There must be a leak somewhere in the system. I have let cars sit for a couple of years and no problems so something is bad.
Check the master for leaks at the firewall inside the cab.
Check the fronts and the rears by looking for any drips on the bottom of the drums or sweating on the backing plates.
Its a closed system when fluid goes out even a t5iny amount air gets in.
89 2.5 TBI, 4" RC lift, 1" Booms, SYE, Adams Driveshafts front and rear, Ford 8.8 with 4:88s, BFG 33X12.50X15, and a ton of other stuff.
AF Jeep Club Tail Number 184