No brake pressure in standard brakes?

When I stomp the brakes on my 56 Willys Wagon, there seems to be no resistance(pressure). However, when I pump the brakes pressure builds and it stops the truck promptly. I have bled the brakes several times. I have been told there may be air in the master cylinder, but I have bled the brakes several times already so wouldn't that air be out? Would it be the master cylinder? Anyone know!?