Truck Repair: pressing brake pedal shuts off engine/shorts out, chevy s10 blazer, s10 blazer

Question
Hi - have a 87 Chevy S10 Blazer 4x4, 2.8l, with some type of electrical problem.  Every time the brake pedal is pressed, the engine not only shuts off, but it drains the battery so that in order to get it started again it has to be jumped.  I've replaced the brake light switch, cruise control switch, had the battery & altenator checked, and have yet to find the problem.  Was wondering if your had any advice or ideas on the subject.  Thanks!

Answer
Hello Duckie,
I am wondering if it really drains the battery, or just has some effect on the wires, like where they all pass through the firewall.
Any chance also, that the vacuum brake booster is leaking and killing the engine ?
Put your foot down on the floorboard, up close to the brake pedal rod, and push real hard on the floor. See if just the pressure moving the floorboard might be causing it, rather that application of the brakes. I am thinking in the area of a missing ground wire on the firewall, but also a wire being pinched between the firewall and engine.

Van