Paul, the belt tension should be that you can move it about 1cm (or less) with a firm push.
I think the manual is referring to a belt slipping casing inconsistent pressure. This would be a really loose belt and you would be able to manually eye the slippage.
Is it possible you have a leak in the high pressure side?
With best regards,
Sailing Vessel - Cream Puff - SM2K - #275
Currently cruising - Guadeloupe
Ya Fohi, Amel 54 #98