Have you looked at your master cylinder to identify the problem? Is it a corroded bore that allows fluid to leak back past the piston seals? Or are the seals bad?
On my master cylinder the corrosion was in the form of a shallow groove running for part of the length of the cylinder at the lowest point of the bore. You need to take the cylinder apart and look inside with a good light and a magnifier. Took me about a half hour before I could get everything so I could see the damage in mine, but then, I don't have such good eyes anymore.
If it is the bore you can send it out to be sleeved. I had mine done for something like $100 which is a lot less than a new cylinder. Can't think of the place that did it off the top of my head, but they were someplace out in Cali. Found them on the internet. If the corrosion isn't too bad I have heard of guys cleaning up the bore with a bit of wet or dry wrapped around a dowel. Don't go too coarse or the roughness will eat the seals on the piston.
Another option is eBay. There seems to always be a master cylinder or two on there. Try to get the seller to guarantee that it works so you aren't getting stuck with another bad one.
