Have you tried dripped the wheel in water, most tyre places can do this, see if and where it's leaking.
I presume the valve has been changed each time, sometimes you can get grit in it and it'll slowly leak.
I have a wheel on another car that slowly leaked around the rim, where the tyre bead fitted was a little corroded, kept having to wire brush the flacky shi'te off and use a bead sealer, Prem do a sealer that just paints on TYRE BEAD SEALER at AP-Tech
Hard to tell what wheels you have, but mines a 2004 SE 2.0D and have 17" Caymans, might be worth replacing the wheel altogether
17" Cayman Alloy Wheel & Tyre - 225/45/R17 - X-Type Parts
I presume the valve has been changed each time, sometimes you can get grit in it and it'll slowly leak.
I have a wheel on another car that slowly leaked around the rim, where the tyre bead fitted was a little corroded, kept having to wire brush the flacky shi'te off and use a bead sealer, Prem do a sealer that just paints on TYRE BEAD SEALER at AP-Tech
Hard to tell what wheels you have, but mines a 2004 SE 2.0D and have 17" Caymans, might be worth replacing the wheel altogether
17" Cayman Alloy Wheel & Tyre - 225/45/R17 - X-Type Parts