Proportioning Valve Problems

Some may have a residual pressure valve in the master cylinder. I've never witness a failure in those but anything is possible.
Frozen park brake cables are much more common.
The strangest fault I've ever seen was severe steps worn into the touch pads in backing plates. Brakes worked fine until the linings wore down enough to allow the shoe to drop behind that step.
In fact I've heard tales of flex fluid lines plugging but first time I witnessed this was about 2 weeks ago. 99 Town and Country, right front hose. I had automatically assumed frozen caliper. When I broke the banjo bolt loose at the caliper I automatically had a new diagnosis ( squirt ) LOL. For the sake of knowing I had my daughter get inside and try to push fluid out that disconnected hose. I got what resembled tears for fluid flow. Disconnected its other end from the hard line and the reservoir drained.
I added that to suggest the diagnosis procedure. Try to turn the drums by hand, break the fluid circuit, try to turn the drums again. Good luck