The left tag brake starting dragging during our recent travels. I could see that the power shaft would not returned to the fully released position. Tapping the end of the slack adjuster would cause the power shaft to fully release. Problem was intermittent.
The pads were not hanging up on the slide pins nor was there any other signs of pad or piston binding. It was as if there was something gumming up the works inside the caliper mechanism. Take a look at the YouTube video Barry posted to get an idea of all the parts of the disc brake system.
Meritor Disc Brakes (https://www.youtube.com/watch?v=7N-St4Px8cs)
After waiting 3 weeks for an appointment at RnR RV Service in Liberty Lake, Washington, the caliper was removed and torn down. Inside was a corroded mess. The piston boot had a hole that had allowed water to infiltrate into the caliper mechanism. The other side tag showed the same boot failure and had less corrosion (but still a lot). I had both caliper mechanisms replaced. The drive axle calipers were inspected but showed no sign of boot failure, so they were not replaced (a decision I may live to regret).
So, four days later and $8,000 ($3k parts, $5k labor), I am on the road again.
Just thought someone might be interested. Something to keep you awake at night.
Oh, coach has about 150,000 miles. As I bought it used three years ago, I have no idea of it's history regarding past brake issues.
I would at least have the calipers and slack adjusters serviced. Posts are on the Forum on how to do it yourself.