please help!
 

99 blazer 2wd . when i hit the brakes it shuts off. what can this be?!

I'd say an ignition/security issue - almost for sure! Do you have a remote starter installed?
With our vehicles if it's started with the remote & you get in & don't put the key in & step on the brake to put it in gear the vehicle will shut off.
Maybe the valet switch has been switched?

Search around in here for Security & Ignition I'd swear that's what's causing it.

hmm i wonder if the ignition switch would cause symptoms like this ?

have you seen any other symptoms like no dash lights or radio ?

never had electric start on it. just remote locks. sometimes when i first start it it will start then immediately shut off. after about the third try, it will stay running. but when it does shut off, i lose all radio pre-sets. so its losing all power i guess. battery connections are good and no lose wires. could the brake booster maybe be causing this? im just curious

sounds electrical


check your grounds
get the battery load tested also


read this

Anatomy of the Ignition Switch - Blazer Forum - Chevy Blazer Forums

diagnose the ignition switch its worth a shot

The security system is mucking up. Some sort of electrical issue with the security system. It's intermittent so possibly a ground or a short?