1965 Power Disc Brakes Pedal Goes To Floor after bleeding system.

[ManiacD]

We have a 1965 Ford Thunderbird. When purchased, a front disc was dragging. We rebuilt the calipers and installed a rebuilt master cylinder and rebuilt brake booster. When bleeding the system, we seem able to purge all the air from the system. However when we start the engine, the brake pedal goes all the way to the floor, as if there was air in the lines. The system has been bled repeatedly. Has anyone experienced this? Thank you.

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[dmsfrr]

I'm totally guessing on those newer Birds but... is there a small (missing) pushrod between the booster & master cylinder?

Are you using a pressure/vacuum bleeder appliance to bleed the brakes or pumping the pedal?
Has the power booster been confirmed good since the rebuild?

 

[Ward 57]

We have a 1965 Ford Thunderbird. When purchased, a front disc was dragging. We rebuilt the calipers and installed a rebuilt master cylinder and rebuilt brake booster. When bleeding the system, we seem able to purge all the air from the system. However when we start the engine, the brake pedal goes all the way to the floor, as if there was air in the lines. The system has been bled repeatedly. Has anyone experienced this? Thank you.

So if I get this right, you have a hard pedal after bleeding with the engine off, but it drops to the floor when you start the engine. I would take another look at the booster. It's the only thing that changes with the engine.

 

[74 Harley]

I have a different thought, when bleeding can you stand on the pedal for 30 seconds without it going down at all?
You installed a rebuilt master cylinder, did you bench bleed it before installing it on the car?
A bad booster will not overpower a brake system.
If you truly got all the air out you probably got a bad master.
Try this. Car off. Jack up each front tire and try to turn it while your bleeding partner pushes the pedal. You could also have a faulty or incorrectly assembled caliper.

 

[Ward 57]

I have a different thought, when bleeding can you stand on the pedal for 30 seconds without it going down at all?
You installed a rebuilt master cylinder, did you bench bleed it before installing it on the car?
A bad booster will not overpower a brake system.
If you truly got all the air out you probably got a bad master.
Try this. Car off. Jack up each front tire and try to turn it while your bleeding partner pushes the pedal. You could also have a faulty or incorrectly assembled caliper.

Could collapsed hoses also contribute to this? No mention if they were replaced. Too many people pinch off the lines when replacing calipers to keep them from dripping instead of just tying them up out of the way. A dragging caliper is a sign of collapsed hoses also.