air leak on a auto siphon

I don't know if you did a search and didn't like the threads you found, but I recall several threads asking this question. Seems like many answers were 'not to worry', but read thru these to see if the scenarios in the threads fit yours.
https://www.beeradvocate.com/community/search/117705936/?q=siphon&o=date&c[title_only]=1&c[node]=8

 

I've had two such experiences with mini auto-siphons over the years.

1) after years of service ... air began leaking around the rubberized gasket at the bottom of the plunger. No could fix.

2) air began leaking around the junction between the plunger and the hose b/c the hose opening had become stretched.

Two fixes: wrapped a course of blue painter's tape around the junction. Also wrapped a course of blue painter's tape around the plunger tip end before inserting it into the hose. Both stopped the bubbles; however ... found the second fix worked the best.

 

Check to see If the gasket has cracks or dry rot. If it does it'll need to be replaced. I don't know if/where the gasket can be purchased separately.
Also inspect the barrel for cracks or other deformities, paying attention to where the beer level was on the barrel when you first noticed it was drawing air.

I had a similar leak to HerbMeowing's first issue....leaking around the black gasket at the bottom of the plunger. Mine was deformed due how it was stored.
If the gasket is only deformed and not cracked, try soaking in some really hot water to soften then immediately putting the plunger/gasket back in the barrel to re-form it. That worked for me.

Depending on how bad the leak is, a quick-fix is to add some distilled water to the barrel on top of the plunger (starts at 45 seconds into the video):
http://beersmith.com/blog/2016/01/18/how-to-fix-a-leaking-autosiphon-seal-beer-brewing-quick-tip/

 

My experience with that was air getting sucked in where the hose attaches to the plunger. It seems the hose was getting brittle with age/use. I put a dab of Keg Lube on the tube and reinstalled the hose. Worked well enough until I bought a new length of hose.

 

Just looked at one of mine. That part of the contraption is a three-piece assembly; center tube, gasket and a gasket ‘holder’.
Still don’t know where to get just a gasket, but spare center tubes are available for 1/2” and 3/8”. Search for Auto Siphon (or Auto-Siphon) spare center tube.

On mine it looks like the gasket ‘holder’ is just glued to the center tube. Maybe the glue failed.

 

I had two Auto Siphon units fail, both leaked at the gasket. After that I gave up and returned to my old method. Fill the racking tube with Star San solution (water works but I'm a neat freak and don't want to risk contamination), lower the output end to a waste container and let it flow until wort fills the tube, then finish racking into the bottling bucket. Works every time, no bubble worries.

 

not trying to convince anyone...but I was so close to pulling the trigger one of the new stainless siphons at homebrew con. I don't use my auto-siphon enough to justify the $, but they do seem really nice...
https://www.brewsssential.com/products/brewsssiphon

 

Those don't work mine didn't. Use gravity or your mouth...haha I could make quite a list of brewing equipment that doesn't do what it supposed to do.

 

Sorry to hear your autosiphon didn't work. But most do.

 

Get some aquarium grade silicone from the pet store to seal the leaks you find. It's cleaner than food grade because most fish are very sensitive to impurities in their water. Use soapy water to find the leaks. Of course, air injection might be beneficial when siphoning wort; not so much with finished beer.