rpms drop while holding the brake

at the title states , recently ive been noticing then when im stopped at the light and hit my brakes my rpm dips down from 750 to about 500 and then after a few seconds back upto normal.

what could be the cause for this flucktuation?

i have a 91 5speed SE if that helps.

Vacuum leak.

Check all vacuum hose connections.

Download the FSM if you have to.