Check the sidewalls of the tyres, they will give loads of info including max tyre pressure. If you stick to 5 - 10% below that you will be fine until you can phone a tyre or DAF company to confirm the correct pressures.
The manual should tell you and if you don't have a hard copy you should be able to find an electronic version somewhere (we had an ancient tractor and bought an old manual for it off e-bay).
90 psi should be fine you could go to 100 in the fronts check the max presure on the sidewall, under infaltion is the main cause of blowouts , if you have any concerns about the tyres take it to a tyre place which does truck tyres...