'93 Ford Ranger Slave Cylinder failure?... AGAIN??!

Picked up a '93 Ranger 4.0 4wd last November and have been plugging
away at it's inherent 25 year old, Canadian winter-driving problems.

First to fail was the slave cylinder. Started having trouble shifting, checked
the fluid (was low), and ultimately fully spilled the hydraulic guts in my
driveway. $800 or so later -- thanks to the labour required -- and I had a new-
feeling drive! YAY!! The operation also afforded the opportunity for the
mechanic and I to take a look at the clutch which looked like it had been
done recently and still in great shape.

Cut to last week (roughly 10 months since the new slave), when returning
from a little weekend roadtrip, she started having trouble shifting. Fluid level
was full, no visible leaks from the slave cylinder or anywhere along the
hydraulic line, but proceeded to gravity bleed anyway. Little-to-no air bubbles
present, noticed a slight improvement which lasted a little no more than an
hour, then back to difficult shifting.

I brought the truck to a local shop to have it vacuum bled again with little-to-
no air present, and almost no difference in the issue. The mechanic on hand
claims, "that's just how these slaves are; the spring in them gets fatigued and
doesn't function as it should."

...Now... I did study some metallurgy back in the day and I am aware that
metal fatigue is a real thing (I need only look at my leaf-springs to be
reminded), but less than a year for the new slave to fail? That seems a little
ridiculous and I just can't swallow it.

I've taken a look through the bellhousing window and the clutch and slave
look fine. I have read that it's possible air gets trapped in the top of the
master cylinder (as it is oriented at an upward angle), but without any
apparent leaks, should I suspect it? I'll probably try detaching it from the
firewall and expelling any air as it should be an easy home fix (I'll update if it
works).

Any other ideas, folks? Thank you in advance for any contributions to this! :D