Posting for future readers' archival reference - assuming our new overlords don't manage to kill the forum through continued mismanagement...
There are only 3 tumblers in the tank lock if memory serves me correctly...it shouldn't be hard to pick. I'd be surprised if a locksmith took more than 5 seconds and charged you for it if you took it to them.
Turns out it actually has five pins. Here's the cylinder as you extract it from the lock core, with three of the wedges removed.
Here's the removed pins and their springs out of the cylinder.
You can use your fingers to remove the pins but you will likely need a pick to remove the springs.
If the cap assembly has already been removed from the tank, you do *not* require the key to remove the lock cylinder. If you want to make this an "any key or a screwdriver can open this" cap, remove all the pins and springs and put it back together. Either way, if you're having problems with your lock, you can strip it down this far, clean up the ancient grease it shipped with, regrease and reinstall.
Here's the lock cylinder reinstalled in the (stripped) core.
The core and cylinder with the rubber cover/diaphragm back over the serpentine fume vent/air equalization vent path.
This is the back of the cap that you screw the core down onto.
This tab with a square hole on the core is the outlet of the tank vent path.