checking the integrity of & changing the O-ring on a SVEA 123R

I would just replace them all.

You may want to ck the “rope wick” that extends into the fuel container also,  because if it is burn or singed  the stove will not work properly. Easy to replace once you have a replacement.

I would replace the fuel cap washer and the safety release ‘pip’.

Search https://classiccampstoves.com/ for all the advice you are ever likely to need.

https://www.ebay.com/itm/123800060064?hash=item1cd30f00a0:g:69IAAOSwiqBcU3wF

Thanks, all, for your help!

If it ain’t broke, don’t fix it!

eBay to the rescue!

I was going to mention the little air pumps that make priming them easier.  It lets you pre-pressurize the tank first and then fill the priming ring just by opening the throttle (not dribbling from your fuel bottle or using an eye-dropper), and causes it to kick off a little sooner and to function better in really cold weather.

But thought I’d check availability of them first.  $39 used ones and a classic NIB one for $99 on Etsy and eBay but also BRAND NEW aftermarket ones.  For $14 + $10 S&H.  With the fuel-tank adaptor cap (which the pump requires – it’s a replacement cap with a check valve).  So you get a brand new cap and gasket plus the air pump for easier starting.

https://www.ebay.com/itm/113782083977

In the 1970s and 80s, I always brought the pump with me – it made it all easier and made for less spilled fuel.