These boosters rust up or stick when setting around. I drove my 54 for almost 5yrs without the unit working. Let it set over winter, and next spring it started to work by itself again.
Rebuilds are expensive, and if a Midland, you have to send out, they won't sell you parts. Yes, you can drive w/out boost, just brakes harder, and have to start slowing earlier, than if you had working unit.
