[chemnick]

Hi Guys:
Regarding an upgrade to front disc brakes on my '67 GTO, I have been noticing most booster/master cylinder combinations have the valve mounted next to the master. My question is what is normally done about the existing valve down on the frame rail? Do these kits come with a pass-through block to put there? In checking with a vendor regarding a new (OEM type) prop valve, I am told that it is the same valve for drum or disc front brakes, true or false? If I go with the newer style setup for the M/C then is there an available line set that can be obtained for this? Kinda confusing.
Chemnick

[mrennie]

My understanding is that on the 1972 and older cars, the block on the frame is a distribution block only (regardless if drum/drum or disc/drum), and if you had front disc brakes then a proportioning valve was added near the master cylinder. There may also be a hold off valve mounted somewhere (near the back?) for the rear drums.

[lust4speed]

Not sure about the early disc brake cars not having a proportioning valve. Think that a proportioning valve always went along with disc brakes. Basically takes much less pressure for drum brakes to grab, and some equalizing force always had to be there to balance the disc/drum combination. The manufacturers probably never used any sort of proportioning valve for drum/drum systems since they usually just changed the diameter of the wheel cylinders to modulate braking force front to rear.

To simplify things, if an aftermarket master cylinder with the proportioning valve up near the M/C is used, then you should remove whatever is currently on the frame rail. Doesn't really matter whether it is a T, a distribution block, or a proportioning valve because they are being replaced with the new system, and what is currently there can cause problems. I've taken the easy way out before by simply using a coupling and a short length of tubing to lengthen the rear brake line up to the new valve, and then a T fitting combining the two front brake lines together with a short length of tubing up to the valve.

Do new master cylinders have a means of duplicating what the original hold-off valve accomplished? Didn't the stand alone hold-off valve disappear a few years after GM started offering disc brakes? My blue '67 had factory disc brakes and had the hold-off valve. When it failed in the 1980's there were no rebuild kits or reproductions that I could find at that time so I did the only thing I could - took it off and ran without it. I never did feel any difference in braking with or without it, so don't know if it ever was that important. Wonder if it wasn't like the coil springs behind each piston in the early disc brakes that disappeared on the next generation caliper.