Very old thread, but thanks for the info. I'm having the same problem, put the canister in a bench vice to compress the spring and remove the clip, but can not get the cap off. Just wanted to check if I missed something before pulling and twisting too much. I have a feeling this has never been done before. the PO bypassed the CLASS and installed schrader valves. I did not like that, and decided to fix the CLASS controller instead. luckily the manifold and compressor were still there but finding replacement hardware was a bit challenging. Thanx to Amazon I was able to get some banjo fittings and the correct bolts, and spliced with barb fittings into the old hoses.
I was having trouble with the rear timing out when trying to go to high, and wouldn't get above 40ish psi and had a slow leak from somewhere. I got a little hyper focused on the banjo fitting because the bolt was bottoming before the banjo was tight. I took the wrong approach first and cut the bolt shorter, and when I tried again I cut it again and now it's too short and am waiting on replacements. The real problem was that the female threads in the manifold were a little jacked up doh! So in the mean time I've been tackling other minor issues while I wait on parts. I've been reading also about the solenoids maybe being dirty and filters being dirty not allowing enough airflow.
Anyway back to the garage to try and get this thing apart. I thanks for letting me tell my story, nobody in my house cares about what I'm doing and I've been working from home for two weeks and into the foreseeable future so none of my typical technical banter with coworkers is happening.