Either a bad master cylinder or leaky wheel cylinder.

There is a junction block up front that has the rear and front brake lines hooked to it a lot of them have a cut off in case you loose the front or rear brakes this has to be reset by pushing in a button on the junction block.

I have reset the the button on the junction block, now I am having trouble with the rear brakes locking up.

Make sure all air is bled out , or it will sink.

If ur having problems with thht n uve replaced all those parts n bled ur brakes i would say try bleeding ur abs moduel i had tht happen to me on a malibu or u could have a leak some where .but also id take ur master cylinder back off n bench bleed it by hand cuz some times they can be at a angle in the vehicle in wont bleed all the air out . n while its off check ur adjustment on the push rod tht goes to the back side of the master cylinder from the brake booster tht will cause ur brake lock up u said u were having if it out of adjustment