1.) Disassemble the pump/motor assembly located on the top of the unit: a.) Remove outermost plastic shroud. You will need a nut driver with an extension to get to one hex screw - a good Xcelite set will do. b.) Remove inner plastic shroud with filter sponge. If filter sponge is clogged, replace it. c.) With plastic shrouds removed, begin disassembling the metal pump components. Start by removing the four screws around the dome cover. d.) Remove last two screws holding valve plate to piston housing case and remove. e.) Remove the piston cylinder from the piston housing by lifting up. f.) Loosen the clamp screw holding the piston rod to the bearing. Pull the piston away from the bearing - you might have to wiggle or pry on it a bit to slip it off. If you have to pry it off, pry only the piston rod itself - take care not to damage the bearing.
2.) Inspect gaskets for wear or damage - replace as necessary. For Piston and Cylinder assembly kit purchased from Parts Select, a gasket mated between the cylinder and valve plate will be included.
3.) Inspect valve plate for bent valve reeds or for debris caught between the valve reeds and the plate. Remove any material found and replace valve reeds as necessary.
4.) Reassemble by following instructions from step 1 in reverse. Mind the torque specifications - images are available online for these torque specifications.