Brake problem

Have you fully checked your brake system ?
(Calipers, hard lines, flexible lines)Have you flushed the brake fluid recently ?

If the problem is really the master cylinder, then it's time to consider one of the replacement units, as the stock brake master cylinder is not currently rebuildable.

If your booster is useable, you can mount one of several masters to an adapter, or you can replace the booster and master as a unit.
I did this, and wrote an article on how, appearing in the next Pantera International.

Chuck

if yure not losing any fluid anywhere, the master cylinder seems the likely culprit. follow chuck's adwice & get ane from your favorite pantera vendor... ;~)