Out of the blue, the passenger door on my '74 Delta 88 convertible will not open. Difficult to unlock and door handle thumb button fully depressed. Interior handle also will not open it. I can tell something is not disengaging when I pull on the door. This newbie would welcome suggestions. Thanks.

 

First thing I'd do is roll the window all the way down and look thru the slot to inspect the latch linkage to see if anything is binding or become detached. Spray all visible pivot points with lubricant while you're at it. Are you able to get the lock button all the way up? The fact you're getting resistance would imply rusty pivot points on the latch lever...or, maybe the outer spring has broken and is jamming it from moving freely. If the lock button is all the way up, you should be feeling resistance when pushing the door handle button or pulling on the interior handle as they push against another lever that would allow the release of the latch "jaws" from the striker post. If you're not feeling that resistance, the latch is still in lock position. If you have resistance, problem could be rusty/ sticky latch "jaws".