Many VW components use normal round-shaped o-rings.  They start out
round in cross-section, but get compressed and flattened into the
squared groove upon assembly.  When they've been in service for many
years, they harden and take a set into this square shape, and it's only
a matter of time before they start leaking, as they no longer are the
same size or shape.  You may be able to get it to seal up again by
adding some sealant and re-assembling the parts, but it really just
needs a new o-ring.

Mike B.

On 11/1/2012 11:41 AM, JRodgers wrote:
> There is a gasket that seals the valve to the body of the heater. The
> gasket is not like an "O"ring - round in cross section, but is instead
> square in cross section. It fits nicely into a square shaped groove.