The spring and ball is not very special, a 1/4" ball and the spring is about 7mm in diameter with 11 coils and about 24mm long. My spring that I got here is not very firm, about 1,4 kg of pressure when the spring is out and compressed fully and the force pressing down the ball mounted in the seat is 0,4-0,5kg. But it's not that important what the spring looks like as long as you find a spring that fits the hole and gives about the same seat pressure. That seat pressure should not make it too hard to press down the pump plunger when filled with oil.
I've been thinking about why the oil petcock in needed, that might be because of the pumping vaacum from the crankcase draws oil through the ball valve and the factory might have had trouble to construct a automatic valve that can be tight yet let oil pass easy enough with the hand pump?
