You need to find out where its leaking... THEN order parts.
ORing kits are a ripoff, you can get a bag of 100 of each common size for the HW100 for less than a kit.
The fill valves can leak, but usually aren't a common leak point on the HW100. The noise you hear is likely air leaking around the fill probe orings, which would also explain the slow filling since you have air leaking out instead of filling the gun.
Here's what I suggest: Remove the cylinder from the gun, and fill it. See if the tube leaks on it's own. If so, there ya go and no need to tear the gun apart.
If it doesn't, the leak is in the gun and you need to start at the brass inlet valve and go in from there. If the leak is in the gun, it's likely in the brass main valve there. Again, if it's here there is no need to tear the gun apart further.
They are easy guns to service, but a little unconventional. Tearing into the regulator (which is built onto the action) for no reason without a reg tester can be frustrating to set back correctly without a regulator tester. So don't blow it apart unless it's actually leaking. Again, why I highly recommend pinpointing the leak first, instead of tearing the gun completely apart.
The cylinder is a bit of a PITA to get apart without damage, so make damn sure it's actually leaking before you go messing with it. You'll need some heat and a way to hold the ends securely as they are usually VERY tight from the factory. Also the retaining screw for the fill valve plunger is very tight and brass and easy to marr up.
In closing, don't tear the gun apart until you find the leak. This is not one you want to "just replace everything and it will be fixed" with, and almost always it's something fairly simple on the outside.
