Hydraulic bleeding

There are six non-DH MTB in the house and 5 of them have a Reverb post. I also have the Reverb from my old Superfly which is a non-stealth model, else it could go in my Slash. It is fair to say, despite the occasional problem, I like Reverbs a lot. Four of these have the "new" shift lever controller (as opposed to the original biro-clicker button).

One of them must be leaking somewhere in the system, but I have no idea where. There are no oil leaks. If air is going in, I would expect some oil to be seeping out. The lever loses function over a few weeks, despite continuously turning up the "speed" control. When bled a bit of air comes out and then it's good as new for a few more weeks. I checked all the fittings and they're snug. Other than this one, they all seem to be very low maintenance.

I recently returned the push-button post to service in my single-speed. It has been out of the bike for perhaps 2 years. It works like a charm even after sitting. No bleed necessary there.

And as a result of swapping brakes off the Slash to the Ripley and installing new brakes on the Ripley, I got to bleed four SRAM "bleeding edge" brakes in a row. Three worked perfectly. Super-solid lever feel and working perfectly. One, a new one, was bled twice and still has air in it.
Where from?
Why no fluid leak?
My method is based on the SRAM guidelines and works in all the other brakes.

When I was a car mechanic I bled more than a few brakes there too. We used a manual procedure. But now you can get a vacuum tool that sucks the air out - doesn't need a second person to push the brake pedal. Fast and efficient.
Makes me think that a vacuum process for MTB (and now road) brakes would be better. I generally push the fluid through with the syringes. But one can suction the fluid through in the same direction by pulling the plunger on the other syringe. I probably don't try this enough.

Speaking of bleeding. I don't like the Shimano gravity feed bleeding process very much. You can easily let air in if the top funnel runs dry, which can happen if you aren't looking, which is easy because you're working at the caliper rather than lever. With one brake in particular, I could never get the syphon action to begin. So, I like to push fluid in from the caliper and have it come out into the funnel at the lever. Aside from set-up and cleaning after, it takes about 30 seconds to get a clean bleed with one syringe at the caliper on Shimano brakes.