I feel you on trying to build up that intra abdominal pressure. I played college ball at 295 and cut down to 235 within a year and a half of finishing up. When I was on big boy status, pushing into a belt was cake, but trying to push into a belt when you don't have a belly anymore is tough.
That said, the most I ever hit was 525 as a player, and even though my strength took a hit while I was cutting, I just this week hit 535 for a double, and turned around the next set and hit 555 for a single. I weighed in at 238 the night I hit it. Keep working at it and good luck getting that 315.