Sound's like the master cylinder on mine.
I did go down the route of cleaning it in an ultra sonic bath. Cleaned it up and repainted, but when fitting what turned out to be the wrong seal kit (tip always use genuine parts) I managed to damage the cylinder.
So I gave up and sent it to Past Parts in Bury St Edmund's who did a top job of refurbishing it including fitting a new sight glass. Not cheap but cheaper than a new one , and buying second hand I would feel obliged fit a new set of seals anyway.
They also did the master cylinder on last year's Raffle Kat
Replacing with one from a newer machine will work. The main thing to look for is the bore should be the same.
Either 14mm or 5/8" I think.
I have found that with a refurbished master cylinder, braided hoses, new pads and seals; the braking is certainly as good as can be expected for a 33 year old bike