Originally Posted by AuSeeker

If you have no fluid pressure at the back brakes, you either have a blockage in the brake lines or the master cylinder has an issue.

I would first check the rubber brake line coming down from the body to the rear axle, loosen it where it connects to the steel line up at the body and see if the fluid will flow then at that junction, sometime rubber brake lines deteriorate on the inside and can block fluid flow when bits of rubber breaks loose on the inside, if you get flow there then it's most likely the rubber brake line, if you get no flow there then I would look at the master cylinder.

The master cylinder has 2 sections one for the front and the other for the rear, there are 2 main brake lines on the side of the master cylinder, figure out which one goes to the rear and loosen it at the master cylinder and see if you have pressure/flow there, if not that's the problem, if it does have pressure then there a blockage somewhere between the master cylinder and the rear rubber brake line at the rear axle.