I have a 92 Sport with 115k on it.
After I bleed the brakes I get about 500 miles before I start to lose pedal. By 750-1000 my pedal is low enough that I need to rebleed, after which I have firm pedal and it starts again.
There is a new master cylinder, calipers and rear cylinders w/in the past 6 months. I am not losing fluid and can't really detect a lot of air in the system when I bleed.
I don't want to throw any more real money into a truck this old if it won't solve the problem, so any help in tracking it down would be great!