New seals on cracked pistons are no good...the cracks will tear up the new seals. A cracked piston will sometimes leak behind the head, other times will leak water into the oil. Check your oil...if it's milky, there's water in it.
When you pull the head back off to replace the pistons, check the front of the crankcase where the seal retainers sit. If the aluminum is beat up around the hole, the seal retainer will be allowed to move back and forth with the piston, and the o ring will leak. The seal retainer will also look "beat up". In this case, you're better off buying a new pump, or finding a good used crankcase. The other options are to replace the crankcase with a new one or strip it out and have it milled at a machine shop to make the front flat again. If you take the rear of the pump apart, make sure all the rods go back in exactly the same position. After the parts wear in together, if they're relocated or flipped over, chances are likely your pump will lock up. Also, get new oil seals...might as well put them in now as tear it back down to fix an oil leak in the future.
When you replace the pistons, always use new hardware (washers, o ring, backup ring). The copper washers serve 2 purposes: they are a seal against water getting in the piston, and are a "crush surface" to absorb any imperfections in the mating surfaces. I have cracked new pistons early in my career by trying to use old washers. They only crush once, after that there's no give to make up for imperfections.
Feel free to give me a call if I can be of assistance.
