I originally had a stock 15/16 XJ MC in mine, with stock '88 front discs and Cadillac calipers in the rear. Once properly bled (the Caddy calipers can be a bitch!), my only problem was a pedal lower than I preferred. Too small of MC WIL NOT result in a pedal going to the floor, unless the calipers are HUGE compared to stock. Low pedal, yes, but firm, if properly bled.
I latter upgraded my MC to a 1" bore. Then I upgraded my front calipers to '79Chevy, which have a much larger volume requirement than the OEM XJ's. My 1" MC--which I believe is the same size as the ZJ MC, worked fine, except, again, due to need for more volume of the new front calipers, resulted in a pedal that was too low for me--but "firm". No way did the 1" MC result in a pedal that "slowly" leaked to the floor. The MC piston size will not cause that to happen.
Being able to "pump up" the pressure is a sign of air in the system. The only way the pedal will settle while under constant pressure, is a leak--external, or internal.