As the pressure in the valve builds it requires more force to pull the trigger to release the air. It is a sintered steel trigger and when the sear engagemnt is minimized by trigger take up it can chip the tip of the trigger which engages the sear. Don't get greedy as the system is simple and thus has it's inherant limitations.
Reducing the trigger return spring tension will have a greater effect at low pump values but when the pressure is up the spring is a very small percentage of the total force involved.
The 1400 is a gun you do not want to push to the breaking point as finding spare levers and pistons are rather difficult since the parts have now been discontinued for a few decades. Keep it lubed and don't overpump it. It is a good gun and will last for a long time if cared for correctly but when your lever is toast your not going to be able to find one.
"NO GUNS WOULD BE A RIOT"