True. Also if a brake booster takes a shit or just that check valve that connects the vaccum line that comes off the booster it could cause them to lock. Is it all 4 brakes or just the rears? And have you replaced the little button on your brake pedal that cuts the brake lights on and off? If you did that and it is too big it could be pressing the pedal down a little bit the whole time and that would explain why it takes some to lock up.