Hi Paul,
With all that you've done at the same time, it is easy to not be able to pinpoint any one item for this issue.
I would tend to focus on the proportioning valve or even the rebuilt master cylinder. If the pressure builds and the front brakes lock, it could be more that the fluid is not returning to the resevoir keeping the cylinders extended. The things that would cause that are the return restrictors in the master cylinder (fluid does not return as quickly as it arrives at the wheels to help the driver better control the brakes) or the proportioning valve, which blocks-off one system in the event it senses a failure (front or back; left front/rightrear or vice versa, depending on how you car is plumbed).
The other thing to check would be to make sure that the pistons in the front calipers are not too tight and binding as they get hot with use and the loosening as they cool. This can happen especially if the pistons were replaced without honing or bushing the calipers. From what you've said, I think if you find nothing else obvious, consider having the calipers professionally rebuilt is the next best step.
You migt check with Apple Hydraulics in NY. They specialze in classic car hydraulics, with attention to LBC's.
Their contact info is below:
Apple Hydraulics, 1610 Middle Road, Calverton, NY 11933
Voice: 800-882-7753, 631-369-9515, Fax: 631-369-9516
e-mail: info@applehydraulics.com | home page:
www.applehydraulics.com I hope that helps you. Let me know how you make out.
Art