So when VMC/IMC how do you "know" when your static source has a loose connection and is reading cabin pressure? Had it in a 76 once.

Gypsymagpie - are you sure it wasn't a combined static/pitot probe? Most systems have the water drains in the plastic tubes running from the probe to the instruments/ADCs.

The SODPUD is an old mnemonic from days when separate pitot and static ports were the norm.

I’m not qualified on the D model but I recall that the S76s I flew (A+, B, C & C+, all SPIFR) all had dual static systems, vented on each side of the tail cone, which were are heated same as the pitot system to prevent icing. I don’t recall there being a switch because in effect there was redundancy from the start.