[RMcNeal]

Be very careful cleaning the Hepvo valve. The membrane is quite fragile so any snake or brush you push through there could easily tear it. Some have had luck with just simply stomping on the shower drain with water to force air through it, or flushing with a leaf blower.
If there is water backed up into the shower, removing the Hepvo valve would allow the water to drain into your basement, so bring a bucket with you if you remove the valve for cleaning.