I recently had a mechanic tell me that if I used a hand pump to siphon out some ATF from the fill tube, I could accidently add air bubbles to the transmission and possibly ruin it. Any truth to this? I think he was trying to sell me a flush after the pan drop. I know the pan drop doesn't get all of the fluid out, that's why I was going to siphon out some of the "new" + "old" and add new myself to eventually have it all be 100% new.