A couple of years ago, I had a similar problem with the driver's side door
on my '79 Bus.  I really had to turn the key hard to get it to pop up the
lock button.  Sometimes I would have to turn it clockwise and then
counter-clockwise until it popped up.  The problem got worse and worse.

Finally, I carefully popped the inside door paneling snap things out of
their holes, one by one, and removed the window crank, inside door handle
hardware, etc.  I really couldn't get to the guts of the lock mechanism
easily, so, out of frustration, I just plugged the little red hose into the
WD-40 nozzle and sprayed all up inside there, under the door lock pop-up button.

It worked!  The lock popped up and down like the day it rolled off the
showroom floor.  I then inserted the red hose into each keyhole on the Bus
and sprayed, following that with a good dose of powdered graphite lock
lubricant.  No lock problems since.

Cheese