Loosen the steel line at the master cylinder outlet a tiny bit, like 1/8 turn or less. Top off the reservoir, and give the brake pedal 3 or 4 quick pumps. Follow the pedal all the way to the floor, and prop a piece of broomstick or something similar between the seat and the brake pedal to hold it down. Roll under the truck and tighten the line fitting. This works as well as a bench bleed if the master cylinder is in pretty good shape. Now go find the leak- - - - -master cylinders don't run dry unless the fluid is going somewhere- - - -usually at a leaky wheel cylinder or a rust pinhole in a steel line.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!