Thank you for joining us here on Ferrari Life and thank you for your very kind post
We've been already exchanging quite a bit regarding your F1 issues and I'm glad that replacing the maintenance parts (pump, accumulator and E-DIFF solenoid valve) made already a big difference. It was really worth doing this because these parts won't last longer than approx 10 years.
Regarding the pressure you mentioned, usually the pump starts when the pressure drops below 42 bar. I don't think you have an issue with the pressure sensor which is reliable and I've seen only very rare cases where they failed. I'm pretty certain that there is a short time lag between the diagnostic tool and the TCU and that's why you saw the pump priming at 39.6 bar while in reality it was about 42 bar.
Regarding the random big pressure losses, I see only 2 or 3 possibilities:
1) TOB seal failure causing random leaks. There is chance this also makes your clutch slip excessively when this happens.
2) Selespeed actuator internal leak.
3) Banjo bolt or hydraulic hose failure.
Pressure loss doesn't mean that you'll have a noticeable hydraulic fluid leak. A very small invisible leak can already cause big pressure drops. Although, what you can try is to check the fluid level in your tank at equal pressure rate by measuring the fluid level from the tank neck. It's important to do that each time at the exact same pressure (check with your Launch) because the fluid level greatly varies depending on the pressure. If you see that the fluid level is getting progressively lower, that would confirm an internal system leak.
I hope this helps and thank you for letting us know how it goes.
Happy holidays and take care.