Tubeless Setup Issues - Slow Leaks

Posted by Jerry Chabot on

I get a lot of questions from customers a year down the road from a wheel purchase looking for help setting up a fresh pair of tubeless tires. There are many MANY misconceptions about what is happening with your wheel and tire when you are faced with a troublesome slow leaker or struggling to get a new tire set up. I am going to limit this particular discussion to getting consistent pressure to hold, as opposed to mounting tires. That is a whole other discussion.

So I get a call from someone saying my new tire sets up but won't hold air. OK. Very common. I ask a few quick questions just to take away any incorrect assumptions on my end: Is the tire new? Did you add sealant? 

If the answer to the above is yes, I am 85% sure the issue is the tape, and I say so. That always triggers a long convoluted response as the customer tries to convince me that there is something weird going on elsewhere, because they have pumped this thing up so many times and spent so much time "tracing" the leak, and have convinced themselves that it is something complicated, other than THE TAPE. 

TL:DR It's the tape. 

At this point I have them confirm that the valve stem nut is pretty tight, and that the valve CORE is actually tight and did not unscrew a tiny bit when they pulled off the pump. These new (I am old) screw on pump heads are a bit of a PITA, in my opinion, and the oldest pump head which is also the simplest pump head is still the BEST pump head - the big brass Silca push on. They can pump up track tires to 160 pounds they can damn well get 55 PSI in your gravel wheels. But here we are. Unscrewing valve cores like it is our job. 

If both of those checks pass, I am 99% convinced it is the tape. Ok sure, you can have a faulty new tire with some kind of bead issue, but the sealant should really seal that up.

At this point the normal response is "no no, it is the valve stem, it is leaking out of valve stem I can feel it". Sometimes people have tried putting a piece of inner tube or bigger o-ring under the valve stem lock nut trying to seal it up. Often times it helps which just reinforces the thinking that its not the tape. 


OK, so here is all you really need to know:

  1. The air belongs between the tire and the tape and no where else
  2. The sealant belongs between the tire and the tape and no where else
  3. If air is coming out ANYWHERE but out of the tire casing or the bead area (technically still part of the casing), IT IS THE TAPE
  4. That is it. Stop thinking. 

Why do I know it is the tape? OK, so here is the thing, every time you have a tape failure, the air comes out around the valve stem. Every time. Each time. Always. Every every. No, seriously. That does NOT mean that you have a leaky valve stem! Please do NOT try to just tighten the nut more with pliers until you damage the rim. Please do NOT try to add a gasket of sorts under the nut to stop the air. IT IS THE TAPE!

The dirty little secret here is that the spoke nipples actually seal up against the rim and hold air. Those little suckers are holder 120kg of force, they are TIGHT. No air of any significance is able to leak out around the nipples. Instead, all the air just goes out that big old hole in the rim where the valve stem pokes out! Duh. 

A leak in the tape will allow air INTO the rim cavity, and it try to get out, and the easiest way out is the valve stem hole. Valve stems normally come with a nice little o-ring to protect the rim from scratches. That is the ONLY REASON THAT O-RING IS THERE. The stem seals on the interior of the rim and the big rubber bung is very easy to seal and does not need to be super tight. If you try to make the o-ring seal, or add a gasket in there (like a piece of inner tube... sigh...) all you are doing is SEALING UP THE RIM CAVITY. 

Pop Quiz: Where does air belong?


If you have a tape leak and you seal the stem hole you are now turning your entire rim into an inner tube. They are not meant to be inner tubes. The fact that the nipples seal means that if you tighten that nut enough you can probably turn your rim into an inner tube. Please don't do that. FIX YOUR TAPE

Here is a wheel I fixed recently up because it would bleed down every couple days. It held pretty good, it only got soft after several DAYS... Tire looked good. Sealant was fresh. 

Wheel had my (great) valve stems with the nice fat o-ring installed. Nut was normal hand tight. Here is a photo after I backed it off.

Quick inspection shows telltale fluid around the nipples where a little sealant has migrated out of the tire, into the rim cavity, and been pushed out the nipple hole and dried and collected dust. I thought nipples sealed? They do, but if you put 95PSI behind them and then put sealant fluid on top of them, it is going to get pushed out a little bit. Your nipples are also going to corrode and fail, just FYI. 

I removed the tire (looked new) and inspected the tape....

Hmm, that looks suspect... lets poke it...

Yup, that is a wide open hole. This is a tape failure mode. This is pretty common on road wheels with only one layer of a thin tape. I always use two wraps of tape on high pressure wheels because the tape will stretch under pressure and eventually rip like this. Two layers, not for tire fitment, but to withstand the pressure of a road tire. Even with two wraps, the tape can rip during set up if you hit it with a high pressure bottle or compressor. 

So what happened here was the darn nut sealed up the interior of the rim like a giant auxiliary air tank and allowed the tire to hold air pretty well, actually. Lesser quality rims have been know to blow up and split when this happens, especially deeper road rims. You can pop a deep rim like a balloon by trying to stop that pesky air leaking out around your valve stem.

Stop. Think. Follow the money (the air). How is the air getting to the valve stem hole? What is the ONLY way it can end up at the valve stem? THROUGH THE TAPE! That is the only way. Check your tape! Better, every time you replace tires, just put on new tape, please. It really does not cost that much (I will sell you a 60yd roll for 20 bucks). It is just good practice to freshen the tape, tire, and sealant together while you have it all apart. Maybe lube the freehub while you are that deep into it, ok? Your wheel builder will thank you.  

That's really it! To review, repeat after me: It. Is. The. Tape. 

Thanks for reading!