This is a new symptom that's started after driving truck for about a year and 8k miles:
If I am going at least 15-20 mph, and I quickly stab and hold the brakes for a very slight braking effort, the pedal will pulsate like ABS while providing the expected slight braking effort. I can clearly hear "air pumping in and out" when this happens - it is clearly audible in the cab and also vibrates everything because it is also a braking force oscillation too.
This flutter can be maintained indefinitely going down a hill if the precise amount of pedal pressure is established - and increase and the flutter stops and the brakes work as expected.
Crazy Engineering Theory: a kind of mechanical-servo instability (oscillation). The culprit could be air in he lines which acts like a spring. Since this only happens when I jab the pedal, the jab would kick the mass of the system off it's equilibrium point and you'd get an oscillation. The PBU assist would then cycle Yes, No, Yes, No... Slow application of pedal usually does not cause this flutter. As far as air goes, the brakes don't pump up which indicates there isn't any air in the system.
Some History may help identify what is happening:
TRUCK SAT: for 10-20 year.
POWER BRAKE UNIT STICKY CHECK VALVE:
The PBU (power brake unit) had a sticky check valve that resulting in the brakes not be released by the PBU - you pushed the pedal and when you released the pedal, it remained at its depressed position. Then with some kicking, pumping, etc. I would hear a "click" and the pedal would return.
At first, I thought something was hanging up in the linkage. I had the booster out a couple of times and cycled it on the bench with an external vacuum source. It seemed to work just fine.
I don't remember how at some point I figured out it was a check valve (I confirmed the existence of his check valve by studying the PBU operation). So I removed the boot and cover from the rear of the unit and spared some dry teflon garage door lube into the booster. That seemed to make it work OK.
POWER BRAKE UNIT NOT SPONGY:
Every PB I've driven has a spongy feel - like there is a balloon between your foot and the pedal. This unit has a barely detectable sponge at times - it feels like a manual brake pedal. But it definitely has power. Again, bad memory, I seem to recall when I first started driving after fixing up, there was more sponge.
POWER BRAKE HOLDS VACUUM 10-15 sec AFTER ENGINE OFF:
Apply brake and hold with foot
10-15 seconds later, pedal pushes foot back.