Tires: pressure loss in all four tires

Question
I've Michelin latitude tour P225/R65/17 100T on my 2007 Honda CR-V. I put all four tires in Feb 2012 and have about 19k miles (mix of city/hwy) on them. For last few months I've noticed that all four tire are losing pressure at constant rate. I fill 32psi and they will go to about 28psi in ~15 days (couple weeks). I've noticed it for last 3-4 months (possible that it was like that earlier also). I've done the rotation and have them check valves also. This winter temp are fluctuating a lot in Florida (-/+20F) and tire shop is saying that is the reason. I understand pressure loss because of cold but I'm not sure if that is the cause here as I don't see it in my other car. Will appreciate any pointers or things to check.

Thanks

Answer
Themteverst,

That is unusual.  

On the one side, leakage due to the wheel corrosion or tire imperfections wouldn't be consistent.  One or 2 tires should be considerably worse than the others.

On the other side, if the temperature were causing this, we're talking about 40°F difference - and surely it doesn't fluctuate that much between filling the tires and measuring them later.

So try this:  Increase the inflation pressure to 40 psi.  Drive home and let the car sit overnight.  (Obviously, you'll want to fill the tires immediately before driving home.)  Adjust the pressure the next day to 32 psi.  Then wait a week and measure again.

The purpose of the over inflation is to try to seat the beads thoroughly, just in case that is the problem.  Also allowing the vehicle to sit overnight before adjusting is to make sure it has settled down and to provide a consistent reading.  

If the loss is still there, then try it again.  If if that doesn't work, try a leak test.  Take a tire off and immerse it is a kiddie pool and check for leaks.  The leaks will be slow and hard to see, and patience is the key to this.

Good Luck