rear brakes

anyone have this issue:
I cannot get the back brakes to lock up on wet grass.
They are in adjustment, fresh drums/ shoes, cylinders.
Thay've always been weak.
I have never done the master cylinder yet.
it's an '89, I do all the maintainence. The stupid back brakes just don't feel strong, I'm thinking of replacing the master cylinder. If that does'nt tighten them up I'll try an adjustable combo valve.
:repair:

I can only assume that you have vacuum bled the entire system and have replaced ALL of the old brake fluid. If you off road it you could have also dented a hardline. I can also assume that you have the mechanical adjustors set so if the rear end of the jeep is off the ground you run the shoes out so they just barely touch the drums when you rotate the wheels.
It could be a bad master cylinder, I just have seen too many simple issues overlooked and compensated by expensive parts. Is the pedal soft??, does the jeep pull wierd when you come to a hard stop?
Good Luck.

I would check lines and then consider proportioning valve....