My jag looses small amounts of coolant. ( 50ml / 100km, when driving at full speed )
I changed coolant reservoir, an O-Ring at the reservoir, the cap of the reservoir with no success.
So I bought a pressure tester, but I wonder how much pressure can be applied to the system ?
( I want to fix it, not blow up my coolant system ;-)

 

15 psi is generally the maximum. The reservoir cap should have the stated max pressure.

 

Is this also the limit for pressuring to find a leak ?
I read of dealers for other brands, which apply much more than the nominal pressure to find leaks.
But for this I need some limit not to damage something..

 

Yes. You do not want to over pressurize the system to find a leak. You'll just create a new one--blow out a heater core, gasket, etc.