Originally Posted by Gord&Fran
Truck has power brakes with booster and dual master cyl under the floor.
My conclusion is that master cyl has to be replaced. Reasonable conclusion or did I miss something?

Do you have residual pressure valves in the ports of the master cylinder? If you're running discs up front, they need a 2# valve, and drum brakes need a 10# residual pressure. If the master cylinder is below the level of the brakes, the residual valves keep enough pressure in the cylinders that air can't creep in and put a bubble in the lines. Even a tiny air bubble will cause the symptoms you're having.
Jerry

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