bad idle if you touch break
 

on a 1994 silverado if you depress break pedal, the motor will almost die. Have checked for vacuum leaks but found none. any suggestions?

Its a vacuum leak for sure.................but it could be hidden by A. a bad brake booster i.e. if the diaphram in the booster is bad) seen it several times.....B. intake manifold (check with carb cleaner) also check the vacuum "tap" for the brake booster(where it comes out of the intake)...if it is cracked it will cause major problems like that........have you "given" it a vac leak to see if the idle changes? this will tell you if you have an active vac leak that is made worse by the brake booster.........................these are areas I would start with if I had this problem

When I pull a vacuum line off the Idle increases, sometimes a lot.

ok then I would say you have a bad brake booster then

I agree it is a leak internally in the booster allowing too much of a vacuum leak when the brake is depressed.

Yeah when press the brake pedal you might hear the normal air noise but louder. An indicator the booster is bad.