I suggest that you take the truck to a reputable brake repair shop to get to the bottom of your troubles. A '59 is modern enough that they will have no problem at all at getting you back on the road and safe.
I like to do my own repairs as much as possible, but when we are talking about brakes, a guy needs to have those 100% right.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission