77 Peugeot GL Master clinder leaked brake oil back into the

My 77 Peugeot GL Master clinder leaked brake oil back into the vacuum bboster. The diaphragm got sticky.
So i had teh diaphragm replaced and also replaced the master cylinder.
The problem is that the vacuum booster got crazy.
One time the break is soft and great. The other time, the pedal is hard to push as if the booster is not working at all. When i force it down for 2 cms or something, it goes down a little lighter but I feel it as if it is pushing my leg back.
I do nt know if the diaphragm is put in a right manner. I suspect also the system needs to be bled. I think also may be the actuator rod from the booster going into the master cylinder needs adjustment, especially that the rear wheels lock before the front.
Any help out there.....
Ahmed Bleeding the brakes is absolutly necessary after changing the master cylinder. That could explain the mushy pedal. The hard pedal is probably the result of a loss of vacuum in the booster due to a leak or improper installation. Get that all figured out before you investigate why the rear wheels are locking up prematurely, that could be the result of not having bled the system and may be fixed when you do bleed.