Use spray brake cleaner and a Q-tip or brush to clean the area. You'll need brake cleaner to clean the disassembled unit anyway. Once clean, you should see the end of the piston (#3 in the diagram) sticking through the washer (#7) with the circlip (#25) holding the washer in. The spring clip (#5) that held in the rubber boot thingy might still be in there, it's easy to pull out with a small screwdriver or a dental pick.
You need an internal circlip tool with very very small pins and a long reach, not a standard combination unfortunately. Sears has an angled model that you can file the pins down (to fit the tiny holes) and use.
Once apart, clean the housing thoroughly with a bottle brush and soapy wate and dry. Clean out the bypass hole with a strand of copper wire and then spray inside the cylinder liberally with brake cleaner and let it dry without wiping (so no lint from your wiper gets in). Lay out the new parts in order, then spray them individually and leave to air dry. Put some newspaper under a clean shop towel and get a clean bowl. Pour new brake fluid through the cylinder into the bowl, coating the internal parts. Set it down on the towel, then take each internal part in order and dip it in the fluid in the bowl then insert into the cylinder.
Once the piston and washer are in, you get to insert the new circlip. This is easier than getting the old one out but still a feat. If you get it set square in the outer bore you can slide it down into the groove if you're very careful with a screwdriver or easily if you have some tubing that will push the clip and clear the piston. Check it's fully seated before installing the rubber boot and spring clip.
