How many atmospheres (ATM) do the rear inflattable tyres need to be adjusted at in terms of pressure?
The manual, unfortunately, doesn't say that, and I cannot know this by heart for every stroller. I do know, though, that the information should be on the wheels - on the outer tires, so try check-in on those.
Apart from that - if there's nothing, the general pressure for stroller wheels is usually between 1 and 1.8 ATM, but it can vary of course.
Or you can just go to a nearest bike shop and ask them to help, they should have a grip on the right pressure in all kinds of wheels.
Your -very berry- Eli.