I usually store our bikes for the winter hanging upside down by the wheels from the rafters. I would assume that I can't do this with a hydraulic disc brake bike: am I correct?
When turning the bicycle upside down or on its side the brake system may have some air bubbles inside the reservoir tank which are still there when the reservoir tank cover is replaced, or which accumulate in various parts of the brake system when it is used for long periods. The M755 disc brake system is not designed to be turned upside down. If the bicycle is turned upside down or on its side, the air bubbles inside the reservoir tank may move in the direction of the calipers. If the bicycle is ridden in this condition, there is the danger that the brakes may not operate and a serious accident could occur.
If the bicycle has been turned upside down or on its side, be sure to operate the brake lever a few times to check that the brakes operate normally before riding the bicycle. If the brakes do not operate normally, adjust them by the following procedure.
< If brake operation is sluggish when the lever is depressed >
Gently depress the brake lever several times and wait for the bubbles to return to the reservoir tank. It is recommended that you then remove the reservoir tank cover and fill the reservoir tank with brake fluid until no bubbles remain. If the brakes still operate sluggishly, bleed the air from the brake system. (Refer to "Adding the brake fluid and bleeding air".)
I store my bicycles this way for the summer (winter fat tyre bikes) and winter (non-commuter mountain and road bikes). I haven't ever had any issues with this and the shop I worked at previously stored all the most expensive bikes this way as well (less expensive bikes were warehoused). I haven't ever had any issues where my brakes suddenly didn't work bringing bikes out of storage. That being said, I tend toward higher end brakes on almost all of my rigs. Hydraulic disc brakes are intended to be a closed system. It's either sealed or it isn't. Cheap brakes with poor seals on the lever may leak when stored this way. However, cheap brakes with poor seals on the caliper will leak when stored upright as well.
Storing bikes in this manner will not be an issue unless there is already an existing issue with the brakes (poor quality or damage).
There may be another aspect, not just whether it will leak or not. Air may travel from the reservoir into the brake lines making them spongy or have too much travel on the brake lever. I'm told that dirt bike owners are advised not to store their motorcycles other than level, so it MIGHT apply to bicycles too. Should be an easy fix if it does happen, just bleed the brakes, or maybe just store them upright for a time before using them to let the air bubble back to the reservoir.
I stored a disc bike upside down for a couple of months and the brakes certainly needed bleeding. I now hang up my disc bikes from a D ring into which the pointy bit of the saddle goes and a piece of 5mm rope around the stem. Both are attached to ceiling hooks. This also has the advantage of keeping the bars out of the way.