The proportioning valve from my 65 Gt was rebuilt by a specialty brake shop in Southern CA. I believe the kit was about $35 and labor $45. It works like new.
I soaked it in penetrating oil for several weeks before I dismantled it, and realized my skills weren't up to putting a new kit in.
Good luck.
Mark Mosteller
Garden Grove, CA
"Everything that is really great and inspiring is created by the individual who can labor in freedom." Albert Einstein
65 Fastback GT. 65 Convertible. 84GT, 90 5.0 LX Convertible. MCA. Southern, CA