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hey guys, thanks in advance for any and all advice. I have a 74 Roadrunner with rebuilt 440. Got it running, although it seems to be May e have a cylinder misfire. But that's another topic. I am having brake issues at the moment. I have bled the brakes, starting at furthest point, R/R, L/R, R/F, L/F and I am getting full fluid on every line, no air. But as soon as I crank the car, the brakes are still going to the floor and feel like a wet sponge. Is this a Master Cylinder issue or a Proportianate Valve problem, or both. It has a brake booster installed as well. Thanks for your time.
 

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If the pedal is going to the floor, to make sure it isn't the master cylinder to start with, is verify the bleeding one more time, sometimes it seems OK, but it isn't, let the car sit without starting it, check the pedal again. Power brakes take a lot of extra pressure to make them work when the booster isn't energized (engine running and vacuum assisting the pedal), so you really have to stand on the pedal and push a lot harder. If once you verified they are bled good, with a buddy, put the car out of park (engine off), stand on the brake and see if a buddy can push the car. If he can't, things should be OK. Now, you didn't mention if you just bled the brakes or if you just bled them, so you don't know the condition of the master cylinder itself. If it was empty and you bled it, they do tend to leak at the firewall (or front of the master cylinder against the brake booster) when they do go bad (and bleed past the plungers internally), to which time to replace. There is also, if not mistaken, an adjustment to the rod going from the back of the booster through the firewall to the brake pedal that may be out to lunch, too. Going to have to dig into it a little more to get this figured out.
 

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Could be a vacuum leak. Make sure the plastic piece on the brake booster is ok, as well as the hose from it to the intake.
 

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If the pedal is going to the floor, the problem is in the hydraulic side of the system, not the vacuum boost side. Could still need bleeding, but if you think it's bled, you can try to find the problem by disconnecting a brake line from the master cylinder and plugging the port. If you get a firm pedal after plugging off a port, then you know the circuit with the problem. If you can't get a firm pedal with both ports plugged, bad master cylinder.
 
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We also had an issue a couple years back where the guy had a flexible line running to a front brake that went upwards so much that an air bubble was in the hose itself and was very difficult to bleed.
 

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This can also be a mechanical problem. While a helper pushes the brake pedal, watch the calipers for movement. They should only move slightly as they compress against the pads. Any loose, worn or binding pivot can cause excessive caliper piston movement. This will show up as excessive brake pedal movement, like going to the floor.
In the rear, shoe-to-drum adjustment or bent/disconnected hardware can cause the same problem as above. You can use a large screwdriver to block shoe movement. Holding the front shoe should cause the rear shoe to extend.
 
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