Power Brake Question

[Ron61]

I have a Corvette power brake booster I think it is on my car and have had since I built it in 1996. My question is this. Lately after I drive the car and use the brakes a few times I have noticed that the harder I push the brake pedal the more it drags the motor down until it will actually kill the engine. I have great brakes, they will lock all four wheels if I try, the pedal has no mushiness or spongy feel, I can push it down and hold it and it will stay at that point all day, so there is no bleed down. I never have to add any brake fluid, no leaks, no vacuum noises around any of the hose from the plate under the carb to the unit, and it won't do this at all when first started or when just sitting at a stop sign even after being ran for a while. I have also tried pumping the brake pedal a couple of times and holding it there and it just stays. Any ideas ??

Ron