Yes sir, been there, bad rear caliper on a 1996 Nissan, had some brake action though, and it was around town when it hppened, side street.
Yup, just 2 days checking MC on 1955 Fargo, noticed it got low, checked all wheels, and found the culprit, left rear wheel cyl, which is a diff from an 1988 Diplomat, $13.00 for new wheel cyls each, some new brake line, and back in business.
I was not enough of a leak, or I caught it in time, brake shoes are fine, only 400 miles on those shoes.
I did not replace wheel cyls ,when I did brake job, they were lookin okay, guess I should have....