When I apply the brakes my idle drops too much

[kens65fb]

My 350 SBC (mild cam, turbo 400) idles fine, at about 900 RPM.

When I apply the power brakes in PARK, the idle drops about 200 RPM.

When I shift the automatic trans into, DRIVE, or REVERSE, my idle drops an additional 150 RPM, or so, which I believe is probably normal.

It is at this point, the idle, is down to about 550 RPM, and the idle becomes rough, barely running, but does not stall.

I know that is not electrical, as I have completely removed the ALL wiring from the vehicle.

The only electrical wiring left is the ignition switch, with 3 wires.

Any thoughts