Rear brakes lock up before fronts

[LS short box]

If I read the original post correctly you installed front calipers on the rear? Typically the front calipers are quite a bit bigger than the front. Front calipers have larger pistons for more braking pressure. I think that's your problem. The front calipers on the rear are applying way too much pressure. You can try an adjustable prop valve but I don't think will work.