Using a common grease gun I can carefully remove or pressurize the cylinders off the engine. The grease isn't compressible and uncontrollable like air, it won't leak through the piston rings and valve seats like oil. I just unload the valves and turn the stems to rub the leaks out. I use a rod to support the heavy cylinders, keep the side load off the connection rods. 9000 psi will break the rusty rings and corroded aluminum pistons out of the steel barrels of the cylinder. It takes about 8 hours to get one off, its a lot of grease!