Bicycle wheel with tire pump attached

Rotten to the core

Here we go again

Overall Kuroko’s new Panaracer tires have been a huge improvement over the René Herse skins with their weeping sidewalls. But a few days after I first got the GravelKings on, I noticed the rear had a slow leak. It would be fine for the course of a ride, but over a couple of days it would lose most of its pressure. If I left the bike sitting for a week, it would be totally flat.

I’ve let it go for a few weeks because I was fine with topping the tire up before each ride, but I knew that I wanted to sort it out sometime. As this afternoon was warm and wind-free, and I had some free time on my hands, I knew this was the perfect chance for some maintenance.

Bicycle tire resting on bucket of water
Not quite what I envisaged

The time-tested method for finding a slow leak is to inflate the tire and immerse it in water. I pumped the tire up to its limit, filled the bucket to the brim with water, and … the result was laughably unlike what I had envisaged. I suppose I could use the bathtub, but I could imagine Nana’s reaction when I carried the wheel into the bathroom, even after I promised to clean the tub when I was done. Instead, I filled the bucket with soap suds and used a brush to lather the suds all around the tire where it meets the rim.

Soap bubbes emerging from bike tire valve

I inspected the rim carefully all around the bead on both sides of the tire. I didn’t find what I was looking for, and so I lathered up the tire again and had another look. Around and around and … nothing! I set the tire on the floor and pondered my next step. The bathtub suggestion kept peeking its head out from the corners of my thoughts. And then I saw it! Bubbles forming around the valve core!

I took off the valve cap, lathered up the valve again and watched: no mistake, the valve was the source of the leak.

With the source of the leak identified, I quickly loosened the valve core to inspect it. (Note to self: Let the air out of the tire first next time.) There was a bit of grime and latex sealant stuck to it, so I spent some time cleaning it up until it was spotless. I also tried to clean out the valve stem with some twisted paper towel. Then I reinserted the core in the valve and filled the tire.

Once again, the soap bubbles betrayed the leaky valve. I used the tool to make sure the valve core was in as tight as it would go, but it made no difference. The bubbling continued to grow as the air continued to leak.

OK, I have a spare

In order to replace the valve, I needed to remove the tire — at least partly. I worked the tire levers with care to avoid spilling the latex sealant inside the tire. I was mostly successful. Of course a little seeped out onto the rim, but it cleaned up quickly with some paper towel. And I only needed to remove one side to get the valve out.

Bicycle tire half removed from rim showing liquid latex sealant inside
Not a huge amount left

There was not quite as much sealant left inside the tire as I imagined. This allowed me to work without creating a huge mess, but I think I should probably add some more before I get a puncture.

I’m sure if I were to search through the parts bin I could come up with several spare valves, but the first thing that came to mind was the one I’ve got on the spare front wheel, which has been sitting unused on the balcony since my return from England nearly two years ago now. I’d put a new valve in a couple of weeks ago in preparation for mounting a new tire, but as I haven’t seated the new tire yet, it was a moment’s work to get the valve out and insert it into this rim.

Tire levers, tire valve and valve core
The new core (top) and the bad old valve (center left)

With the new valve in place, it was the work of a minute to get the tire back on the rim, wipe up the leaking latex, and prepare for inflation.


I charged up the air reservoir, connected the pump head and turned the tap. Immediately the tire gave a satisfying Pop! Two more, louder Pops! followed in quick succession as the tire seated itself on the rim. I hastily removed the pump, screwed in the valve core, and then inflated the tire once again. In about a minute I was lathering up the new valve and looking for leaks.

Bicycle tire with large soap bubble behind valve
OK now it’s leaking in a new spot

Immediately I noticed some bubbling around the valve, but this time from a new location. With the old valve, the bubbles had been forming around the valve core at the end of the stem. This time the bubbles were coming from the base, where the valve emerged from the rim.

Bicycle tire with small soap bubbles around valve

I picked up the wheel and swirled it around, allowing the sealant to coat the interior in every possible location. After a few seconds of swirling, I set the wheel down and once again lathered up the valve. This time no new bubbles formed. I waited a minute or two, reapplied the soapy water and watched: no new bubbles!

I’m not going anywhere the next couple of days. I’m going to check the tire again tomorrow. If it’s low again I’ll remove the valve core and add some sealant (which I forgot to do in my haste after the tire inflated with the new valve) and try once again. Regardless, I’m confident Kuroko will be better off now that I’ve disposed of that rotten old valve core.

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3 responses to “Rotten to the core”

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  3. […] had the same issue with the rear tire just a couple of weeks before, and had quickly fixed it by replacing the valve with a high-quality unit from Schwalbe. In that […]

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