brake job from hell

,,,,i need some good advice on this one.....my wifes 91 Nissan Stanza needed brakes, so i purchased the front pads and rear shoes...install the fronts[easy] and rears[so so]....notice the rear brake cylinders are a little leaky, so i replace those and bleed the system....problems start....pedal is drifting down to the floor with steady pressure...rebleed the system and no improvement...so its gotta be a shot master cylinder and i install a rebuilt unit, bleed system and the same shiiity pedal....return the master and get another,bench bleed it,bleed system and ...same pedal dropping to the floor with steady pressure.....any advice on this appreciated.

Does it have ABS, if so bleed the ABS as well (usually one or two nipples on top of ABS unit) ...other than that I am not sure...

...no ABS.

possibly the valve goin from the valve cover to the brake booster is shot, or the booster is shot itself...recheck the sytem tho, make sure no leaks anywhere. Still possibly air in the line and u think u got it out.

..I checked the valve and it seems good[can only blow thru it one way]....i put my finger over the power booster vacuum connection and had someone press on the brake.....you can hear a small hiss from the booster rear,are these vented at all or is this a leak?

The brake booster wouldn't have anything to do with the pedal dropping to the floor. Does the fluid level drop when the pedal drops all the way to the floor? Are you having to refill it? Check the rubber lines going to the front calipers. They may have developed cracks and started leaking. Also, you may want to inspect the steel lines running under the car. Over time they have a tendiency to rust through or have holes rubber in them from the vibration. Look under the car while you have someone push the brake pedal. See if you can find any fluid leaking to the ground.

Which way are you bleeding the car.... we've had troubles in the past because we didn't bleed criss-cross, just front then back, then around the other side.

...manual says to do a criss cross bleed pattern.

I guess the easiest way to rule out any leaks is to have you constantly pump the pedal and have the pedal hit the floor. If you have a leak, then sooner or later, the resovoir will eventally go empty and there would be fluid on the ground. If this is not the case, then you most likely still have air in the system. Don't worry, this sometimes happens.
You have 2 options. You can try gravity bleeding, which is actually quite simple. Hook up tubing to each bleeder screw and crack them open. Make sure the resovoir is full of fluid and just monitor the level periodically. The air pockets will eventually work themselves out.
The other option is to bleed out as much air as you can and just drive it for a few blocks. This usually works out the air pockets.

P.S. Just curious, How does the fluid look when bleeding? Do you see a small trail of bubbles almost too small to see? And did you properly adjust the rear shoes? How do the front calipers look? Do they move and slide around easily?

...THANKS FOR THE REPLIES...still not fixed though...i agree the booster should have nothing to do with bleeding the system........with the car not running the pedal should build pressure as its pumped a couple times......i think this second rebuilt master is junk as well......i have bench bleed it and removed all the air bubbles BUT it doesn't forcefully squirt the fluid out of the hoses[no matter how hard the rear piston is pushed in]..with it installed on the car it wont lock any of the wheels...pressure falls off....the fluid is clean as its been bleed at least a new gallons worth....the rear shoes are new[new rear cylinders too] and new front pads[ive rechecked the mounting and its all good].

My car has brakes!!!....it was a defective master cylinder[two]. My advice is to stay away from rebuilt master cylinders and buy new.