We are having a problem with the left caster wheel on our 2001 Metro HP. It will not stay inflated and it seems there is not a leak in the tire itself. The wheel seems to be leaking air between the seal of the tire and the wheel. The manual says that the air pressure of the tire should be 22 psi but the tire can hold up to 50 psi. We only put up to 22 but feel this is not enough pressure to hold the seal. What is your advise?