After few attempts, I managed to reach 3000 lb with 4 seconds left. I know there must be other better solutions outperforming mine, but this should be enough for confused folks to get the achievement they want.

For better illustration, we define A as the limit point of the side tank, B as the limit of the main tank, E and F as what game itself shows on the pressure capacity.

• Margin speed
  As the tank charges faster when the side tank approaches the limit, you'd want to keep staying on point A for more speed.
  Charging to the top costs pressure, so we should stay on point A as long as possible for each attempt.
  
  
• Capacity
  The capacity is actually an expendable resource. Imagine the pressure capacity as your MP, you start consuming MP when you exceed point B, and stop costing it when leaving it. These two moments are the beginning and ending of our each attempt.
  Since we want to maximize the duration each run, we should break point B at "full MP" (point E), and drop below point B when our "MP" is empty (point F)
  
  
• Recovery
  The capacity recovers by time, that means, you are losing capacity if it's empty.

Now we can easily putting the puzzle together:





1. Stay slightly below point B and waiting for the capacity reaches point E

2. Break the limit right before capacity goes empty









3. Charge toward point A

4. Tapping the space bar to stay near point A as long as possible, but DON'T reach it






5. Drop the main tank below point B as late as possible. This is the point you might need practice to beat records. But simply finish within 120 seconds shouldn't be hard.

6. Consume randomly waiting for capacity to drop

7. Repeat the entire process til the end