I am repalacing all lines and wheel cylinders. I removed the master cylinder and it was full of crap. Original brake fluid evidently does last for 38 years. Imagine that. What is a good cleaner to use on the inside of the MC, that will not damage it? Also I tried running fresh brake fluid through it and it just keeps blowing what looks like small particles of rust. The rear reservoir seems to be working ok (except for the rust), but the front acts as if it is partially plugged. I need a better way to flush it out.

Thanks in advance

You should disassemble it and inspect it, or just get a new one. You could flush it with denatured alchohol, but the rust and varnish may be all that's holding it together.

Id probably suggest just getting a new one, unless you're really concerned with originality. I think they only run around $20.00, just had one priced out the other day at O'Reilly's. Maybe it was $30, anyway, not to much cash for a new part.