My 1999 Chevy 3500 7.4 2WD dually has brake issues. For starters, the pedal won’t pump up. It still will go nearly all the way to the floor when I depress the pedal. Also while depressing the pedal and turning the engine off, the pedal will go to the floor. This truck has the hydro booster not the vacuum booster. Also the pedal feels spongy. Have noticed the brake fluid in the reservoir has been getting lower but not seeing any leaks?

 

There is a good chance the Master cyl needs to be replaced Leaking internally.Will not hold pressure.
The fluid has to be going somewhere.. Sometimes it leaks between where the master is mounted and the firewall, . If the vehicle has seen a lot of salt from winter driving , you might look at the brake lines that go under the vehicle especially where there are clamps to anchor the lines to the body.or chassis.
Another clue is to look for any dampness at the backing plates, which could indicate a leaking wheel cyl if the vehicle has drum brakes on the rear.