master cylinders usually don't leak. A master cylinder will tend to loose
it's ability to maintain the seal that causes the wheel cylinders to engage
the brakes. The master cylinder is a sealed unit as far as my experience-
i'm not a mechanic but have done my brakes and a master replace cylinder a
couple of times.
are you talking about the unit on the firewall, or do you mean wheel
calipers. or did you open up the master cylinder and see that the fluid
level needs to add more brake fluid (normal) as the brake material wears
away the wheel calipers need to depress outbound further to engage the brake
material, thus the master cylinder needs more fluid.

ok trevor,

I went out to my truck which was giving me spongy peddle, and the master
cylinder was dry. I don't use the truck for more than to the dumps. But last
trip was spongy peddle. So since my brother is visiting tonight I had him
push the peddle and I loosened the brake bleed nut and by adding brake fluid
and bleeding the peddle is back to firm. And I was noticing that my
differential carrier case is wet with some kind of oil or fluid from the
right rear. I suspect that the right rear brake cylinder is leaking and I
will take a look at it.
See you helped me in a way I became suspecious when I moved my truck and
checked the master cylinder reservior.
good luck
greg