So I was doing a little mudding in the 74 f100 today, trying to pull out a VERY stuck jeep, when all of a sudden the brake pedal went all the way to the floor. I checked all the lines and the backs of the drums for leaks, and found none. So I'm thinking its the master. Its got manual drum brakes all the way around. Let me know what you guys think. Thanks.

 

A sudden loss of brake pedal is usually due to a burst pipe or possibly a major failure of a wheel cylinder. However you can find no leaks? Is the brake fluid resevoir still full? Have you checked where the actuating rod from the pedal enters the master cylinder for leaking fluid?
Another possibility is a failure of a shoe/part inside one of the brake drums, if this was the case then pumping the pedal may get a brake.
Have a check around and post back.

 

resivor is still full, no leaks anywhere, even master, haven't checked inside drums yet, still have to have the truck towed back to the house, which will be tomorrow.

 

Master cylinder is toast