06 Gt rear brakes hanging up

My sons 06 gt is having an issue with the rear brakes intermittently hanging up ,thought it was rear calipers so we put on new calipers and brackets with slides , will be fine for a few days then they will start to hang up and have to crack the bleeders to get them freed up and go again , Any ideas ???

Does he use the e brake? It could be the proportioning valve or even the cup seals in the master cylinder. The master cylinder is a known fault in these cars.

Car is an auto so doesn't use the e brake , Checked that also and cables release all the way , Car was hit in front and we fixed it , could the abs be bad ? seems to happen gradually after a day or so , Don't want to put new rotors on it yet till we get it figured out

Its possible. Are the lines at the abs pump tight?

Yes , Everything tight with no leaks ..

Its got to be the proportioning valve. I dont see both the rear lines collapsing and trapping fluid.

Will check it out , !!Thanks

There is no proportioning valve.

Maybe my terminology isnt right. Isnt there something that distributes brake fluid front to rear?