1967 Shelby / Mustang Proportioning valve

Question: 1967 Mustang / Shelby rear brake proportioning valve. Are these able to be rebuilt or are they available though parts vendors.
Part Number C70A - 2B091 - C.
When i depress the brake pedal, brake fluid leaks passed the rubber O ring.
Is it faulty or is there another issue maybe ? I have just replaced all the lines / rear shoes and wheel cylinders.
Any help would be most appreciated

 

Thanks alot, that saves spending hours looking though parts catalogues.
Cobra Automotive part number: 100-1679. Thanks again.
Thats a very close number to my GT500 only 5 numbers different. Very cool.