Should you find your master cylinder leaking onto the ground, it may be as simple as replacing the seal at the aft end of the piston.  I had installed a brand new master cylinder about one year ago, and shortly afterwards it developed an annoying leak.  The performance of the brake system was not affected, but the leak was a nuisance.

Removing the master cylinder for repair can be quite the chore on a Willys.  Mine was even worse because of the tight clearances with the PTO driveshaft and relocated exhaust pipe.  One thing led to another, and I got to thinking.  Normally dangerous things happen when I get to thinking (It is just social thinking, I can stop any time I want) but this time it worked out well.  It was not necessary to remove the entire master cylinder for this repair.  

Looking at a cross section view of the master cylinder, I knew that the seal at the aft end of the piston was the likely culprit.  This scanned image, from the Willys service manual, shows the seals.  The master cylinder I had was slightly different, with seals #3 and #13 formed in one piece:

To drain the master cylinder, open a wheel cylinder bleeder screw and pump the fluid into a jar for disposal.  When removing the dust boot (#14) and pushrod (#15 link), expect a lot of fluid to spill that was trapped in the boot.  Even with the weep hole at the bottom of the boot, a lot of fluid will collect there.

A retainer spring (#1, a circular clip) holds the piston (#12) in place.  A 3/8" dowel fit in the hole for the pushrod, and let me depress the piston and relieve pressure for removal of the retainer spring.  I used a 36" long dowel, and the far end wedged in place perfectly against the left rear leaf spring.  If that doesn't work, trim the dowel length so it can be sprung into place against any convenient anchor point on the frame.  With the piston depressed, remove the retainer spring and piston stop (#2, a thick washer).  Now the piston can be removed by pulling out the dowel.  Although the spring tension is not too great, don't let the piston fly out when the dowel is freed.

 

A new seal was robbed from a rebuild kit.  Here is a close-up of the piston and seal:

The dowel is used again to allow the reinstallation of the piston stop and retainer spring.  Finish the rest of the job per the manual, including pushrod adjustment and brake bleeding.  I was back in business in considerably less time than if I'd had to remove the master cylinder.  

You may not find this repair of any use to you, especially if the master cylinder is old and has pits in the bore requiring a full rebuild or replacement.  Since I knew the new cylinder has started leaking shortly after installation, the seal was the most likely source.  Remember, this is not a dual circuit system like found on modern vehicles, so if there is any doubt a full rebuild is your best assurance of safe braking.  This repair is only appropriate for a nuisance leak from the rear seal.  Other master cylinder problems would require removal for a full rebuild.