While it is possible it is a bad master cylinder it can also be a bad hose. If they are original they should also (all four) be replaced too. If the brake fluid is dark and nasty looking figure on rebuilding all 4 calipers too. Most brake problems can be avoided by flushing and replacing the brake fluid every other year. If you have a 5-speed do the clutch at the same time. Again, if that fluid is also dark and nasty figure on doing the master and slave cylinders. While doing that, if you still have the plastic hose, time to replace it with the S/S braided one.