How are the valves removed ? One of the press down and turn to deflate buttons has broke and presume the complete valve needs replacing but it's not obvious how this is done .
Trouble with the pliers is that if the valves are stiff you may split the plastic lugs and make removal even more difficult.. Last year bought two new valves, plus a plastic spanner device that fitted in to unscrew the valve bodies, plus some 2-part PVC glue and patch material, for a refit of a six/seven year old PVC inflatable - not cheap but much cheaper than a new dinghy. I bought from a local inflatable repairer, but Polymarine have the stuff - http://valves.polymarine.com/?page_id=103 shows the sort of valves and tool I used.
My local inflatable repairer had a version of the tool at about £15, I seem to remember. I was going to file the ends of a bit of pipe to shape, but the tool was cheap enough to save messing about.