I have run across this problem before...
I suspect you have a frozen wheel cylinder piston, when the brake pedal returns, a vacuum is created, allowing air to suck past the
wheel cylinder cup seal....Check all wheel cylinders for freedom of movement....Give it a try.....
I did brakes for a living for years and that is one I never thought of although I firmly believe now that I ran into it on an International 2-1/2 ton flatbed that we could never get the brakes bled good enough. That one was an old farm rig that sat 10-1/2 months out of the year and then was used to haul fruit in the fall.
Way back when, when I started out as a sophomore in high school autoshop the instructor taught us to adjust the brakes tight so that the wheel didn't turn and then back off 11 clicks. That always seemed to work pretty well on the older rigs.
In my own experience both as a working mechanic and a hobbiest at home I have found that with most rigs that have sat for a while it is better to go ahead and rebuild or replace all of the brake cylinders. They are the part of the car that hates inactivity the most and usually are the first thing to give problems on a rig that sits for long periods of time.
