If a small piece of dirt or rust gets into the unit, it may prevent the dump valve inside the EH valve from closing. The dump valve will then leak fluid into the accumulator when the brakes are applied. The misrouted fluid allows the pedal to drop, and the pedal may go all the way to the floor without applying the brakes. No ABS warning light or fault code will be found either because the limited diagnostics on this system can't tell if the dump valve is fully closed or not.
A small piece of rust or dirt? That would never happen in one of these old trucks would it? Oh I forgot, these are the trucks were the brake line rusts in two midway down the driver's side frame.