Hi and welcome to the forum!

From the owners' handbook:
Tyre pressures in bar (psi)
Front tyres 1.5 (21.

for medium load 1.7 (24.7) fully laden
Rear tyres 2.0 (29.0) for medium load 2.5 (36.2) fully laden
Haynes never made a manual for the BIS only the air cooled model, but if you join the club you can get a BIS workshop manual on CD. (There is also a Polish site which has manuals online.)