97 sierra lights make truck die

[devilr3v]

hey guys new on here. I have this 97 sierra 4.3 and i just fixed it up after sitting for 2 years. After i started her up and pushed on the brakes, the truck just drops down to 200rpm then revs up to 3000. Also when I turn on the nightime lamps it just dies. I check the alternator seems fine, change the tail light boards, put new bulbs in, fuses are good and relay is also fine. Any ideas?

Thanks in advance

 

[98trck]

brake peddle problem is a vacuum leak in the brake booster ( booster has gone bad)
with the engine running have somebody step on the brake peddle while you listen for hissing coming for the booster

 

[devilr3v]

not a vacuum leak. it's electrical somewhere. since when you unplug the tail lights and push the brake pedal it is fine.