fan question

I have an 07 Volvo 670 . When I started it this morning I had under 50 psi usually, once it goes pass 60 PSI the engine fan cuts off or disengages. Today I had a hundred and 120 psi and I noticed the fan was still engaged, the engine temperature was slightly less than normal. and the fan was still engaged. I cut the truck off and checked the fan there was no play it didn't move. So i cut the truck back on and it was normal it had disengaged on its own. So I take it too a shop they put the key on the on position and hear the air leaking from the fan. Take it apart and see that it's an air fitting leaking. My question is will a leaky air fitting stop the fan from disengaging at the correct psi, and why did it disengage once I cut the truck off and then back on?