[SCOTI]

Originally Posted by 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.

^^This^^

Mongocanfly might be on to something w/the line sizes but your master cyl is equal size (not a stock replacement) so the line fittings connect where they fit. I could see an issue if you bought everything from separate sources & were trying to piece parts together (lines from source #1; master cyl from source #2; prop valve from source #3) but it seems you're working w/a kit of some sort.

A disc/disc set-up has proportionally smaller piston sizes for the rears on OE set-ups & even most aftermarket kits. If your set-up is utilizing D52 calipers front & rear, you're going to need to adjust the hydraulics to dial things in.