Corrision, corrision, corrision! Its worked its way into your calipers
thru a old seal on the parking lever atop the caliper.
Same thing happened to my 82 5KT. The dirt and water got into the rear
calipers from an old seal and caused the innerds of the calipers to rust
to the point of almost locking the rear wheels!
If its not too bad, you might be able to rebuild your rear calipers.
M