1988 240 rear door will unlock but not lock

[carrie240]

Just bought 1988 240. The rear passenger door knob is very stiff. Have to use a wrench to push it down. Won't lock at all with central lock/using key in driver's door. But it will unlock fine using central lock/key. Should I assume the mechanism is gunked up or something is stuck down there? Figure it can't be electrical if it unlocks fine.