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1970 Mark III !@#$% Brake Problem

I have a 1970 MK III. Current problem started with what seemed like a bad master cylinder; peddle would go to the floor if you hit it hard.
I took it to my long time mechanic who has been in the business decades, and worked on this car as well as its predecessor 1971 MKIII. He checked all the other brake elements and agreed, the master cylinder should be changed.
I had acquired a “new-old stock” master cylinder that was installed. No improvement.
He observed that the “brake light warning switch” seemed to be leaking. It was replaced with an overpriced after-market unit. No improvement. And question if in fact the replacement unit was defective because it seemed to leak.
Completely by-passed the Sure-Trac system. No improvement.
Rear brakes have been adjusted properly, calipers centered properly, booster rod proper length. No improvement.
System bled numerous times after various steps above; good stream. No improvement.
Installed a brand new master cylinder. SOME improvement; push peddle once and goes almost to floor, push second time and comes up pretty good. But not right.
Greetings Paul –
If the system has been bled properly and NO air is remaining in the system the fluid would need to leak out externally and be visible if the pedal goes to the floor. However, if the master cylinder is bypassing internally the pedal will go to the floor and no leaks will be visible and the pedal can be “pumped up” somewhat only to go to the floor again if held down under pressure. Therefore because of your recent experience as you have described I would remove the master cylinder and bench test it for bypassing. You would need to bleed it and seal the outlet ports and then apply pressure to the piston in order to observe if the piston travels to the applied position or not. If it does under heavy pressure it would be bypassing as you are describing because of failed internal seals. Let us know what you find.
We should have proportioning valve parts available if you still need them.

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