[midniteblues]

heres my 2 cents
i think with that much air in the lines would've been noticed right away not 4 months 3k mi. later.
air bubbles allways travel up unless forced down. 3k mi. bounceing down the road any air would've made it up too the mc and the rubber in the cap would displace the escaped air in the system.

i would fill the master and bleed the entire system and be positive there is no leaks ANYWHERE!(if its non power be sure too check in the cab behind the pedal).

if there is no leaks...
i would replace the master and be absolutely sure the brake push rod is adjusted too the proper length. it could be a lil' long and pushing the piston a lil' too far over the past few months eventually blowing or hanging up the last in line seal on the piston hence the reason fluid is presureizeing the resivior instead of the lines.

also be sure the valve on the prop valve is centered

again just my 2 cents