This won't be exhaustive, and probably in no sensible order for that matter, but here are a few things to consider: -
Back o-ring (1st, counting from the back) - this seals air in to the back of the marker. A leak around the back of the pin can be resolved by cleaning and greasing - or if necessary, replacing - this o-ring.
2nd o-ring - this o-ring separates low pressure air from the solenoid, from HP air entering the back of the bolt kit. If this o-ring leaks, you will see a pressure increase on the LP gauge, and your bolt will be pushed forwards.
This may lead to your marker firing when you gas it up.
Middle o-ring (3rd from the back) - this o-ring also separates LP and HP air, though this time, the HP air in question is stored in the dump chamber. Again, if this o-ring leaks, your bolt will be forced forwards, and the marker may fire when gassed up.
Hard to tell which of these two o-rings is at fault, as the symptoms are the same, so you can either switch each out in turn, and see which fixes the problem, or you may just prefer to switch them both out - after all, if they are both the same age, they probably have similar levels of wear.
4th o-ring - this o-ring stops more air entering the dump chamber when the bolt is forwards, ensuring that the marker shoots from a fixed volume of air. If you have lower than expected shot counts when running long dwell settings, this o-ring may need cleaning or replacing.
Front o-ring (5th from back) - this o-ring seals air in the dump chamber prior to the marker firing. A leak down the barrel could be down to this o-ring.
Rear o-ring - this is the bolt sail o-ring - the o-ring on which the LP air acts to cycle the bolt back and forth. If this o-ring is dirty or damaged, the bolt may not cycle, or it may be pushed forwards. Again, this could lead to the marker firing when it is gassed up.
The difference between this and the pin o-rings, is that a pressure change will not register on the LP gauge.
Middle o-ring - this is another o-ring that separates HP and LP air. If this o-ring is dirty or needs replacing, you will see the LP gauge pressure rise, but in this instance, the bolt will stay in the rear position.
Front o-ring - if this o-ring is dirty or needs replacing, it will cause a leak down the barrel. Again, it is hard to know whether it is this o-ring or the front pin o-ring that is the cause of a leak down the barrel, so you will need to either try each o-ring in turn, or replace both.