My car bucks sometimes when I step on the brakes?

- I was driving this week and on two separate occasion the car would "buck" hard from a high and slow speeds when I applied the brake. Also at a traffic light while the car was completely stopped after a minute I would feel the car buck again slightly. This will continue to happen until I shut off the car and then it will work fine again until a couple of days have past. The check engine light came on this week before all this happen, but is not on and hasn't come on since. I brought it to a transmission shop today, but he didn't get any codes with his scanner. It showed that I had disconnected the battery and that he said a few weeks would have to past to get a true reading. He did say that if the Solenoid Pack was bad it would show up as a check engine light or with his scanner. What is the possible problems-thanks Anthony