So far I've added 1 qt. of transmission fluid to my new truck, 14,000 since August. There are no signs of leaks anywhere and it seems to perform the same as my '99 and '00 did. Most of the miles have been highway miles with about 5,000 of them towing a dog trailer out West.

 

Crawl under the truck with a good light and check the trans and surrounding area for signs of spray. If it is a small leak it might be hard to find. Another thought is was it full tostart with? Maybe the predilivery check was not so good.

 

If you know it was full to start with and it is definitely losing fluid maybe the vacuum modulator is leaking. It can suck tranny fluid into the engine through the vacuum line. Since it is all burned in the engine there is no evidence of a leak. You can usually check it by pulling the vacuum hose off the modulator a putting a hand vacuum pump on it to see if you can suck fluid out. This is not very likely to be the problem on your truck though since it is so new.

-Jon

 

It's extremely unlikely to be a vacuum modulator. There hasn't been a vacuum modulator on F150's in years!

Mark

 

Doh! I need to stop working on old vehicles!

-Jon