There are two major things you need to check before going any further, fuel pressure and engine manifold vacuum at idle. Fuel pressure should be no higher than 6.5 psi or you're overloading the floats and forcing fuel past them. Check your manifold vacuum at idle and see if it's dropping as you apply the brakes, record that number. Then you need to check your power valve and see what it's rated at, it will have numbers stamped on it. For instance, if you have an 8.5 power valve and your vacuum drops below that when applying the brakes your power valve is opening causing your problem.