Chris
Inline Tube has them, They call them a pro valve in one section and a hold off in another. Check it out.
http://www.inlinetube.com/
PR-114
Proportioning Valve - GM
1967-68 A-body Chevelle / El Camino / GTO / 442 / GS
1967-68 F-body Camaro / Firebird
1967-68 X-body Nova
1968 Pontiac Lemans / GTO Factory Hold Off Valve
1967-68 Olds Cutlass / 442 Factory Hold Off Valve
1967-68 Buick Skylark
1967-68 Chevy Chevelle / El Camino Factory Hold Off Valve
Cast Iron
$89.00
1967-70 GM Hold-Off Valve
Details:
This is the hold-off valve that mounts on the stud of the master cylinder on all factory 1967-70 disc brake cars. It is used in conjunction with the distribution block mounted on the frame, which is the same on factory disc and drum cars. This valve holds off the front brakes to give time for the rears to activate. At a calibrated pressure, the valve allows fluid to pass and engages the front brakes. This valve is the same on all GM cars, but the bracket is different for applications NOT listed below.
They are on Ebay alot too.
Mike
