Brakes not locking

I have a 99 XJ with a chry 8.25, and for some reason the rear brakes wont lock up. I put the jeep on jacks and run it in drive, hit the brakes and the rears slowly come to a stop. (i am running 33" trxus MT) The drums are adjusted correctly. Is this a problem with the rear brakes or are the tires just too heavy and create too much momentum for the drums to lock up? Shoes are new, brakes are bled, and i put a new Master cylinder. BTW the front dics work perfect. I know drums are junk but still i would expect them to be able to lock up. Thanks.

I just finished a post.... I bet your rear cylinders are leaking. Mine stops like a BMW now. And I had assumed for a long time bigger tires just meant I couldn't stop... I was very wrong. Read the below post.

http://www.naxja.org/forum/showthread.php?t=100838&highlight=new+brakes

Good stopping to ya,
Jason

either you arent getting enough pressure for the cylinders due to leakage or your pads are not properly adjusted.

Ah i dont see any leaks. Could the cylinder be leaking internally with being visable from the out side? (as in the piston in the cylinder is worn and allowing fluid to pass on the outside of the piston. Just like a loss in engine compression?) What about the booster? I am getting perfect braking up front, however inspite of this is it possible for the booster to be bad which is not allowing rear brakes to get enough pressure? its very strange.

Brake vacuum

What about the brake vacuum system? Could that be a cause for the rear brakes not working? Whats the best way to test to see if it is working properly?

sometimes the purpotioning valve sticksto the front side. when it senses the rears have to much of a preesure loss the valve will compesate and send the pressure to the front. after the brake job was done they (the purpotioning valve) sometimes sticks.