A hissing sound occurs probably due to a crack in the valve and the crack on your valve will contribute to low suction issue.
Some of the valves wear out faster because they may be overly irrigated with water during the washing process or the use of our teat cleaner was not observed and other cleaning nipple sponges/fingers 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. Instead, you can try using the teat cleaner (the other end of the brush) as it has the grooves that are meant to help with removing milk residues without applying too much force.
So 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. And do add some undiluted bottle detergent before washing.