lodihoseman

I should have mentioned, the brake booster uses vacuum to boost the brakes.

I'm not positive on a Chevy, but my Ford diesel has a vacuum pump that runs everything that needs vacuum. A diesel engine doesn't create vacuum on its own as the speed of the engine is regulated by the amount of fuel given to the motor, where a gasoline engine is regulated by the amount of air supplied. There is no restriction to the air supply on a diesel, which doesn't provide any vacuum. On a gas engine you would know immediately if you weren't providing enough vacuum as the engine would not run properly, but a diesel engine won't give you that indication. I would look there for your issue. Vacuum pump, or some of the lines from the vacuum pump.

Brake fluid absorbs water and should be replaced every couple of years. Water is heavier than brake fluid, so the water collects in the wheel cylinder. The water causes the cylinder to pit, and eventually leak.