Loss of brakes and van steering shaking

I kept losing my braking ability. It felt like something was blocking when I pushed on the pedal. I was told it was the Booster. this was replaced. I have 4 new tires, whith wheel balancing and alignment. I was told my brakes were fine. My steering wheel shakes badly when I brake. I still lose my brakes at times an then the transaction kicks in and then the stabilazation light comes on. this happens all the time ,even in dry weather.