Sean,
I have done exactly what you are doing. I solved this with a Summit/Willwood adjustable proportioning valve and two Swagelok fittings I bought at a local Swagelok distributor in Pelham.
here's their contact info:
Alabama Fluid System Technologies Inc.
237 Cahaba Valley Parkway Phone: 205-988-4812
Pelham, AL 35124 Fax: 205-988-3852
You will need two "S-300-1-2". Call me and you can come see mine if you want. Here's some pictures. You can buy a propo-valve locally at Seco but expect to pay more.
Part Number Search Results
Part Number : SS-300-1-2
Description : SS Swagelok Tube Fitting, Male Connector, 3/16 in. Tube OD x 1/8 in. Male NPT
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Quantity :
Stainless Steel Brakes A0707 - Stainless Steel Brakes Brake Proportioning Valves - summitracing.com
Summit SUM-G3905 - Summit Brake Proportioning Valves - summitracing.com
Did you put that PV only in the rear brake lines?
Thanks