I’ve been losing 1-2psi every day whether I’m riding or not. I’ve taken it to a garage. They checked the tyre and the valve, but could not find any leaks and suggested I use tyre sealant slime. I have done that now, but the situation remains unchanged. I’ve also replaced the valve cap and used two different pressure gauges.
Slime is nasty shit and a lot of places will charge extra to clean that crap out when the tyre is replaced. The best way to find the leak is too over pressure the tyre and put in a trough of water and look for air bubbles. Or use really soapy water.
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How old is the bike? It could be a rim leak if the rim is corroded on the inside. They are harder to find. They can clean the rim to prevent it happening in future. That requires taking the tyre off.
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I bought a small kiddies inflatable paddling pool that I slip under the wheel and then fill with water. Saves having to remove the wheel. For one particular leak it worked really well but generally a spray bottle with a very very weak solution of washing up liquid works better. Just spray on and wait.
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Sealant slime is Bad News. I learned many years ago that fitters hate it with a passion and as Kevsta says they sometimes charge extra to clean it up.
A spray bottle with soapy water is a Good Idea, you need patience though if it’s a really slow leak…
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I’ve bought a new wheel in the past with good tyre on it for less than the cost of a new tyre fitted - that might be worth a go. In my case I was also able to sell the old wheel. Of course, depending on your bike this might not be an economical solution
I’ve heard about slime being a bit of a pita to clean for fitters, but if even a garage can’t find the leak and end up suggesting to use slime… might as well.
I tried looking for the leak myself with the methods suggested, but hadn’t had any luck either.
That being said, the leak has stopped. No pressure loss in two weeks now. I haven’t done anything apart from applying slime, but the tyre was still losing pressure for 2-3 weeks after that, which seem suspect. I guess I shall call it divine will and praise the tyre gods.
The slime held 
But I managed to get a couple more punctures in addition to the one I already had fixed, so the tyre had to go 
Had it changed a month ago. Not sure if I didn’t notice it at first, or it was fine, but the leak is back 
Triple checked the tyre - no punctures.
I take it might be a rim leak like @Joby suggested. A new oem rim is a cool £1000. A used one is about £45. I’m trying to decided if I should buy a used one or ask a garage to fix the existing one. Which option has a greater chance of success?
When I had rim leaks on my car Kwikfit just cleaned the rim which fixed it. It was mostly internal corrosion. I had all 4 wheels stripped and refurbished at Lepsons and I’ve not had a rim leak since then.
@Slacker From what you have described it appears that the leak has been caused by a build up of soap on the rim from everytime the tyre has been changed. A quick removal of the tyre, clean all the crud off and it should be fine, no need for a new wheel.
Be careful with the internal sealants such as slime. I have dealt with a number of crashes which have been caused by the sealant causing imbalance in the wheel leading to a wobble and weave situation as it counters the balance weights put on when the tyre is new and balanced.
And as mentioned, many dealers do hate this stuff for a whole number of reasons. Not least because even if cleaned it can be a nightmare to get the new tyre to seal and the residual can also cause the tyre to turn on the rim (back to another imbalance issue).
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As above, had the same issue with The 250’s rear wheel. Rim leaks caused by a build up of left over fluids used to lubricate the bead/rim from previous tyre fittings. Solved by deep cleaning the bead seating on the rim with soap, water and gentle agitation with a scotch pad. A quick job carried out by my tyre fitter at no extra cost to their £10 fitting charge.
Others areas where pressure can be lost are at the valve seat and valve core, remove valve cap to check. Soapy water sprayed around the rims and valve will reveal all, be patient with your observations as some leaks are not immediately apparent.
FWIW, these guys are amazing
Took the bike to the garage and asked them to clean up the wheel and replace the valve. Sadly, that did not help. After 1 week of riding (~200miles) I’ve lost 8psi in my rear and close to 0 in my front.
So, I’m back to square one. I either chance it by buying a used wheel off ebay and get that fitted (about £70 all in all) or ask the Motoliner guys mentioned to repair it which will set me back £170.
If it were me, i’d go option 2, as you could go option 1 and have a similar issue. If 2 fails you still have option 1, and even if you had to do both still cheaper than an OEM replacement.
Schoolboy error in telling the garage how to do the job, asking a garage to clean up the wheel and replace the valve is very different from asking them to cure an air leak! Note too a puncture is an air leak but not all air leaks are punctures. The first job any reputable garage would be doing is verifying the customers complaint. Once the garage has verified an air leak the cause and means of repair should become obvious. For the future you should only be telling the garage what you know and what you want i.e. “the rear tyre is losing 8psi over the course of 100 miles in 7 days, can you fix it and what will the cost be?”
Fair enough, though, I was being a bit brief here. I did explain to the garage that I have a slow leak, and I had it also on my previous tyre. That another garage could not find a puncture, so I suspected there might be a leak in the rim which some cleaning might help. That I’ve never had the valve replaced (in the 10+ years) and it’s been abused by me quite a bit so might as well get that replaced. The mechanic said he tried ovepressuring the tyre and could not find a leak, so he just did what I asked. It was worth a shot I guess.
It surprises me that a weekly loss of 8 psi cannot be detected. Paint the entire tyre, rim and valve (cap off) with soapy water, keep wet and look for the bubbles. Those bubbles in that first image look suspiciously like the point of a rim leak to me.
Well, who knows how they checked 
That’s how I did it last time. No bubbles from the valve or on the tyre. But in a few places on both sides they were bubbles on the rim.
How is it you can find the air leak and the garage cant? You need a garage that knows what they’re doing. I’d take it back and kick up a right royal fuss.
If I was a gambling man I’d wager, for whatever reason, the previous tyre was fitted using a tyre bead sealant to seal the bead, you got a puncture, the low tyre pressure allowed the tyre to flex or rotate on the rim breaking the seal, puncture repaired leaving you with a rim leak. When the new tyre was fitted the bead seating was not cleaned and/or the tyre was not fitted using a tyre bead sealant.