A hissing sound occurs probably due to hairline crack in the valve which can lead to reduced suction during pumping.
Some of the valves wear out faster because they may be overly irrigated with water during the washing process or the use of unsuitable cleaning tools that had caused the valves to unduly expand or crack.
We wouldn't advise using the bristles to wash the valves as the valves are quite soft. You may wash the valves using undiluted baby bottle detergent and gently rubbing the internal and outer valves with your fingers, this help with removing milk residues without applying too much force.
Additionally, you would want to make sure that you hold onto the duck bills during washing to avoid overly irrigating the valves such that water is forced out through the duck bills.
Here’s a video guide that you can refer to on How to wash the Hegen Breast Pump Valves: