Load test the battery, if it checks good then clean and tighten all terminals, if you still experience slow cranking it's possible the starter motor is dragging. They do fail and age is an issue..
You guys are right, it is just a trouble shooting procedure not a permanent fix. All it does is tell you that the tank isn't venting as designed.. I've not experienced an integrated system as op described..
I bought a carter last year and it's done a good job this summer and I think I may get a spare just in case..
Looks like you're in need of some new pulley belts as well... don't wait for it to break the engine will overheat quickly...
I didn't realize they existed until this thread showed up. I suppose it was an attempt to make a maintenance free engine but I don't much care for the idea..