Strange brake problem

[Snake2998]

Have posted this on another forum as well - lets see who comes up with the answer:

Have picked up a strange one (to me anyway) problem on my Cobra lookalike.

First some relevant background info:

383 Chevy Motor - hot cam.
Jag XJ6 Brake system
Willwood front Calipers
TH400 Automatic Gearbox with standard torque converter.

The problem:

When you brake on a trailing throttle or when idling brakes work well and pedal is normal.

When there is some throttle applied as in left foot braking or revving it up against the torque converter, brake pedal goes very soft and brakes don't work so well.

Question:

Why would brakes go soft when there is very low vacuum? My first thought is that the one way valve in the vacuum line isn't working but why would lack of vacuum cause the pedal to go soft when in theory it should go hard?