Sounds like the diaphram in the booster is shot, so it leaks when you step on the brake. The booster is supposed to be evacualted, and when you step on the brake, it is progressively vented. This applies pressure on the brake master cylinder.

If it hisses when you apply the brake, it's busted -- this happened on my old Dodge Aries in Canda in bitter cold weather -- engine would die when I stepped on the brake (manny tranny). Sure enough, when I listened carefully, it was hissing when brakes were applied. New booster fixed it.

The rise in idle is a givaway, too -- it means you have more air going down the manifold, in this case from the booster and not the idle circuit.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!