Valve sets should be done once a year. Cheap maintenance. You can run into substantial problems if it's not done regularly. You mileage slowly goes downhill as well.
Ill add a few other things to look out for:
When you're under the truck, bang every brake chamber at the spring end. If it rattles, the spring is broken, and the chamber must be replaced. Change it with "life seal" chambers only! Cost about twice as much, but they're SEALED and the spring wont rust and break. Come with a huge warranty as well.
Change your power steering fluid/filter regularly and put a GOOD synthetic ATF in there. This is very cheap maintenance.
If you have a hydraulic clutch (probably) change the fluid every 1-2 years and put a good high quality brake fluid in there. How does the clutch pedal feel right now?
Change driveline fluids with synthetics only, there's no reason to cheap out on this.
Keep an eye on driveline leaks, any type of leak can be a huge problem. Stay on top of this. Stay on top of wheel seal leaks too.
Check your front hubs regularly. I'm going to assume that they're oil-filled with a plug on the end. Stick your CLEAN finger in there, and jam it down to the bottom, try to find any metal shavings. Pull your finger out and inspect, the oil should be fairly clean. If it's not, you may have problems.
Grease grease grease, now here is where we run into a potential problem. Has anyone ever told you that there are many types of grease that are incompatible with each other? I am the only one that greases my truck for this very reason.
I misled you, it's not a c-clip, its a snap ring. You need a special tool to take them off.
http://www.canadiantire.ca/browse/pr...=1179349568824
Like that, although I do not advise getting that exact one. Get a good one, you'll thank me later.
After the snap-ring comes off, the slack adjuster should slide right off provided it's disconnected from the clevis.