Been doing the pipe and chain my whole life, I even went back a few years ago and cut teeth in the pipe end to make the bite even better on the slick “gum” tree’s and shrubs
if I remember correctly a 870 is a dual clutch setup, so the PTO is a separate drive disk. Cranking and driving it around while applying the brakes hard repeatedly many time will break the disc loose. Resist the urge to spray any lubes or penetrates in through the access plate
ASAP All States has several 4100's around the states in their yards
https://www.tractorpartsasap.com/salvage-equipment/john-deere-salvage/4100-salvage.html?product_list_limit=32
Those should be wet multi-disc brakes the disc are applied by the “twisting”of two plates with balls running in a ramp. Kinda sounds like the balls are not always returning to the released position. Hopefully others with more hands on knowledge will be here soon.