While troubleshooting my general brake issues, I found numerous gaps in my own knowledge. So here is my current understanding of the general E30 brake system (within reason). To start, late model E30s (not sure about before) have a 17mm rear and 22mm front sized Girling master cylinder. I’m surprised by the 17mm rear. To my knowledge, the ABS unit provides nothing other than ABS functionality. I think late model E36s and all E46s use the ABS unit to proportion as well as perform ABS functions.
E30s have 1 proportioning valve behind the driver’s shock tower on the rear line. I don’t know what its specifications are or why it’s there. Here’s the part number to delete it. (M10x1.0) though I havent tested this. But it’s there. My googling shows it as a common issue with the SpecE30 crowd. The ABS unit itself receives both inputs from the master and splits only the front lines out (see diagram below).
The rear line follows along the fuel lines to the back of the car where it wraps above the subframe and is T-ed out above the diff/subframe combo (T part number 34341111435).
The front passenger’s line run against the firewall under the heater core flap. The driver’s side simply exits on the shock tower and in front of the proportioning valve for the rear. The above diagram properly notes which port of the master cylinder goes front or rear (V front, H rear). I did not fully note down the ABS unit ports. Lower case “h” is for the rear but I don’t recall what “I” or “J” goes to (it’s left or right).