How old is the soft line going to the tee at the rear axle which feeds the rear brake lines? The diameter of the hole in that line is tiny and they are known to collapse and act like a one way valve, causing your issue.
You can crack a bleeder on one of the wheel cylinders to see if that frees up both rear wheels. If it frees up the wheels, replace the soft line.


1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission