... The level is good and the color looks fine, but it is 5 years old now. Is it possible that moisture or anything could build in it over several years? ...
Absolutely yes...and the color in the reservoir doesn't mean it's that clean/clear in the caliper/wheel cylinders/lines. 5 years is a bit much for something so critical to safety.
Do it yourself for about $15 and less than an hour of your time..it's really easy since the truck is so high up. Just need couple big bottles of DOT3, a length of hose, preferably clear, and an empty coke bottle.
Start at the back right (furthest), slip the hose over a bleed nipple and open it up with a wrench..then pump the brake pedal until if comes out clear, putting fresh fluid in the master cylinder every so often. Then left rear, right front, left front. Then again backwards: right front, left rear, right rear.
Keep the hose on the bleed nipple and in the old fluid at the bottom of the coke bottle until you tighten it to keep from drawing air back into the wheel cylinders before moving on to the next one. It won't draw the old fluid up.
Do not be stingy with the fluid...it's really cheap when you think about what it does!