the vacuum pipe off the brake servo hose controlls the boost solenoid, check this with a mitty vac when started it should fly round to max, also check the boost hoses for splits, quick test is to rev hard and listen for a rush of air as if split hose.we just replaced a vac pump because of no boost even though brakes were ok this was the problem.
also these bost valves are common for damage,ie snaped pipes.