Has it got any oil in it fella?Its not meant to drian down like it did at first,its now operating fine by the sounds of things so I`d check that first.
Also,why did you change the pressure switch?It seems a bit high pressurising to 8 bar as well,its usually no more than 6?
Try physically switching it off at 6 bar or 90 psi,use it a bit so pressure drops down to say 5 bar then try and fire it again,if it doesnt kick in then i`m not really sure without seeing it.
Do you know what make and model it is?