Hey.
I have the oilpump out of the car, and i am now trying to get the pressure releaf valve out, but i can't. What is the best method of getting it out?

There are 2 ways to get the piston out if it is stuck either because of it being crudded up, rusted in by neglect, or the bore is scored and pitted....

Some pistons have a couple of slots cut in the sides and you can use a wire hook, or dental hook to coax it out (be careful not to scratch the bore).

If really deft, you can use a set of needle nose pliers to probe into the cup of the piston where the spring resides and open the nose up and wiggle it out....

If all else fails, or you have some time and tools on your hands... you can pull the rotor cover plate, drop the ring out (inspect for scoring) and try to reach it that way with an awl or probe.... if you need more room... heat the pump gear with a torch (propane or Mapp gas fine, use leather gloves), it will expand and you can pull the gear off push the rotor out the other side and now you can get to the hole better.... assemble in reverse, and make sure to put a new O ring in the groove before putting the plate back on. Follow FSM for cover bolt torque spec.