Toyota Repair: 1994 T-100 brakes, rear drums, rear brakes

Question
I have all most no rear brakes. Steping on the brake pedal would make you think that the master cylinder is bleeding by, but has been replaced twice with the same results. I then tried replacing the proportioning valve and still have the same problem. The roters are turned , new pads the rear drums have been turned and new shoes installed. If you are on any slick surface the fronts lock up and no matter how hard you stand on the brakes the rear just keep turning. Only buy being in four wheel drive can you stop on slick roads, with the front brakes doing all the work through the drivelines. I do like my local dealership and the tecks, but can't seem to get a handle on the problem. Your answer will go to them, as both they and myself are out of options. Thanks Ken

Answer
The load sensing proportioning valve may need to be adjusted, there is a specific operation in the factory repair manual that shows the procedure, it's on page BR-55, it's quite involved and requires special equipment but it just may solve the problem, by the way, when bleeding the brakes does any fluid come out of the rear bleeder valves?