New master cylinder from the dealer and we're back in business. That was the problem all along.

 

Glad to hear

 

I think the problem had been there for a while but didn't come to a crashing halt until last week. My truck has zero rust on it, and I had been noticing that the bottom half of the power brake booster had been developing surface rust for the past couple years. I always assumed it was from salt spray-up from underneath the vehicle. Now I realize that brake fluid had been slowly leaking from the back of the master cylinder and coating the lower half of the brake booster, where the two bolt up to each other. Stripped the paint off, as brake fluid does, and thus began the rust.