What's wrong with the valve caps Ptown?
Presta: They don't do anything apart from stop the valve puncturing the tube when rolled up. When the tube is in the wheel they serve no purpose.
Schrader: They prevent dirt and water building up in the valve mouth, which can be blown into the tube when inflated. People use them.
Does a valve cap on presta also not prevent corrosion of the valve itself?
No. They're stainless, I think.
Someone will correct me I'm sure.
Either way I've never had one corrode. I'm sure the rubber will perish before that could happen.
@dazzrie started
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Presta: They don't do anything apart from stop the valve puncturing the tube when rolled up. When the tube is in the wheel they serve no purpose.
Schrader: They prevent dirt and water building up in the valve mouth, which can be blown into the tube when inflated. People use them.