Lear aircon/pressurisation
Don't know anything about the Lear, but a bit more info would help. You said that when you "start up" the cabin is "climbing at 750'/minhen you put the pack on then falls down slowly" and ask "Is this a normal situation on the ground"?
Would need to know more info, like does "start up" mean starting a climb? Can you set, manually, a desired cabin altitude for cruise or is it all auto?