Seem to have a problem with eith my wheel or tyre, as the tyre pressure keeps dropping to 15psi.

There is no damage at all to the tyre, and no visible cracks on the wheel, and the valve isn't leaking.

A friend of mine who works at a tyre place has submerged the wheel and tyre in a tub of water and has spend over an hour checking them both and cannot find any leak, so is obviously only loosing air when the weight of the car is on it.

Anyone know what could cause this?

The mystery has deepened as the spare wheel i have fitted to the car is now doing the same, but i haven't checked that one for any leaks yet.

The car is parked on a flat driveway, so not as if it's parked up the kerb at an angle at all.

 

HOw long does it take to go flat... maybe someone's playing mind games with you and letting it down overnight.....??

 

Thats what i thought at first, but it only holds pressure for about half an hour. Pumped it up the other morning when i left home and was down to 15 psi again by the time i got to work

 

Could the valve be damaged, and leaking under pressure once its on the car

 

They checked that. Even inflated to 50 psi and submerged in water there is no sign of any leak

 

Sounds like you'll have to submerge the whole car then....

 

Is it possible the spring which keeps the valve closed is weak? Then the centrigugal force of driving causes it to open. Replace the valve (not the stem) and see what happens.