Originally Posted by Hirnbeiss

I would start by trying to isolate other sources of vacuum leaks. It could be that you have low vacuum, and using the brakes just puts it over the edge and triggers the light. You can disconnect the vac hoses one by one and cover the vac nipple with a rubber cap. Then dirve the car to see if the problem is still there - the one that doesn't give it is a vac leak source.
I would also verify with a MityVac that there is no leak at the booster.
If those don't do it, I would get the mixture checked and adjusted. It could be that it is fairly lean, and using the brake booster is enough to put it over the edge.