What about the quality of the different R-12 brands out there? Is there any difference? I've seen a few different brands:

DuPont
Sercon
ColdShot

Saw these brands for sale at reasonable prices on Ebay. All state that proof of certification is required for sale. Anyone know if they actually verify your certification? If so, how difficult is it to study for and pass the 609 cert exam?

TIA!

~Paul

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'91 300E, 212K miles