If you've checked all else and still have no ideas, it could be the master cyl (although not too likely, it can happen that it doesn't pressurize the rear enough) - test it by switching one of the front lines with the rear line.

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k