I have a tire that goes flat every few days. I thought neighborhood kids were messing with me so I inflated it to 35psi and put it in the trunk. Two days later it's at 18psi. I don't see any punctures on the tread or sidewall. There might be a patch on the inside that's failing. But first I'd like to replace the valve.

1) Can I swap valve innards with the spare or does removing the innards ruin them? I've done this with bicycle valves.

2) Can I replace the entire valve with basic tools? I have a tool to pull and seat a new valve but can I press the bead off the rim without damaging the tire or myself? I'm 99.44% sure my compressor will [re]seat the bead.

Thanks,
Sixto
MB-less

I would try swabbing the tire and the valve with soapy water before going forward. If it's losing 18 PSI in 48 hours, you should see some evidence of bubbling.

you can swap out the innards just like a bike tire. You could also change the whole valve if you can break the bead. using a jack and your car as the weight, you can often break a bead. Cut off the inside portion of the valve and it will remove easily. If you can't reset the bead with your compressor, spray some starter fluid inside the tire, spray a tail away from the tire and light it up with a lighter. The explosion will usually set the bead. Lots of videos on that technique on youtube. But you should confirm it's bad first using pc's method.

__________________
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Don't forget to swab the wheel as well. A cracked rim will also allow air to escape.

What is this for? A lawn tractor, fine. A truck tire that is E rated, probably not.

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Since it is off the car I would just take it to a tire shop and tell them to find the leak. Around here that is $10.

Use an old spray or squeeze bottle of some kind and mix some water and a little bit of dish washing soap. Inflate the tire and spray the mixture all around and look for bubbles. Check the valve stem and core first. If the core is leaking you probably can just tighten it up. Check all around the bead and everywhere.

Don't assume a leak because you can't find it elsewhere.

If the soap shows the leak around the valve stem and it needs to be changed, you will need to break the bead. Not an easy task without a tire machine. If you can find an old fashion bumper jack and something to lift with it, set the base on the bead with the valve core removed and you can break the bead and push it down enough at the valve stem to cut the old one out and pull a new lubricated one in place. You will need to lubricate the bead before seating it. Most any compressor will make enough pressure to seat the bead if the tire properly fits the wheel with the correct tire and wheel widths.

If the problem is only the valve core, just screw in another.

Be warned that even WITH a tire machine this hard work. With a bumper jack or some other makeshift equipment, it's a back breaking job.

When I was a teenager with no money I changed tires all the time with nothing but a bumper jack and a bumper jack handle. It was the only way I could keep my old 48 Chevy drivable.

I would expect that a tire store would charge less than ten bucks to take care of this.

Hope this helps.

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Annoy a Liberal, Read the Constitution

Tire bead leaks are probably the most common today. As for the valve just put some moisture on it as surface tension will make it bridge the small stem diameter. If the bubble expamds upward you are leaking through the valve.

Human spit has enough viscosity to bridge the hole of the valve stem easily. No sanitary issues as nobody is going to be mouthing the valve stem. In fact a dog may wash it off when on the car.

Since the tire is off the car. Just inflate it. Put it on its side and fill the bead area with water or a soapy solution. Nothing bubbles? Flip the tire over and do the other side.

Certainly it could be the valve stem but odds are it is not your issue.