start by getting a good pin punch that will only just go through the hose entry, then boil the whole thing in water for about five minutes, withdraw from the water using protective gloves etc, get to your pinch vice ASAP, prepare to catch the bits, and give the guts a good wack, its a crush washer (one time only use) which takes a wallop to shift, you are (kind of) turning that washer inside out to get the guts out, then clean the body up with brake cleaner and soap and water, be sure that the tiny holes visible in the reservoir are clear and you have blown through with airline or can of aerosol carb stuff et,when replacing with the new stuff, be very careful to get another good fitting drift, that only just goes into the cylinder (a long type extended socket usually works) and dont knock it in too far, use the lever (or quadrant) as your gauge to when its in far enough, clean and grease the pivot pin in the lever or quadrant, and a tiny smear on the piston face.
CLEM