Seems a bit low I would have thought 1 Bar + (15 psi) maybe near to 20psi on a good day, certainly not as low as 5psi under load up hill? Not just blipping the throttle stationary.
No signs of leaks etc. hoses , intercooler?
On a later Doblo I used a blanking kit and pressure tested the hoses and intercooler and found around 20 psi the clips started to leak. However my actually problem was a leak on the EGR pipe in the inlet manifold.
I did not have the direct experience to comment, but a bit of research on eLearn for testing on the 2.8jtd returns required test results in the region of
1 Bar.
