1974 b200 brake problem

having a problem with my 1974 dodge b200 brakes. when i have my foot on the break pedal while sitting at a light or stopped the pedal will slowly go down. the brakes work fine when stopping otherwise.
any thoughts?

Either you have a leaking wheel cylinder or the internal seal for the master cylinder is worn out. If it's a wheel cylinder you should be able to look under the van at each wheel and see if there's a wetness from the brake fluid on the backing plate. If all wheels are dry then it probably time for a new master cylinder.