if it's leaking around the cap at the top,it's usually because of a rough casting. At the shop where I work,we always flat sand the master cylinder upside down-we place a piece of tan 80 grit 3m sandpaper rough side up on our flat bench,then with NO fluid in the master, sand it until it's smooth. It's amazing how rough the current crop of cast iron masters are on top. Once it's sanded flat , we carefully wipe any residuals off it while it's still open side down and then flip it over,bench bleed it like normal, put it on the car and re-bleed all the brakes. Note that we only do this with problem masters or new master cylinders. if it's leaking at the back,there is a problem with the seals and needs replacement. You can get a spongy brake pedal if you would lose all your brake fluid during normal operation and induce air into the system-i.e. if the top casting is rough.and each time you hit the brake the brake fluid squirts at the inside of the cap from the compensating ports and it slowly leaks out you could get a problem once all the fluid is out of that chamber. Since you have manual brakes, look inside the car by the brake pedal and see if it's wet by the pushrod=if it's wet there,the back seal is bad and time for new. Hope this helps
