OK! I've decided to return to my 1982 R100RT project bike that had suffered the $2000 dollar O-ring problem and I'm making good progress. On checking the rear caliper reservoir, I found it empty. Added fluid and pumped but got nothing was flowing. I removed the master cylinder and found the plunger frozen in the compressed position. A sharp rap dislodged it and the plunger spring returned. I figured I'd better check it out for pitting and found some rust on the piston with pretty worn down gaskets, but still a pretty sooth bore surface after my cleaning and sanding by spinning a gray scotch bright pad on the interior. So I've purchased a new rebuild kit from euromotoelectrics which consists of a new plunger and an external end seal that will need to be pressed into the master Cylinder's end. So my question is how should I accomplish this given one chance to do it right and no second chances! The kit furnishes a slightly over sized washer and a rubber gasket that precedes it in the assembly process and compresses against the plunger. I could use a long socket to push it in with a vise or a hammer, but the lip on sockets are kind of narrow and rounded, but I'm wondering if I should be using a special mandrel the keeps the washer from deforming or that deforms it in a specific way. I'm also wondering if such a tool exist through the club and is able to be loaned or rented. By the way, I reset the old piston and seal with a hammer on a socket to see if it could be reused. I turned my back to get something and heard a twang as the plungers spring launched the seal and plunger out of the cylinder..... I never found it. Are you surprised?