You might want to try a disc/drum combination valve from CPP. They can fix you up with a bracket to mount it close to the master cylinder and have the short hard lines to hook it up. You could try an auto supply for the same piece but most don't carry it. The one you need is for a 71-72 Chevelle.

A 64 Chevelle would have had a single reservoir master cylinder and no shuttle or combination valve. So if you got it off that, somebody besides the factory put it there. The type of valve you suspect you have would have come on 67-69 Camaro, Nova, Chevelle, or Impala with drum brakes.

Look at the CPP catalog and see if your valve looks like the one they sell. If there's looks bigger, then all I wrote above is true. If it looks the same, then you don't need to do anything.

 

[Rick_L]

With no proportioning valve, you have the same pressure at each wheel.

The proportioning valves cut pressure to the rear drums. Seems backwards since disc brakes are "better" than drums. They are, but drum brakes are self energizing and actually require less pressure than discs. The superiority of disc brakes is that they don't heat up as quickly and even when they do, they cool off quicker. So they are just a steadier more reliable deal, especially under harsher conditions.

The other half of this scenario is that when you have rear disc brakes, you often have to reduce the pressure to them also.

 

[Rick_L]

"one of the functions of the combination (Proptioning ) valve ....was to delay the onset of pressure to the disc's to give the drums a head start as things had to move farther . IS that right?"

There is a metering valve in the combination valve that lets a bit of pressure build in the rears before applying pressure to the fronts. This is for wet/icy roads and keeps the car going straight when you first apply the brakes. This is done on both disc/drum and disc/disc combination valves. The difference in brake application is not really noticeable on dry roads. It's a pretty small effect anyway.

Disc/drum combination valves also have a residual pressure valve in the outlet for the rear brakes, and a shuttle valve that shuts off one end if there's a leak there. The shuttle also activates a switch that powers a warning light (if you have the light).