REar brakes not working

Ok my rear brakes dont work. They are adjusted correctly and bled. Now if i put on jacks i have to press really hard to keep the tires from turning with the car in Drive. First question is how do i know if its the boost pump or the master cylinder? Second question is if i have to replace which company in the after market makes good break parts. I dont want to replace it with stock parts. Thanks in advance.

It might be that the wheel cylindar is just locked up and thus not putting as much pressure on the shoes as it should be.

How does the peddle feel (i.e firm, hard, soft)

But it could be other problems.

Air getting into the line from loose fittings or pin holes in the line
Vacuum line not hooked up to the manifold.
Master cylindar gone bad
Bad seal on the resivor
Automatic adjusters not working (I had this one time and the brakes would tighten up on the drum)

I just use the AZ brand of brakes and they do a good job of stopping my XJ with 35's on it.

peddle feels normal. not too hard and not too soft. If there was pinhole then their would be air in lines. When i bled it again today there was no air in lines. Where is this vacuum line to the manifold? Now i know my front breaks are working good. The vacuum line applies only to rear? and a seperate line for the front? thanks