with all 3 hyd. systems pressurized, going from 3000 to 1700psi and back to 3000 in a matter of a second or two
sounds more than just the blue?
if it is just the blue, have a look at the accumulator charge pressure, normally around 1800psi, if it is 0 then a fair bet the bladder has gone. There are checks in the MM to do. However that is unlikely to cause fluctuating pressure. Sounds more like a pump problem or one of the valves in the braking system shuttling? Or the age old problem of air in the system and a proper bleed is needed.