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I ride my cruiser to the supermarket and back, using either a trailer or a basket over the front wheel for carrying groceries. The bike has developed a persistent slow leak on the front wheel, and I've replaced the tube without it helping. I can't see anything puncturing the tire, and the spokes aren't poking through the rim tape that I can see.
These are 26" wheels with low pressure (around 40 - 50 psi). What else can I look for that might be causing this?
Mark your tire where the tube's valve stem is located, remove the tube, inflate it and put it in a bucket of water. Look for bubbles. This is where your leak is. Now inspect the tire and rim at the correlating point for something that may be causing the leak.
Good Luck.
Another trick for finding leaks that I don't see mentioned here is soapy water. Pull the tube out, and make a preparation of very soapy water. I particularly like dish detergent (e.g. dawn) because it is so concentrated, but any soap should work. When you brush soapy water over the tube, the air escaping from any leaks will form soap bubbles. If you don't have a great place to submerge it, this trick can be a lifesaver.
When you're looking for a slow leak, you want to find the cause of the leak. To do this, it's important to mark the tire, the wheel, and the tube in some way so you can figure out exactly where the hole came from. You can use the valve stem on the tube and the rim to figure out where they fit together, but you should mark the tire to indicate where the tire sits in relation to the valve stem. If you do this, and keep them oriented the same (i.e. don't flip them over as you work on them), you should be able to find the leak and to find the place in the rim and the tire that's near the leak.
Once you find the spot, inspect the rim and the tire. Any problems with the rim should be fairly obvious if you run your finger over them; check it and move on to the tire if you don't find anything. When you are inspecting the tire, start by running your finger over the offending surface. If you feel something sharp, be sure to dig it out. If that doesn't work (and with a really slow leak, it often won't), you'll need to get more aggressive. Turn the tire inside out, and fold it over in that area. The fold should stretch the inner rubber wall, and any holes caused by punctures will be more obvious. Find the hole, and then dig around in the hole with a small screwdriver or something. I've often found little shards of glass or bits of wire embedded in the tire itself. You can't feel it, you can't see it, but it can still puncture your tube when you go over a bump just wrong.
Before buying a new tube, check and see what your problem might be. Slow leaks can be caused by a bunch of things. First of which is obviously something making a tiny hole. The next could be that your tube is old/degraded and air simply leaks from it. Or you may have some damage around the valve.
It might also help to know how often you ride your bike. For some reason that still alludes many cyclists, air just happens to leak out of your tire quicker when you're not riding it. Compare your tire pressure after riding everyday, to having put the bike down for a week or two. There's a significant difference in pressure.
Anyhow, on to business. Remove your tire from the rim, and check all along the inside of the tire, as well as the sidewalls of the tire to see if you feel anything sharp, or piercing into the tire, as this could be causing your air leak. Check if the edges of your rim are sharp, and check to see that your tire isn't pinching your tube. I also recommend checking your tube for any holes. I suggest pumping your tube (outside of the tire) up until it expands beyond its expected size. There is no harm in doing this. The more pressure inside the tube the easier it will be to hear the air escaping. Of course if you don't hear any air escaping it will help to place it in water and look for bubbles, and like someone else said before, move your tube around, bend it in different shapes as some holes are so small, or in such a fashion that they only show under those conditions.
It's possible that there is a small shard of glass or metal imbedded in the tire that you're missing. That's the only thing I can think of since you've checked the other obvious potential problem areas. Mark the position of the valve stem to the tire and the remove the tube a check for that slow leak. Hopefully you'll find a small hole that's causing the problem. Then allign the valve stem mark to the tire so you can check the area of the tire where the leak showed up for a shard of glass or metal. If nothing is visible from the outside, carefully feel the tire carcass with a finger on the inside to see if you can feel anything that is sharp poking through the carcass. Remove anything you feel and hopefully you can patch the tube and not have that slow leak any more.
Ok, as an addition to all the answers.
There is a possibility of the puncture being too tiny. And cannot be found easily, through submerging techniques or any other.
On that case, only one last thing remains, inflate the tire at least 150% or double it up, if it is resistive. And then submerge it in the water. You will surely find the puncture.
This sort of puncture occurs in old tires or bad quality tire. And this method should only be applied as last option.
happened once ;)