We had the same problem with our 1998 V70. I sprayed a small amount of silicone lubricant (using the plastic tubing that came with the can I bought) behind the release button. Then I carefully was able to work the release free (had to pry on it slightly to get it to pop free). Sprayed a little more ...

Mystery solved! It was water in the fuel tank! Condensation from sitting all them years. This year I had to drain the tank and the bowl on my power washer for the same reason. Keep that in mind the next time your lawn mower will not start up after being stored for the winter. US Vodka .... that stu...