Props are routinely rotated backwards during maintenance during magneto timing with no adverse effect.
The quickest way to kill a vacuum pump is to let oil/solvents get onto the vanes.
Well, yes, and no. Certainly solvents or oil or other contamination in the pump is the fastest way to start breaking vanes...which then tend to cause breakage on the other vanes.
Some engines can be rotated backward, and some can't. If one has condensation inside the vacum pump casing and rotates the engine, it can cause vane breakage and pump failure in short order. This is more likely when rotating the wrong direction, but also possible in the event of rotating in the correct direction...solution there is to ensure the engine is preheated first...though moisture in a dry carbon vane pump can still cause pump failure whether it's frozen or not.
Mike Berry wrote a good article on the subject of pumps, a few years ago, which can be read here:
Vacuum Pumps: Wet vs. Dry
Well Blackhand must fall into the category of UNknowledgeable mechanic because I understand that compression is very important to piston engine performance ( Health).
Your comments would indicate this to be the case; most people tend to blindly believe that high compression numbers are good, lower compression numbers are bad, and take a fairly generic view toward what compression really is.
Even with a calibrated test set and a qualified technician performing an actual compression test, the values received are highly subjective, and generally not repeatable Compression values go up with time, depending on who's doing the test, and they go down. Point is...actual compression doesn't mean all that much. The engine needs it to run, but the values thereof...don't really mean much.
Hearing someone talk about an engine with references such as "it's got great compressions!" is somewhat like hearing them talk about buying a car by saying "it's got cup holders!" A compression test, again with calibrated equipment being performed by a qualified technician, is still subjective to numerous factors previously discussed. When I perform this test, I'm not nearly so interested in the values as I am to listening to the exhuast, and induction, and crankcase through the oil filler cap, for leaks. I couldn't begin to tell you what's wrong with the engine at that point...it's simply an elementary diagnostic tool that gets me started. To suggest that one could do anything by pulling it through by hand on the line with their magic calibrated fingers is...ridiculous.
You're not going to get this information when turning the prop by hand without pressurizing the cylinders. A compression stroke without the addition of external pressurized air is not adequate to determine or diagnose the problem, certainly not as a pilot pulling the prop through by hand.
Pilots have all kinds of excuses for pulling the propeller through by hand. My favorite, usually passed down from one instructor to student who becomes an instructor and passes it to another student ad nauseam, is "I'm limbering up the engine," or "I'm limbering up the oil." This is of course, a statement of ignorance, as one is doing no such thing. The design of the engine doesn't permit such a thing, and all the actor is really doing is scraping protective lubricant off cylinder walls and other such places (valve stems, etc).
As far as feeling "relative compressions," what is it that magically makes the engine fail by hand that doesn't reveal itself in operation? The engine ran fine on shutdown, but while sitting idle, begins to stick and have other such problems? Ever stop to think that perhaps by pulling it through without adequate pressurized lubrication, you're creating the problems you're trying to find in the first place? We've been told by one offended poster that he learns things by pulling the engine through that can't be found when it's running...a truly amazing concept...as that's the best time to find the problem.
Pulling an engine through to check for hydraulic lock...valid in some cases, but not all...because even in cases where the check is necessary, often the starter motor provides more protection for the engine by employing a clutch...whereas one can bend a rod pulling on the propeller for leverage.
Pulling an engine through to find the compression stroke prior to hand propping...legitimate, though one certainly won't be able to tell the compression of the engine while doing this, or be able to diagnose it.
Pulling an engine through while performing maintenance...also legitimate under the proper circumstance.
Pulling an engine through to limber it up, to magically feel compressions and to hear and diagnose as a pilot on the line...not legitimate, and not very smart, either. Especially with a charge of fuel in the cylinders which can wash off lubricants on the cylinder walls before engine start (when the most wear occurs), cause the engine to fire with greater power inadvertently, leak into the exhaust and cause an exhaust (or induction) fire on start, and which is wasteful.
So far as pulling backward, it's not only a potential issue with accessories like the vacum pump (circumstance and pump dependent), but also with generator brushes, which can be chipped or broken by turning them the wrong way. It can be inappropriate for certain gear trains in accessory drives, or even in propeller reduction drives.
So far as preflighting the engine, again, one should make as thorough an inspection as possible. This includes tugging on everything that can be touched or felt, and considering even small oil leaks for their source. If the "checklist" says "Engine, Check," then this isn't adequate. No law or principle of reason suggests that one should do the bare minimum; check everything. This may include pulling the propeller through...just not for the reason that some enthusiastic, but misinformed posters have advocated.