this a diesel truck?
see if there is a vacum booster on the master cylinder and also check for a vacum pump on the engine somewhere, usually mounted on the rear of the alternator/generator. check any lines in those systems and the operation of the components themselves.

do the basics first though, make sure the master cyl rod freeplay is properly adjusted, and the wheel adjusters are adjusted up correctly on both drive wheels. once this is correct if problem still exists then check the brake vacum assist for problems.