Brakes work but motor doesn't

I have run across a new problem with my Jimmy. I could really use a hand with this as I have never seen this before. When ever I hit my brakes and my truck starts to slow my Motor dies. No noises no warnings no nothing this just started this morning and really sucks with all my city driving that i do. I have a 1992 Jimmy 4X4 4.6 Vortec

Check for vac leaks at the booster, check the hose from the booster to the intake. Check the rest of your vac lines. If all looks good, pull your upper plenum and check for washing.