Sometimes difficult to diagnose with the tyre fitted as there is no visible damage to either wheel or tyre. Normally occurs with older wheels as multiple tyre changes inevitably leads to corrosion of the channel between inner and outer rim, where the tyre sits. This corrosion results in the tyre not sitting flush on the wheel and air seeps out between the two.
Remedial work involves removing corrosion, repairing the channel and recoating to ensure a good seal between tyre and wheel. The tyre can then be refitted, valve replaced and the wheel rebalanced.