Re: Proportioning Valve on one side, how to center it?
Your new system certainly has the "newer" combination valve like used on the second gen and later cars. This combination valve incorporates all the separate functions of the separate valves used on the first gen cars including distribution, the warning light switch, rear proportioning and the metering/hold off function. Thus, there is no separate metering or hold off valve in you system to worry about. As already mentioned, do confirm that you don't have the OEM proportioning valve that would be mounted to the subframe under the driver seat area of the car. I'd call the manufacturer and ask them some questions about the problem.
'69 Z/28, Lemans Blue/White Stripes, DZ302, Tremec Magnum, 4.10, Procar Elite Seats,
Custom Upholstery, Dynatech headers, 17" VWW V45's, 235/45, 275/40, C5 Front Brakes