A 15 watt iron is a bit small for what you're trying to do, a 25 or 30 watt would be better. Newer pedals are made with lead-free solder, and the lead-free melts at a higher temp than good ol' lead-tin solder. Make sure the tip is CLEAN - it should be a shiny silver color.
...adding new solder, then using the plunger to remove it all as needed.
This is completely confusing to me, if you're trying to remove something, why are you adding more solder?
I presuming this is a leaded and not a surface-mount component. Apply heat, wait for the solder to liquefy - it should just take a few seconds - and using some needle nose pliers, yank the old cap's lead wire out of the hole. Repeat the procedure for the other cap lead. Don't heat the wire any longer than needed, excess heat can 'cook' the printed circuit board and cause the copper traces to lift off.
When the cap's out, use de-soldering braid or your solder sucker to clean up any excess solder left around the holes. Then solder in your new cap. A 1 uf cap is almost certainly polarized, so make sure it's installed with the correct orientation.