Hi there this is my first post so let me just introduce myself real quick.
Im a out of shape 200lb norwegian who have not touched my GT for almost 15 years now. This summer I thought to myself, what the heck man, you need to shape up so put together that old GT you have hidden in a dark room for way too many years now and just ride again.
Well, that was exactly what I did. Bought myself a complete group upgrade kit with everything included wheels, tires, derailleurs, shifters, suspension fork....and disc brakes...
First of all, the A2Z uni mount fits like a glove one my old GT frame, no problem at all.
Whats not cool is the front brake handle. On my very first test ride just to check everything like gears tire pressure etc, I pressed my front break lever to see how they worked, and after a few tries I heard a loud click sound. What the heck was my first thought thinking it was the pads settling in, so I tried again and this time I could press the lever almost completely in without much breaking force(I could still pedal my way forward while I did this).
Removed the whole system(master cylinder and caliper) and took it to my dealer where I bought everything. His first comment was air in the system due to me pressing the lever while there was no disk or wedge/bleed block in place. I asked him how air could come in just because the pistons move 1,6mm out(disc thickness), but he had no good answer for me, it was a fact I should know about in his mind.(I havent found anything mention this in owners manual) I told him its just BS and beside not even the point. The handle failed while it was on the bike and it worked just like it should until it failed but he was convinced it was my fault and that I had messed it up playing with it. Not to brag but Im a maintenance technician and my job is to solve problems and find faulty stuff all the time, handling much more complex systems than these in my mind, simple hydraulic brake system and from what I can see on the parts diagram there is a threaded bold which comes from the master cylinder and are screwed in a threaded sleeve attached to the brake lever by a knob.

My guess is that either the sleeve or the bolt part has faulty threads so I guess my question is, is this a common error on these breaks? Should I also ditch my rear breaks and buy the famous XT disc brakes in stead and not bother spending time convincing this dealer that its not just trapped air due to "misuse"? They spent, by the way half a day today to bleed them without any luck and they have still not called me back telling me I can come and get them.

BTW how many years are MTBR now?