Perfect firm brakes one day, pedal to the floor the next???

Howdy yall. this is reguarding my 74 super. Two days ago i had adjusted the brakes, and bled them, and the pedal was nice and firm. i slammed on the brakes, and all four wheels locked up. and when braking hard, it kept straight, didn't pull to either side or anything. They were perfect. and the next day was still perfect. and i made sure the resivoir was full. today i get in, and hit the brakes, and the pedal goes to the floor??? any ideas? what could cause the brakes to be perfect one day, and then have no pressure the next? i went under the car this morning trying to see if someone maybe cut the lines or something, but i didn't see any cuts, or any puddles of fluid, but then again, i park in the leaves, so i prolly wouldn't have noticed if there was a puddle. I also did NOT check the resivoir to see if it was empty, but i don't see how after two days. any ideas?

Do you park on flat ground? If you are on an incline sometimes the fluid will drain back to the reservoir. If you pump the brakes does the pedal comes back?

Oh yeah, got my engine the other day from IAS in Forest Park. I asked if you had been there and Jason described your car, Oh yeah "Turbo Mike"!!! Car running good now but wont stop!

yeah, i park flat, and NO, when i pump the brakes, nothing, tho usually it alway did bring pressure, but now it wont. but i notice when the pedal stops (cause of the mastercylinder), it does enguage the brakes slighty, enough so that when i down shift to slow down, its enough to bring the car to a stop. but i feel absolutely no pressure no matter how much i pump it up.

Check and see if you have brake fluid under your floormat!

Did you recently put new brake shoes on? You may want to try adjusting the shoes again. I had a hell of a time getting a firm pedal when I redid my brakes. I had a great pedal at first, but then the shoes "seated" in their holders and that created more travel room for the shoes which in turn gave the pedal alot more travel. The pedal would go all the way to the floor. This is a common problem when replacing shoes, but even if you didn't replace the shoes, you may still want to check it.

First though, I'd definitely check the resevoir. If it's low, do like Digger said and check all along the tunnel for leaks. Also get a flash light and look at the back of each backing plate to see if you have a busted wheel cylinder.

dude i had this prob. 4 2 weeks straight i bled them everyday to hav brakes..i put knew brakes all the way around,new brake lines, a new mastercylinder and i still had the same prob.. then i looked at all 8 of the brake cylinders and one of the rubber covers was busted and when i mashed the brakes all the fluid wud just pore out of that cylinder. so i went ahead and replaced all brake cylinders and i had the best brakes ever

damn, i thought i responded to this, hahaha, my bad.

Turns out it was one of the wheel cylinders. It just went out, and was leaking fluid all over the brakes. One of the rubber cap things on the INSIDE of the cylinder tore. got that replaced, and all is well now.