A couple of the center booms are hard to turn. What kind of maintenance can I do to these to make easier to use? Looks like there is a wheel of some sort up there, do those require replacement at certain intervals?
There's a grease fitting in there, but judging by the ring on the plate left by the wheel I'd bet they're done. I run a wash with those same booms, most of them are extremely stiff, so I'm replacing them all with Dralco SS ones.
Those look exactly like our Mark VII booms. We did not have grease fittings until I added them. Prior to the grease fittings I would pull them apart every couple of years and clean up the pipe/shaft in the middle. It is a chore unless you have a socket and impact. The nut is nylon filled and tight all the way out. 1-13/16" socket and a 1/2" drive impact makes it easy.
There is a bushing on the shaft that is delrin or some similar material. Mine are worn, but work fine when everything is clean.
You might look into dual Z booms instead. With the dual center booms customers have to pass one hose around the other. I can't even imagine that battle when the people in my area put the gun away like this: