Well, I removed the valve key cover and inspected the gaskets. In retrospect, the o-ring could only have caused leaking around the seam in the key cover, and not from under the handle itself, where I am experiencing it.

The spider gasket "appears" OK. It hasn't deteriorated, and doesn't appear especially deformed. I didn't want to remove it from its groove and inspect it because I have nothing to seal it back with handy.

This may all be a moot point as last night I submitted a service question on the Hayward website. A while ago I received a reply...

Parts for the 711 valve are not available. The valve
has been replaced by SP0714T

Regards,

Richard Burns
Technical Service
Hayward Pool Products
1 Hayward Industrial Drive
Clemmons N.C. 27012 (908) 355-7995
Soooo, looks like I'm getting a new valve after all.