I recently had a scary experience with the air brakes on my older (1995) coach. On a fairly heavily traveled Texas secondary road (speed limit 55) I came to a traffic light turning yellow while I was about 300+ feet away. I hit the brakes hard and only decelerated very gradually, coming to a stop with my nose in the intersection just as the light turned red.
I cautiously, and very slowly, made my way to a shop that I knew to do HD truck brake work. The mechanic inspected the brakes, but found no obvious problem. He then liberally greased the two lube points (slack adjuster and S cam) with standard chassis grease and sent me on my way.
The brakes seemed to work better after that, but still not really well. I found that if I applied them really aggressively a couple of times it improved performance to an almost acceptable level but with some delay and grabbing.
I was not really satisfied, so managed to get the coach to Motorhomes of Texas and a mechanic I respected.
When I described the problems I was having with the brakes both the service writer and the mechanic had the same response -- "you've been getting lube at truck stops right". The mechanic went on to explain that most truck shops use the same grease (typically a lithium base) for all chassis points, including the air brakes and that's OK for trucks because they're used daily. However, with the more typical coach operating cycle -- run it up hot for a day, or a few, then sit for weeks, or more -- the typical lithium lube with an oil carrier starts to separate and congeal, especially in the fairly intense heat area of the brakes.
The mechanics solution was to displace all the lithium type grease with a clay based grease with which they have had good experience. This necessitated a minor adjustment of the brakes for the different lube characteristics.
After these steps I went for a test drive with the mechanic and there was a dramatic improvement in braking performance with him driving and I could feel it, as well when I finally drove away from the shop.
The mechanic's parting advice was two points: 1. If you get chassis lube with your regular oil changes don't let them touch the brakes, and 2. the brakes do not need a lot of grease so only relube with the clay based product every 2-3 years.
I am gradually getting an education in the maintenance and operation of these unique vehicles called motorhomes and pass this on for information and comment.