IF I BRAKE HARD AND COME TO A STOP MY CAR DIES. WHAT CAUSES THIS?

Check for vacuum leaks

From the FAQ:

Car Stalls When Brakes Applied: Vacuum Leak or FI Relay. [Query:] I am having a problem stalling under hard braking. I have had it checked for vacuum leaks and they appear OK. It only happens when the car is warm and I apply the brakes hard. [Response 1:] I would look in the direction of the power brake booster. Right after you brake, you fill up the booster with air, which has to be evacuated by the engine. If your idle is somewhat shaky, throttle plate is out of adjustment or you have a lazy idle air motor the idle can drop enough to stall it. The check valve in hose from intake manifold to booster has an arrow on it indicating the direction of installation Take it out of the line and suck on it to see it opens and shuts correctly. [Response 2:] Check the solder joints on the fuel pump relay. Cracked joints will sometimes cause a warm relay to drop out when the engine speed drops and the alternator slows and the system voltage drops.
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

Just thinking about what happens in a hard stop. I can think of three ideas, maybe you can think of more.

1) The manifold vacuum is at its highest, and the power brake booster is working its hardest. There is a check valve in that vacuum line, but I don't know what happens when it fails.

2) Something with some weight to it is moving forward. In a slow stop, it may move but not as far. If it is a wiring part, maybe it hits something momentarily and shorts out something.

3) The throttle plate slams shut and the throttle position switch kicks on the Idle Air Control. If the IAC is gummed up and slow to open up then the engine is starved for air until the IAC is working properly.

Hope this helps.

Good Luck,

Bob

:>)