Daf LF air brake pressure?

Hi I have just purchased a 7.5t Daf LF on a 52 plate to convert into a race home its been on the drive from november and i just move it backwards and forwards to work on it, the air brake pressure is strugling to get out of the red now some one told me the tanks could be iced up but its been a little milder and no different.

I tried again today and also tried the tanks very little moisture really but since i drained the air tanks the gauges wont go past the 5 mark only takes a couple of minutes of reving from nothing to 5 but will not go past that before i drained them only getting to around 7.5 and when i picked it up they were at 10 constantly.

The pressure does not drop overnight when left any ideas

Thanks