generally if you can pump the pedal and it comes up there is air in the system somewhere.
Does this clark have the vertical mount mastercylinder in it? If so then its going to be fun bleeding all the air out of it, pressure bleeding it may be helpful. If its the usual horizontal mount and is just on a tilted angle it can be bench bled or even use a pressure bleeder on it as well. Sometimes you can bleed it out while installed on the truck through the lines @ the mastercylinder itself but if its that vertical mounted one even that will not work well.

I've had many systems that are horrible when it comes to bleeding out sometimes. They get an air pocket in them that is very difficult to get out.

I doubt it is the booster, its only purpose is to power assist the brake system and shouldnt cause a "pump up" symptom.