68 Brake Failure

Cougar Community Classic Cougar Maintenance Restoration and Repair
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Hello everyone, I am struggling with brake failure on my 68 with front disc, rear drum. I have had the Bendix booster rebuilt, rebuilt the proportioning valve, 3 master cylinders later (currently a Dorman, bench bleed), adjusted the rod, and have good fluid at all four corners and no fluid leaks and master bowl levels are maintaining level.

Here is the problem, with the car off pedal builds to a nice firm pedal, as soon as I start the car, I lose all pedal completely. When I pull the vacuum line off, the booster makes a nice swoosh sound and once capped, has pressure and the pedal returns somewhat, but not enough to drive. Basically, you can stop the car at idle if you pump it about 5 times. I had a buddy, recently retired after 40 years in the brake and alignment business, help me. He has checked all four corners and has eliminated any issues there, believes the proportioning valve is working properly, adjusted the rear drums, the rod to the master, etc. and he is stumped.

Everything was rebuilt less than 1000 miles ago and this all started because I was feeling something in the pedal and had the brakes professionally checked. They recommended I replace the front calipers. I did the calipers and the lines, and I now have this issue.

Anyone have a similar problem they solved or ideas on how to fix this?

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Calipers are likely on the wrong side. The bleeder must face to the rear of the car. If it faces upward, you cannot get all of the air out.

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Have you replaced the three flex brake lines?? When they get really old the lines will collapse resisting fluid to flow… Rear flex brake line is also known to get week and balloon up taking up all the brake fluid pressure… Just a couple of things to check…

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Ive seen an issue with bench bleeding. The piston is pushed too far in & the seals get nicked by the bore Only push in about an inch or two. I use a screwdriver with a piece of tape marking the depth.

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Thanks Nolen. I am replacing the rear line since I already replaced the fronts. I have good fluid flow to the back, but I’m replacing it either way. I appreciate the input regardless.

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Thanks Bill. I did bench bleed it and its been properly adjusted. I have eliminated the master as the problem. I am leaning towards the proportioning valve or the booster, both of which have been rebuilt. I appreciate the input regardless.

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Thanks Hemi. Working my way out to the calipers, but they aren’t on the wrong side. I did check that. I am starting to question if they are the correct ones for the car. I’ll keep you posted.

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Stupid question: Could you have a drum brake master cylinder installed by mistake? rear reservoir would be noticeably larger on a disc brake car.

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Thanks Manyc. No that isn’t the problem. It is a correct master cylinder.

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Finally had a minute to update, I am making no progress on the issue. I did replace the rear brake line after much fighting to get it off. That wasn’t the issue. At this point I have replaced everything except the hard lines. I have no leaks, I have fluid at all four corners and no bubbles, the pedal starts to get hard but still lose all pedal when the car is started. Any other suggestions?

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Its possible you have a DOA part too. Just because it was new or rebuilt recently doesn’t mean its still ok. or even ok out of the box. While rare it does happen. Ive been bit by that snake way too many times in my career!

I have had bad master cylinders cause similar issues. First post mentions your on your 3rd master? Did you have the same issue with all 3? what quality of masters where you getting? New or Rebuilt?