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My Cortina Mk3 2000 will idle at about 800 rpm, choke off, fully warmed up, and it will be real lumpy. If I hit the brake, the idle will smooth out and the rpm will rise to about 1200 rpm. It does this when cold too with the choke off. I know the booster is functioning. Could it be a vacuum leak between the booster and the manifold? The hose is 40 years old...

Definitely sounds like a vacuum leak but not the hose. That should leak all the time, not just with the brakes applied. I would really suspect the diaphram.

It sounds like an over-rich mixture; pressing the brake pedal draws air from the servo to weaken it. The problem might be in the carburettor.