First and foremost I would get ahold of the contractor by any means. He needs to fix the issue.
Second, I dont know the code in your area and therefore dont know the type of system installed, so I can only comment on what is in my area.
No one has mentioned this yet, but septics in my area have a sand filter BEFORE the leech field. And the entrance to the sand filter is what is called the distribution box. It has 1 inlet coming from the tank, and two outlets, cause you have two sand filters. One of them normally has just an elbow on it pointing up, which forces the water/waste into the other filter. Twice a year (when clocks change) it is reccomended to switch filter beds.
These two filter beds converge back together and drain into one leech field/trench. Which should also have a permiter drain around it (tile laid in stone) and connected to a known good tile to prevent rainwater/runoff from saturating the leech field. There is also an inspection cover over the "exit of the sandfilter"/"entrance of the leech field".
So, for further clarification, does this sound like the type of system he has? And are you sure it is backing up in the distribution box?? or is it backing up at the entrance to the leech? A typical system here will have 3 or 4 "covers". Either one or two for the tank, then the distribution box, then the entrance to the leech.
If it is indeed backing up at the distribution box, perhaps try swapping to the other sand filter (move the elbow or plug to the other pipe).
If it is backing up at the leach field AFTER the sand filter, then either there is a collapsed/broken tile in the leech field, or it is saturated. Did they install a perimeter drain?
The pressure build up could be nothing. Could just be the grade, and having water pressure uphill of the cover creating small amount of pressure. You didnt indicate just how much pressure there was?
The type of system I just described doesnt have a vent either. Other than the vent at the house. The tank is not airtight. (concrete with concrete lid).