Brakes intermittently locking when I go in reverse

[dandastic]

When I put my 2008 Cayenne in reverse the antilock brakes kick in. (both rear and driver side) It is worse when I am turning the wheel. (Backing out of parking spot) No codes are showing but I do have the engine light on. I thought it was the gas cap and replaced it. I have replaced all of the brakes.(pad, calipers and sensors) I have replaced the spark plugs and coils. The battery is fine. I have taken it to a porsche shop and no codes come up. They suggest it may be the transfer case. I am curious and would like to hear if this has happened to anyone else before I replace the transfer case.

[Loren]

Sorry your check light is on or off?

 

Do the antilock light (symbols) flash on the instrument cluster when this happens?

Is the parking brake completely off?

Do the brakes work well (normally) when driving forward?

Have you checked the brake booster and the connecting hoses/lines for leaks or wear?

 

Did the shop use a Porsche diagnostic tool?

[dandastic]

No antilock symbols, Yes the parking brake is all the way off and Yes everything is normal while I drive forward and Yes the used a porsche diagnostic tool. I hope this helps and thanks for replying.