I'm having a very similar problem. The one we have is a 5409 something something WD - with a Lister diesel. It's supposed to be 30.7 cfm, but I can only fill 12-15 tanks an hour. I can get the exact model number later if it helps.
The second stage is at 100, third is at about 550. I can't tell what the first is because the gauge is broken (naturally). The owner bought a refurbished Bauer K15 that is doing 18-20 tanks an hour. We're doing about 200 tank fills every day at the moment and will be for the next month. He bought the Bauer because the Mako is running so slow. Yes, I know it probably would have been cheaper to simply fix the Mako instead, but that's not how they think. I don't know if it ever ran properly to be perfectly honest. They may have bought it broken and had low expectations ever since. But it would be very helpful to get this thing running at 30 cfm instead of 15. I can definitely tell you it has been neglected and has probably lacked maintenance since they bought it.
The auto drain is disabled and bypassed (naturally)(as are ALL the electronics), so when I shut it off at 3300 (the banks don't work anymore, naturally, so we fill direct), I don't hear any leaks. There is no rpm meter for the diesel engine, but it seems like it revs pretty high and already runs pretty hot, so I don't want to mess with that. But I'm thinking there is definitely a problem between the stages anyway.
Can you tell me if this is something I can check out/fix on my own? I don't have much time to mess with it as we are too busy to have it out of service for any amount of time. I'm not a compressor tech - I'm just the manager, but I know more than anyone else at the resort about compressors, so if it's going to get fixed, it's going to be me doing it. (We're in Utila, Honduras, unfortunately we can only work with what we have, which isn't much).
Any guidance on checking the valves would be very much appreciated. Any technical manuals would be awesome too so I can actually see what I'm doing. I'll end up taking something apart on it one way or another. Tuesday is an off day, so maybe I'll do it on my day off. It could be worn piston rings or broken parts, which I can't do anything about, but if there is something I can tweak, I'll do it. So any help as to what and where to look for would be awesome.
Thanks for reading and helping!!!
Mike