Our 15-year-old Toto Aquia II CST416M toilet had been making a hissing noise at the Dual-Max flushing system valve, seem to allow water to leak past it. Just turn the water supply off, lift off the green plastic cover to access the ballcock cap – I cleaned it, but the hissing continued after it was reassembled and the water supply turned back on.

Ballcock cap is listed as THU302 “Diaphragm Cap – Lavelle” in an older parts list from 2008, but it appears removed from Toto’s 2017 parts list – go figure. Park Supply of America sold me a Lavelle Industry Inc. Korky R528 Ballcock Cap Assembly replacement (UPC 749306528039) that was easy to install and immediately corrected the issue. Thanks, PSA and LII!