Find the master cylinder and top off the fluid and bleed the brakes. Once you get the brakes bled have someone pump the brake peddle and crawl around under there tracing the lines looking for leaks.
I have a 2 1/2 ton chevy truck that does the same thing if I leave it sit for months at a time. One wheel cylinder leaks a little bit. That would be my guess your problem is. The ones I have replaced on my pickup were pretty cheap and no to bad to install.
93 Jayco RK 325
07 Chevy classic crew cab long box LBZ Duramax
Farmer, Trucker, Equipment operator, Mechanic