3-4 inches water left in dishwasher after running

Removed 4 screws in washer then removed cover. Then piston and nut assembly which just needs turned loose.Pull out and then,put new one back in,tighten up, put cover back on put 4 screws back in. Then unhooked water line, power cord, drain hose. Pull washer out and drained water out of system with drain hose. put a towel down to catch excess water. Loosen nut at end of drain hose with a pliers, remove flapper put new one back in tighten drain hose . Put washer back in hook water, power and drain hose. Run thourgh cycle and it should work much better.