Idle surge when applying power brakes

[fatbass]

I was noticing that when I push on the brake pedal when idling in neutral, the RPM increases about 2-300 rpm. This was during my weekly "start it and run through the gears until it reaches operating temperature".
I haven't yet cleaned off the snow to have a look underhood but I was wondering if this is common or if someone knows what the likely fix is. I'm guessing vacuum leak. Truck is a '71 C20 with power brakes.