For what it's worth, I submit the following, although from experience your symptoms are text book sticking caliper, (not necessarily a frozen piston). The brake flex line should not be ruled out either, they can delaminate internally and act like a one way valve. ie allow pressure flow to brake but restrict the reverse flow leaving caliper pressurized.
As your inner pad is so much more worn than the outer though, I think something is preventing the caliper from floating freely. I would completely disassemble and clean the rust and dirt off of all the caliper parts and also check caliper pins and bushings. Something is preventing the caliper from moving from side to side.
I can't imagine you having corrosion problems in your environment, I see you are 175 miles from the ocean so I doubt that you have to be concerned about Salt corrosion.
We are in an area where lots of road salt is used 6 months of the year.
Had something similar on 2015 (H.D. brakes), on rear drivers side only. Pads totally rusted solid into caliper, had to remove caliper to hammer pads out. BOTH inner and outer pads worn down to their backing plates.
Found lots of rust under stainless pad clips which just tightened everything up. Ears on pads also rusted in place. Opened bleed nipple and caliper piston retracted (pushed in) fine with rotating tool and almost no force, would have expected to find frozen piston, but not the case.
I cleaned everything up, had to grind rust out of pad carrier, lubricated and reassembled with new hardware and pads. Fortunately there was no sign of rotor warpage.
Disc was a bit rusty from lack of proper pad contact but still flat and smooth across the swept area, so I reused it. I think this is just going to be an on going maintenance problem, will schedule disassembly, clean up and inspection, lubrication etc. every two years from now on.
This seems like excessive maintenance, but I think it is a fact of life with the GEN5 vans. These brakes not too difficult to work on, but in winter - not so much fun.
Total kms 30 000, (under 20,000 miles).
Guide pins like new and still had plenty of original factory lubricant. E Brake still functioning like new, but I use it regularly to keep it free.
I also replaced right rear pads, were about 1/2 worn, but supposed to be replaced in sets. That side just needed some normal cleaning up and re-lubrication.
Kudos on routinely replacing brake fluid, I do the same, I think that overall it helps and is widely considered good practise. Whenever I replace pads, I release the bleeder screw for cylinder push back, then do a good system bleed.
Found a lot of air in right rear cylinder circuit when I bled it, Seems strange, because there was no air in either front L & R or left rear when I flushed fluid there, but this vehicle has always had a soft brake pedal and this is probably why.
Found resistance in left Caliper Piston pushback when bleed nipple got accidentally closed. Sounds like a deteriorated flex line acting like a one way valve.
Final Verdict: 1. LR Brake Pad Wear Excessive:
Caliper piston sticking in place due to restriction in brake flex hose, preventing brake pad release from rotor and lack of movement of pads allowing them to rust in place.
2. Soft Brake Pedal: Air in line to RR Brake.
Edited 2018 04 04&07 Discovered partially blocked brake hose, and replaced.Clarification
Edited by michaelGen5, 11 April 2018 - 09:15 PM.