Hy DIY.MechanicMike, to me it sounds like you have a brake booster problem. When the booster doesn't function it actually requires more pedal pressure to be applied by the truck operator. Try doing the following test, with the trucks engine not running pump the brake pedal several times, after that put your foot on the brake pedal and apply pressure now start the engine, if the booster is functioning correctly the brake pedal should go closer to the floor when the engine starts. Normally the booster vent doesn't make noise as all it is doing is allowing air under atmospheric to enter one side (apply side) of the diaphragm inside the booster. Good luck.
