I have a Mercedes Benz W124 230E 1986 , lowered on 18" alloys,

On the dash there is an engine pressure gauge. It has 3 bars.
When de-accelerating, idling, rolling, sitting still it drops to only 1 and a half bars.

When accelerating it creaps up to the third bar.

When I start the vehicle it rests half way between the third and the second.

What causes this?
Any information would be very muchly appreciated.

Thanks
Gavin.