is required reading.
High Pressure Limit describes the force it takes to engage a water gun's PRV. You can cause it to open by pumping quickly or reaching full pressure. It might not ever engage on a soaker with strong HPL, as the 2000 demonstrates. Squeaking is not a reliable way to find a soaker's HPL - the best way is to see what happens at full pressure. For CPS/HP, if the pump hits a wall and you can't physically pump anymore, the HPL is high. This indicates that the PC casing walls are being hit - the water pressure does not provoke the PRV to open before that point. If you can hear the PRV open and the pump does not ever stop dead, the HPL is lower. In that type of gun, the water pressure is too much for the valve and the bladder never makes it all the way to the walls.
HPL is tougher to figure out in AP guns since their PRVs are all going to open eventually. With pump volumes and PC volumes rarely being the same across the many AP guns there are, direct comparisons are not always available. Generally, you look at the power, since higher pressure obviously makes AP guns shoot stronger. An AP gun with high HPL makes a good candidate for PC replacement.
HPL governs the modability of a soaker and is thus important when power modding CPS. The low HPL CPS 2100 mk2 can't handle 10 balloons in some cases, while the high HPL CPS 2100 mk1 can handle 40 or more.
These are the ones I know of:
CPS 1000: at least one version with high HPL and one with low
CPS 1200 mk1/3: high HPL
CPS 1200 mk2/4: low to moderate HPL, usually low
CPS 1500 all marks: variable, but usually moderate, HPL
CPS 2000 both marks: very high HPL
CPS 2100 mk1: high HPL
CPS 2100 mk2: very low HPL
CPS 2100 mk3: moderate to high HPL
CPS 2500: moderate HPL
CPS 2700: low to moderate HPL
Disabling the PRV shifts HPL from what the valve will tolerate to what the soaker's parts will tolerate before breaking. A CPS 21K with 72 balloons can take 13 pumps safely. At 14, the firing valve exploded where it was narrow/thinnest. Epoxy was no match for the carnage and that soaker was totaled. The only non-complicated, non-mathy way to find this absolute limit is trial and error. This is why we are conservative with K-modded guns and why disabling PRVs has fallen out of favor. Note that if you do a very heavy K-mod, the sheer space the balloons take up allow the bladder to hit the walls before the pressure gets dangerous. A 95 balloon 21K will only accept 9 pumps, but with the pump stopping dead, you cannot overpump it.