I have changed the rear rotors and brake pads, bled the brakes after putting itall back together. Seemed to have brakes but then shortly thereafter they are like a sponge. Last time I tried bleeding them they had no pressure. I have front brakes but no back brakes. Someone had mentioned to me about a valve in the line that could be a problem, where is this located and could it be by passed The brake fluid stays full and the master cylinder seems to be okay.

Hi, bleeding your rear brakes with engine running, you should get enough pressure out of the lines to squirt 3-4 feet. In other words, good pressure. If all you get is a trickle, you probably have a bad master cylinder. Just because its full of fluid, does not mean the seals arent shot. If you have bled them in order, left rear,right rear,left front,right fornt and have bled them enough to get fresh fluid out of each one without achieving good solid pressure at all wheels, replace your master cylinder. There is a proportioning valve, but usually it will affect only one wheel when going out, usually the left rear.
Brake pedal height comes from the rear brakes, so if you have bad pressure there, you will never get a good pedal. Eventually with a bad master, your brakes will go completly out..Mike