Rear brakes lock up before fronts

[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.

[JMD]

I did some digging and found the exact parts I have installed. The master cylinder/booster kit is indeed meant for a front disk, rear drum setup. Even though summit doesn’t mention anywhere that this is the case, it seems to be the general consensus in the product FAQs.
https://www.summitracing.com/parts/sum-760197


The proportioning valve I have is made for a disk/disk setup. I think when I bought these parts 6 years ago my idea was that I could use this proportioning valve to correct the fact that the master cylinder is meant for a disk/drum setup.
https://www.summitracing.com/parts/sum-760185

So maybe switching the two lines going in to the proportioning valve will fix it. Or getting rid of the disk/disk valve and getting an adjustable one.

An adjustable valve only affects the rear brakes, correct? How exactly does that work?