I do a lot of work on my own cars and last weekend I ran into a problem that has me stumped. I did a complete brake job on a 1984 two wheel drive GMC pick-up including the rear wheel cylinders which were leaking. I was finishing the job by bleeding the brakes at all four wheels. There was no problem in the front, but when I tried bleeding the rears no fluid would come out of the bleeders on the new wheel cylinders.
My friend and I spent several hours trying to bleed these brakes. The only possible cause that we can think of would be the master cylinder. I was hoping you might have an answer before I go buy one.
Don't buy the master cylinder, yet. The master is a possibility, but there are a couple of checks to be made before heading for the parts store. First of all, have you looked into the combination valve? The combination valve looks like a junction block where the brake lines are routed from the master cylinder, before they continue on to the appropriate wheels. The term combination is used because it actually contains two valves. One is the "metering valve" which regulates the amount of hydraulic pressure applied to the front brakes. The other is the "proportioning valve" which "proportions" the pressure differential between the front and rear brakes. The combination of the two helps provide brake equalization.
Take a good look at the combination valve and you'll notice a rubber boot on one end. Under the rubber is a pin. When pushed in, the pin will center the proportioning valve and allow the brake fluid to flow to the rear brakes.
When you started bleeding the fronts you built full pressure on one side of the valve and none on the other. This moved the valve to one side and shut down the fluid to the rear brakes.
Some other possibilities: the proportioning valve sometimes gets stuck and will have to be replaced. The rear brake hose may have collapsed and shut down the pressure to the rear brakes. Last, the problem could be the master cylinder. If the master cylinder were the problem you probably would have noticed it while driving the truck before the brake job. The brake pedal going down to the floor is a dead give away.