k9, do you have a standard brake pedal or an air brake Treadle Pedal (looks like a gas pedal)?
My 1956 Chevy LCF has Air Over with a Treadle Pedal. It has two Master Cylinders that are actuated via an air chamber (one serves the front and the other the rear axle). I also have a Trolly valve for the trailer. I've never seen another set up like mine. The set up I have seems like a dangerous system...if I loose air I'm done. If you have the type with a air booster and you loose air you can still stand on the brake pedal to stop the truck because the pedal is connected to the Master Cylinder.
Mike B
