Depends on the problem. A shop manual is a good start. If it's a check ball problem, you can get a new ball and VERY CAREFULLY polish the ball seat to remove the build up. Or it may be a piece of junk is caught under the ball.
If the seat is badly pitted you may need to replace the pump. If you polish the seat, you can start the motor to get the oil out of the pump, but you will need to change it right away. Polishing grit will cause wear in the motor. You can use very fine sandpapaer, but be sure to change the oil.
Or the spring for the ball is too weak to maintain pressure, letting oil bypass when the motor is off.
You can test to see if it's the check ball by putting a known quanity of oil in the tank, now that you have blown the oil out of the cases, and wait to see if the drains into the cases. I would make sure the oil is on the dipstick so it's easy to tell if the level has gone down.