why do my front brakes keep locking up???

ABS thinks your stopping hard... must be movement in front end enough, it thinks your skidding. Look at hub bearings for play...the sensor is stationary above the hubs... if the hub moves to much, SLAM!!!! also, you proportioning valve maybe bad or unplugged... is the brake light on? that is wear all the lines come together from master cylinder, before heading off each wheel. also, to center proportioning valve... you need to gravity bleed all 4 wheels... open all 4 bleeders... close one at a time.. starting with the ones that leak the best. have someone keep the master cylinder full. Do not press pedal!! this is a gravity thing. they should leak out after a few minutes.