While driving down the road, my brake pedal is "pumping" itself. Without touching the pedal, I can see the pedal sink close to an inch without touching it, it can do this every second or so. The brake lights are also flashing off and on due to this affecting the brake light switch. It does this most of the time, but not all the time.

Am I right to think that there is an internal brake booster leak allowing the engine vacuum to pull the pedal down a bit???
Any other ideas on what could be causing this?

You will need an old priest and a young priest.

what year, what model, brake types (disc, drums, both, etc) anti lock? power? a little more info, most helpful.

73 A-body, stock front disc, rear drum, all freshly rebuilt.
Doesn't seem to have the problem at idle, but cruising down the highway, that pedal is dancing.

Did it have a prob before rebuilt? What was replaced? pinch off the brake vac hose w a pair of vise grip w something on the teeth to protect the hose from the serrations and retry it. Good visual (re) check of rotors/discs (wheel bearings tight, no wobble?)

I'm thinking wrong wheel bearings, rotor or spindles.

I agree, had one start applying the brakes while I was driving. didn't realize it at first. then I realized I was at full throttle and slowing down on the freeway.

internal check valve or something died.

I took RapidRoberts advice and tested it tonight. First off, without it pinched off, I noticed the pedal pumping almost always when I let off the throttle and coasted, which I think is when vacuum is highest. Next I pinched off the hose and drove the same road a couple of times the same way and the pedal never moved.

So, it sure seems to be the booster. thanks for the help guys!