I just stumbled across this forum and have been digesting this wealth of information. I have one quick tip that I thought I should pass along to add to the knowledge base here.
My flow switch went bad earlier this year and I didn't want to pay the ridiculous price for the replacement. I thought about trying to build my own etc. Finally I was able to determine who manufactured the switch for Goldline. I was able to call them directly and order the part and they shipped it directly to my house.
I believe the Goldline part number is: GLD-451-1001 or GLX-FLO-RP
Here's the manufacturer info:
Harwil Corporation (Oxnard, CA)
PH Numbner = 805-988-6800
Part Number = Q-12DS-C2 (Description = Goldline-2/No Tee)
The folks at Harwil are very friendly and helpful. I paid $80 (including shipping) for my new switch. This is considerably less that I could find on the net.
Hopefully I can help someone else save a few bucks by posting this on the forum.