I have hot water all over the house and in the master bedroom sink, but no presurre in the master bedroom tub, I have replaced the valve cartridge, crawl under house to check for leaks, I'm at a stand still, any help would be greatly appreciated.

When you replaced the tub valve cartridge, did you flush the lines to clear any debris.

I let the water run , How do you recomed I flush the lines for the tub?

You have a volume issue, it just seems like pressure is the problem. Remove that cartridge AFTER you shut down your main and allow the pressure to be relieved. With the cartdige out cover the opening with a big towel and have someone else open the main just for about 5 seconds. It may get messy but that will clear out the lines. Reinstall the cartidge, turn the main on and test. Letus know how it is