Shimano Brake Leaks?

So my wife's bike has some Deore M615 brakes and I have some SLX M675. We moved from BC to Sask with the bikes packed up in shipping boxes and were subsequently reassembled. In my former life I was a bike tech so I am certain no lubes goop or otherwise hit the pads or rotor, both bleeds were good, and all lines are intact. They were shipped over winter. When we went to use the bikes in the spring the rears lost most to all pressure and there was the telltale signs of contamination of the pads front and back. 

Do these brakes have a habit of leaking at the caliper? Or in cold weather? I'm going to assume there is no point repairing them as always. I would rather stick with Shimano because the ergos work for my wife, so would another set of 615s be a repeat experience? Should I look at Avid instead?

I haven't had a Shimano fail that way (due to cold) but I did have some Juicys need a re-bleed after living on the back of my car while I drove across the country in February.

The only thing that bums me out about Shimano brakes is that the calipers are (technically) non-rebuildable if they get a leak, and this is the only failure I've experienced with them.   A year-old XT piston just cracked on me somehow,  ruining two pairs of pads in the troubleshooting process.  It's probably warranty but I haven't gotten around to dealing with it 'cause I just bolted an old XTR one on there and with a fresh bleed and torched pads it's working real nice.  I had an XTR one fail after 3-4 years of heavy use as well.

Less problematic than any Avid I've ever owned, I don't wanna give my $$ to SRAM anymore, and Shimano's are cheap enough to replace if they DO fail so IME Shimano is the way to go.  Calipers can often be found sold on their own, saving you some dough.  If she likes Deores I'd save my time and just slap a whole new one on there, keeping everything but the caliper for backup/repairs.

I dunno what that picture is but it's awfully pretty.

I have heard that Shimano pads oxidize over time. A simple sanding fixes that aspect. If you ride frequently it's a non issue but a bike sitting for a few weeks or more can feel like contaminated pads. I think that only applies to the metal pads. If pressure is lost, ie lever to bar, must be air in the system.

I always start with the free maintenance. Sand, clean, bleed, lube, adjust or what have you.