@Op:
There various things that could cause this problem. The brake-booster could be it.

To solve the problem, there are a few extra questions:



- On the highway, in highest gear: push the clutch down (an nothin else): How does it return to idle?
It should do something like this: fall down in revs from aprox 4000 rpm orso to 2000rpm quickly, then slowly return to idle speed of about 1000 (or whatever).

If it falls down from 4000 to 500 or even 'dead': Check timing, vacuum-advance system (mushroomshaped thing on the dizzy) etc etc.

Same test: if it idle's ok, going down from 4000 to about 1000 rpm, does it stall directly if you only 'touch' the brake a little (very very slight decelleration), or does it stall only when you brake (and de-cellerate from say 80mph to 40 in 5 secs).

If it only stalls on de-celleration: check your carbs ' brake-air-system-jet ' : the petrol in the swimmer-chamber is pushed forward (braking-G-force), and thus the idle-jet isn't supplied with enough pertol, the engine doesn't get it's proper mixture, and stalls.
--> If the brake-air-system-jet is functioning, It could be the carb flange.. (if it looks ok, it could still be ' leaky '
==> Flange test: let the engine idle, and push the carb to tillt it a little (forward, reverse, and to the sides: if the idle speed changes: vacuum leak

If if dies directly after touching the brake pedal.. it'll probably be the brake-boosters-check-valve..
although, it shouldn't be a problem: The check-valve that should make it a one-way system. If this valve fails, there will be a vacume loss when the brake is pressed,.. true. BUT: I drive a 1991 car with faulty check-valve, and it doesn't give me any problems what so ever... (only thing is: all brake-boost is lost directly when the engine is stopped. (doesn't stall on braking)).

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