Brake pedal goes stiff under heavy braking, no ABS engagement?

[CrankyOldMan]

Basically, under some conditions the brake pedal goes rock hard at partial travel. At no point does the ABS kick in. Once I slow down and let completely off the pedal or accelerate again, it returns to normal operation. The freaky part is that it tends to happen when I'm pointed at a danger area, like a curb or light pole pylon. Has anyone else has experienced this, even with another vehicle?

Here's my thoughts on it. Not that they are correct, but it's the conclusions I've come to. For background: I have a Blitz S/C, custom intake runner w/ cone filter, 1ZZ TB, stock rotors and generic pads.

1. Vacuum lock condition on the brake booster. The brake vacuum line that goes just behind the throttle body might not be a vacuum rated hose, and/or the bend radius is just enough to pinch off under vacuum during rapid throttle release.

2. Too much wheel lift on rough terrain for the ABS controller to deal with it. It tends to happen when going over a section of the course with wavy or uneven pavement and/or loose gravel. Seems like the ABS is designed to deal with this, no?

3. Ground slopes away from the direction of travel. The other common condition is that the braking point is on or near the crest of a slight rise/fall. I'm not talking about the deliberate "jump" at Grattan Raceway, just the normal crown/drainage grading of a parking lot.

Anyone have any idea?