just did a 4 wheel disc conversion on my 70. I ordered all new brake lines as well as new front to rear. The front to rear came in two pieces. Dude where I bought it said there is a hold off valve towards the back that came with the disc cars. Can this be purchased at the local auto store, and what is its purpose. I believe it to regulate how much fluid the rear gets.

The hold off valve on the 70 hes talking about mounts on the rear cross member on the drivers side. Ground up has them I believe. Then a short tube goes from that to the rear brake hose.

just did a 4 wheel disc conversion on my 70. I ordered all new brake lines as well as new front to rear. The front to rear came in two pieces. Dude where I bought it said there is a hold off valve towards the back that came with the disc cars. Can this be purchased at the local auto store, and what is its purpose. I believe it to regulate how much fluid the rear gets.

Double check but I don't think it will be needed on the 4 wheel disc set up. It has something to do with the difference between the discs up front and the rear drum brakes.

Double check but I don't think it will be needed on the 4 wheel disc set up.
:yes: I agree.

anyone with a factory 4 wheel disc setup that could shed some light?

There is a good thread about brake valves here: http://www.chevelles.com/forums/showthread.php?t=227102&highlight=hold+valve

See post #6 regarding the function of the hold off valve. I agree with others that the hold off valve (like the one pictured here http://www.inlinetube.com/Prop%20Valves/PR104d.htm ) is not needed with 4 wheel disc...

Hold-off valve would be installed in the front brake line (or as part of the combo valve) to delay the pressure to the front discs on a disc/drum app. You don't need one of these.

Proportioning valve would be installed in the rear brake line to reduce line pressure to the rear brakes and prevent the rears from locking before the fronts. You most likely DO want one of these. Depending on your kit, this may be part of the distribution block/pressure differential switch that mounts under the master cylinder. If you don't have one, I'd install an adjustable valve in the hard line going to the rear. Most speed shops, etc. will sell you an adjustable prop valve. Just make sure you get the fittings to adapt it to your setup...

I got 4 corner discs on my 70. At the advice of a brake expert, I removed the little proportioning valve near the front of the master and the holdoff valve in the rear on the crossmember. Also installed and adjustable proportioning valve in the line going to the rears approximately about the middle of the drivers door area. Simplifies the system.

Hold-off valve would be installed in the front brake line (or as part of the combo valve) to delay the pressure to the front discs on a disc/drum app. You don't need one of these.

Proportioning valve would be installed in the rear brake line to reduce line pressure to the rear brakes and prevent the rears from locking before the fronts. You most likely DO want one of these. Depending on your kit, this may be part of the distribution block/pressure differential switch that mounts under the master cylinder. If you don't have one, I'd install an adjustable valve in the hard line going to the rear. Most speed shops, etc. will sell you an adjustable prop valve. Just make sure you get the fittings to adapt it to your setup...

:noway: :noway: The hold off valve on a 70 is mounted above the rear end. It is a small brass valve (see the last link in my prior post).

:noway: :noway: The hold off valve on a 70 is mounted above the rear end. It is a small brass valve (see the last link in my prior post).

Well sure, but other than being wrong, all my info was right...:p

GM Notes:

1967-70 cars all came standard with a dual master cylinder and drum brakes. 1967 was also the first year for optional disc brakes. If the car had either drum or disc brakes, there was a distribution block with a built-in brake warning switch down on the frame below the master cylinder. From 1967-69, if the car had factory disc brakes it used the drum distribution block in conjunction with a disc brake hold-off valve, which made the disc cars a 2 part valve system. In 1970, GM added a third valve in the rear line for brake correction, so the 1970 system had three valves. For cars converting to disc brakes, aftermarket replacement 1-piece valves are also available. Conversion valves will not plumb into existing factory lines.

I'm coming from Camaro-land, which uses the "other" hold-off valve that I described (and is listed below the part you mention on the inline tube descriptions: http://www.inlinetube.com/Prop%20Valves/pro_valves.htm)
Pretty special when GM had one type of valve that made the fronts engage before the rear and another which made the rears engage before the fronts....

In any case, everyone seems to agree that a hold-off valve (of either type) is not required for your setup. Hopefully you have a prop valve somewhere in the system.

Am I able to connect the 2 pieces of brake line that I bought to bypass the valve, or am I going to have to buy a new one piece line?