Did you follow the Shop Manual sequence for bleeding the Hydrovac and the brake lines? The sequence is different for Hydrovac trucks' brakes.
My 54 GMC 2-ton's HydroVac brakes would pump up but loose pedal (before/after a full rebuild/bleed of the brake system). I replaced the hydrovac and they then worked fine. I bled the lines following the Shop Manual sequence - and, I did three times/rotations of bleeding. Nice brakes, when they are working right.
