Have a 50 mercury with a 350 small block... manual steering/brakes. Seems when I come to a light press the pedal somewhat firm...rpms drop to idle..car starts rough idling. Pop to neutral...rpms go up a bit and all is fine. pop in drive and with foot off brake all is ok. Only seems when I am close to or holding my foot on the brake at a light in drive.

Is this a vacuum issue since I am using the pedal to stop the car? Any help is appreciated.

thanks kevin