Super Soaker Central

So it cuts off if you just hold the trigger? There's not enough detail here for a help. From what you've said, it seems like the valve is still fully opening ("short burst of water"), but it seems impossible that not all the water would dump out. Or maybe the valve isn't opening all the way and you're getting some water leaking out. (if that's what you mean by "short burst")

Yeah, we'll need more information to make suggestions.

The modification shouldn't have anything to do with this problem. In fact, this modification is so harmless that it's much more likely that you didn't reassemble the water gun correctly or the water gun broke for reasons unrelated to the modification. I'm leaning towards the latter.

Does it have the same problem on the stream nozzle? Both nozzles feed from the same pressure chamber so they should shoot the same amount of water.

Try pumping only 5 times. 1/4 cup is about 2 or 3 pumps so you should notice a difference with 5 pumps if the PCs will hold that much. Perhaps when you pump too much the safety valve engages and most of the water from the pressure chamber is dumped back into the reservoir.

The only other possibility other than that is that a leak developed, but you likely would have noticed a leak so I doubt that.

Maybe the valve is opening and immediately closing for some reason. Though the flood blast is normally so quick that wouldn't really matter.

Also, after it's given the short burst, does another trigger pull yield another? If not, what about the stream nozzle. This might narrow down the problem.
If it seems that PC capacity has actually reduced, I'd suggest either a fault in the pressure release valve, or debris has gotten into the PC. But I think it's more likely to be a trigger valve issue.

Problem fixed itself, thanks for all the help.