Inner tube leaks through valve when on the wheel

[Airlane1979]

I found my front tyre deflated. On examining the tube I found no leak. I inflated the tube and it stayed inflated. However, when I put the deflated tube back into the tyre and on the wheel and re-inflated it, the tube leaked through its Schrader valve. What might be the cause and solution?

 

[T4tomo]

the valve is farked, additional pressure of being constrained inside tyre is making the leakage faster and hence noticeable.
....get a new tube....

 

[Airlane1979]

the valve is farked, additional pressure of being constrained inside tyre is making the leakage faster and hence noticeable.
....get a new tube....

Thanks. I'll get a new tube. I wonder why the valve failed? First time in 50 years' cycling.