Nice job!
First you advise divers to use a regulator until it fails, then toss it. Next you characterize the MK2 in a totally BS manner. The MK2 is an excellent cold water reg, there is ZERO reason to think it's not good for "dirty" water, whatever that is, and it has plenty of flow to supply even a high performance 2nd stage at recreational depths. I assume you're better as a scuba teacher than you are with regulator advice.
Well, aren't you just the sweetest thing. There is a problem with the internet as an information resource: everybody can be right, even if they're wrong...
My observation about TCO strategies for the MK2 was accurate, appropriate and I'll stand by it. The MK2 is cheap enough and reliable enough that skipping the annual service and replacing it when it starts to have problems may very well end up being less expensive than having it overhauled every year. Obviously, YRMV: but the cost of labor (and parts) is not inconsequential and adds up, year after year. I'm not advocating the approach but was, instead, trying to demonstrate one of the potential pitfalls to buying used gear on the internet: that beautiful blond sitting at the end of the bar might be there because her last boyfriend just threw her gold-digging butt out.
There is no alternative universe where a MK2 is suitable for cold-water diving. It's issue that even a knob divemaster should be aware of and understand; as the gas flows through the 1st stage, adiabatic cooling causes a temperature drop than can easily amount to 50F, Do the math; if you're diving in 50F water, this cooling sucks the heat out of the regulator body and can cause the water inside the piston chamber to freeze solid, locking the piston in place. Even if the ice doesn't lock everything up, it tends to cut away at the working soft bits, which means the two piston o-rings have a tendency to fail, sometimes dramatically. If you don't believe me, check with ScubaPro about how effective their T.I.S. experiment has been and what kind of temperatures they recommend for the MK2 - they'll tell you much the same thing. Or you can compare a MK2 with a MK20/25 and see the changes SP has made when they get serious in their attempts to address the problem.
For much the same reasons, you don't want to have a MK2 in dirty water. It's far too easy for crud to get inside the piston chamber and interfere with the operation of the spring/piston. Over the years I've cleaned sand, sticks, snails and worse from the insides of regulators that aren't environmentally sealed. There are multiple evolutions of environmental sealing systems out there, some work better than others. Yeah, there was an optional environmental kit for some of the later MK2's, I don't think that's available anymore.
The MK2 is an unbalanced regulator. Using little words: that means that it doesn't compensate for changes in depth, which means the deeper you go, the harder it will be to breathe from it. I'll plead guilty to a bit of hyperbole: they make great pool regulators and I know several divers using them on 20 foot decompression bottles without issue. Where, exactly, a minor inconvenience transitions to a nuisance and then to a hazard is largely dependent upon the diver but ultimately it's all math and work of breathing. Most divers will notice a significant degradation in performance by the time they get to 3 ata, by the time they get to 5 ata they will be working uncomfortably hard.
I'm a working repair technician and have been for many years. I've been through the ScubaPro training more than once, including their Master level class. For what it's worth, I've been through many of the manufacturer technician training programs over the years, including Atomic, Aqualung/Apeks, Dive Rite, Edge/Hog, Guardian/Aga, Mares, Sherwood/Genesis, TUSA, Ocenanic/Aeris/Hollis, Zeagle, yadda, yadda. In a quiet year I overhaul several hundred regulators of various makes, including a MK2 within the past few days. Heck, I even own a MK2, along with MK5 and a couple of heavily modified MK20's. I'm not a stranger to the MK2, to ScubaPro or to regulators in general. I don't know much about you but I'm very comfortable that my training, knowledge and experience with regulators measures up well against yours. Even if your post count has mine beat.
Of course, even if you buy one new, you're stuck breathing through a <insert snarky metaphor here> that isn't suitable for cold water, dirty water or pretty much anything deeper than a bathtub.
