This is a fun job to do when it's pouring rain. No, nothing is going to come shooting out at you but be forewarned that the inside of your pump is filthy dirty!
I'd suggest plastic gloves, and a vice with wood blocks.
Undo the screws that hold the top of the pump onto the tube, pull out the piston, clamp the shaft in the vice (use wood blocks or rubber so you don't scar it up) and break the locknuts.
Take the replacement leather washer and coat it with grease. Install it cup - side down, replace the jam nuts, and CAREFULLY put it back in the tube - don't crimp it! Then put it all back together.
Also open up the little brass port at the bottom (next to the guage) and clean the check valve. I have not been able to find the check valves as replacement items - they may just be Schrader valve cores - don't know.
Blow out all the moisture and accumulated crud in there.
A properly overhauled Silca floor pump is a pleasure to use! Especially if it's equipped with a Hirame pump head.
Mark Petry
Bainbridge Island, WA