Brewzilla
BrewZilla PPSU Sight Glass O-Ring Kit (4 pack)
$3.99
The PPSU Sight Glass uses radial sealing mechanism in order to not leak, even under pressure from a 25w pump. This is a very similar design to the duotight push in fittings that also use two o-rings. Incase one breaks, or leaks, there is another to save the day.