It is probably the quick connect fitting on the hose that is leaking. Cheapo fitting or bad o-ring. When you disconnect it the oring does not do anything as it only seals to the fitting on the tool. Small leak - probably not worth it. Bigger leak, just replace the fitting. Make sure the male fitting on the tool is smooth and doesn't have any major dings on it, as that could be the source and could damage the o-ring.
And of course, it is quite obvious...you need a bigger compressor! "Honey?.... The guys on TBN said..." :devil: But seriously, if you don't have enough air delivery, a bigger wrench won't help as it will have the same air limitations. Bigger compressor, or more storage volume (extra tanks ganged together) plus a decent sized hose - no 1/4" air nailer hoses for this. 3/8" for sure. Impact wrenches need some decent air for any use beyond simple lug nuts. Doesn't mean you
need a 50A/240V 80gal 2-stage compressor, but the more the better. A pancake compressor made for air nailers just wont' work. Trust me. I've tried it... What do you have?
Get a 1/2" drive breaker bar and a pipe that will fit over it. Easy does it as you can break stuff this way and get hurt. 2-3' pipe is usually plenty, but you can always choke up on a longer pipe. The bigger problem with a long pipe is enough room to swing it. Probably not a big issue on a mower deck like that, but often is for car work. Plus you need to secure everything from moving on you - the deck and blades. You will need to wedge it against something solid or clamp to a sturdy table.
BTW if you don't have a MAPP gas torch, that is another good thing to get. They are real cheap and a lot hotter than propane. It is basically a regular propane torch, just designed and rated for the different gas. For serious fun you need an oxy/fuel torch

I would grab a MAPP torch and bottle when you head back to the store tomorrow. And a can of PB blaster too. These are good things to have on hand for the long haul, as you seem to want to do more things yourself these days.