brake pedal and low idle
 

Hi all, I've searched about this but not one of them seems to give a definative answer so here goes.. I idle fine in hot and cold, but once hot, if I press on the brake pedal the idle drops and wants to die. if I come off the brakes it recovers in about 5 seconds or so. Ive checked the check valve in the hose from the booster to the uim, and seems fine, cant blow the wrong way. I have noticed, that the dash illumination changes ever so slightly when I go on the brakes, could it be electrical?
what other electrical issue would cause that ? I have a massive grounding kit installed so I dont think its a grounding issue, could a weak battery it self cause something like this ?
normal driving is fine as well.

thanks

Torn diaphragm inside the booster?

I would def. look at the booster I had one fail on a blazer once and it wen from normal brakes to manual brakes in no time flat it was not a good feeling whizzing past your driveway knowing the brakes ain't working for s*it on a crooked country road.