brake pedal falling

I have a 69 roadrunner with front disc rear drum using the 73-76 a body setup. I was driving a week ago and the pedal started to fall when I hit the brakes, and the brake light would turn on. I drove it home, and checked for brake fluid leaks. There were no external leaks I could see. The symptoms appeared to be a bad master so I ordered a new one, and put it in. After bleeding the master the pedal is firm with the car stopped, but as soon as you drive one or to stops in the pedal will fall and the light comes on. It comes and goes but its hard to stop the car when its doing it. Anyone have this happen before?

Are the rears adjusted up?That will cause it at times.Power or manual brakes?Does it do it if you dont drive but just apply pressure to the brakes and does the master get low?Pull the drums and check the wheel cyls.Rocky

Sure sounds like the M/C went bad as you surmised and one side has to have way lower pressure for the light to come on but from your descrip it is a part time problem that comes and goes . I suppose it is possible that the rebuilt M/C is bad. I'd give it another good visual from head to toe & have a helper work the pedal with you underneath & I'd cap the M/C with a pair of brass inverted flare plugs from the "Edelman" fittings cabinet at your parts house & if the M/C is good and bled out with it capped it will be rock hard with virtually no travel & stomp it hard and try with light pressure & see how it acts. Holler how it goes.

Sounds very much like a bad master cylinder... Hard pressure on the brake pedal usually gets a solid pedal but light pressure will let the pedal sink to the floor...

I agree plug the ports & see what it does.. If the pedal still sinks under light pressure than the M/C is junk...

Definitely don't re out a bad master cylinder right out the box, I've had it happen to me before with a master cylinder and alternator.

I concur sure sounds like a bad master. mexican rebuilds are not all that cool sometimes.

Is the fluid level still full in replacement master cylinder ? If not make sure the fluid bleeding did not knock out wheel cyliner / caliper

Are you using an adjustable brake pedal rod? I had a similar experience with my adjustable brake rod loosing it's adjustment which resulted the same conditions you are experiencing.

Regarding the brake light. This is related to the pedal making contact with the switch (i.e. pedal dropping) I believe.

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I have a 69 roadrunner with front disc rear drum using the 73-76 a body setup. I was driving a week ago and the pedal started to fall when I hit the brakes, and the brake light would turn on. I drove it home, and checked for brake fluid leaks. There were no external leaks I could see. The symptoms appeared to be a bad master so I ordered a new one, and put it in. After bleeding the master the pedal is firm with the car stopped, but as soon as you drive one or to stops in the pedal will fall and the light comes on. It comes and goes but its hard to stop the car when its doing it. Anyone have this happen before?

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If your car was a original drum brake car front and rear did you add a proportioning valve and remove the residual valve in the master cylinder for the new front disc brakes? If not you should