Clutch slipping when hot and climbing under load

Hello to all, I have this serious issue when my truck or clutch is hot and climbing hills it slips pretty bad. I've tried searching everywhere on the net to find a story that fit my description but to no luck.
Rig is a 2014 prostar eagle with a ISX15, 13 speed F.E.
My boss just got this truck about month in a half ago, and dry it didn't slip at all, but once under a load, hot and hills it slipped a lot, I of course let off the throttle to try to prevent the pucks from burning up. To make it worse, while on the down hill and engine break on it would seem to go into lower gear on it's own and once I tapped on accelerator it would go back into proper gear but them go back to lower gear. I told my boss and even took a video of it. His mechanics looked at it said it was new, even international said it was new and adjusted it, then to say "it's the driver, he doesn't know how to shift it" which pissed me off because I'm no dummy and know if something is wrong. So any how he finally bought a new clutch kit, installed it two days ago, I left the yard and everything was smooth. Oh i didn't mention my clutch got ridiculously hard and as soon as I put my foot on the paddle it would disengage the clutch. But once clutch was changed it was great, barely released the clutch and in engaged. But then while climbing the grapevine under a load it started slipping again and clutch pedal got hard again. I stopped in Parlm Springs, CA for a break and pumped my clutch 5x's after that it didn't slip again BUT I'm concerned that it may.
Can it be a leaking master cylinder? Or a faulty hydraulic part somewhere? It only happens when it's hot and climbing under a load.

Sorry for being so long but wanted to give details. I'm hoping someone has had this experience so I can be guided in the right direction. Thanks for any and all input/advice/knowledge..

 

13 speed Fuller Eaton standard.

 

Hardly any. Maybe 1/2 inch. But from my understanding on hydraulic system's there really isn't much play to begin with. That's when everything cools down. But once system is hot pedal gets hard with zero play. Literally can feel the flywheel at the pedal which is funny because its hydraulic.

 

You know that's interesting, I thought the exact same thing and told my boss. The reservoir states DOT3 or DOT4 break fluid, and there is an adjustment rod from pedal to master cylinder. My boss told the mechanic what I said, basically what you said and mechanic said that wouldn't and couldn't cause it because it's a self adjusting clutch. I told my boss that the self adjusting clutch has nothing to do with the hyd. side of the system, so basically his mechanic is having him toss money into it while guessing. My opinion, if it was my truck, back flush the whole hyd. system, inspect or R and R the master cylinder for worn or any swelling of O-rings and adjuste the clutch pedal rod. But mechanics are so ignorant they don't want to take a suggestion of any kind because I'm not a certified/master mechanic.

 

Truth.
That is what he's going to change out next. I just hope it's not a reman. But I will definitely let you all know the outcome. I'm in AZ now. Going to head back today. Yard is in Yuba City, CA but might p/u load in Diego or L.A then head north.