I have a craftsman limited edition, ys 4500, 22 hps, 42'' deck, hydro trans, does anyone know how to purge the system, it is losing power and will not go up hills.

Hi,
Appears to be a Hydrogear transaxle in your machine. Assuming it has sufficient oil in it(it uses 10W30) then most manuals say to drive at full speed in tight circles first in one direction and then the other about five times in a row in each direction to drive the air out of the system. I would suspect low oil or another problem like a belt of an idler myself.
Peace,
Clarke