I have been buying 12 kilograms of bottle packs once a week and it seems like the valve on back wheel can't handle it. I have been getting valve failures and last time I noticed air leaking but it was only due the valve being loose. It is only short ride but there unavoidable bumps and kerbs to go over. The big basket is on the back and putting a two bottles (4kg) on the front basket doesn't help which naturally makes steering harder. I checked a website for air pressure and that extra weight make a difference but I am sure it is. Note that you can't buy a air pressure gauge in Japan (I intend to buy one online) Is there anything special I can do prevent a valve failure? I weigh about 86 kg. The tire size is 27 inches. According to my reading of the bicycle tire specifications which is the bicycle explanation area (all in Japanese) the maximum carry weight for the rear basket is 4kg and front is 2 kg and pressure range is 800-600 kPa (116 psi- 87 psi) Unclear how specific to this bicycle these details are- codes in explanation not specified elsewhere.
If you can give us a photo of a failed valve, that might be helpful.
I suspect that you have a subtle sharp edge in your rim, at the valve hole. The added weight is causing the tube to grind more on the sharp edge and cutting though. It's curious that you haven't noticed the same problem while riding unladen- should still happen but over a longer time window.
Your fix might be a couple of minutes with a small file or deburring tool to round-off the lip. There's a chance that replacement rimtape might help mitigate the problem, though smoothing the hole is my first suggestion.
It is conceivable that your current and spare tubes are a bit weak in this area too. Good tubes have extra thickness around the valve stem base.
I've run a range of tyres and tubes with heavy loads on my bikes, and have never found loading up to cause problems. An occupied child seat, for example could put 30kg directly over the back wheel, and my light touring setup is heavier than your shopping. I have had rim issues, but that's another story.
This is with a range of tyres from 28 to 35mm, so probably not far off what you've got. The problem will be something different. When you say the valve is "loosening" if you mean the core is coming loose, you can get valve core tools and tighten it up. If you mean the spring (on a Schrader/car type valve) or air pressure (Presta) isn't holding the air in, then I'd wonder about dust in the valve - check your pump, but also let some air out to blow the valve clean.
One possibility that would fit with the valve both failing and being loose is that you're using tubes meant for a smaller hole: Presta tubes in a rim drilled for Schrader will last a while, but will wear around the valve stem and leak. Either get the right tube, or use and adaptor grommet to convert the hole size
Very strange. As far as I’m aware all valve types used on bicycles should be able to handle 10 bar or more. Even on road bikes with narrow 23mm tyres you’ll usually stay below 8.5 bar.
With luggage on the rear rack the most common problem is damage to the rear wheel. Everything from pinch flats (due to insufficient tyre pressure) to broken spokes or bent axles. But I’ve never heard of pressure being a problem.