'Grabby' brakes

I had this same issue once, stumped me for a long time.
Maybe you have the same? I checked all the usual's as mentioned by others, I sure thought it was the booster....
My trouble was, when cutting the rotors, they were cut beyond the allowable limit just barley. and I had not replaced the rear shoes. So when applying brakes, the master cylinder plunger traveled so far (cut on the rotors) that when the rears came on (with old and possibly worn) shoes/ drums they locked up. If you haven't already, check the specs on the rotors and the drums, install new shoes.... bet your ok after that. ...worth a look at least :)