transmission resevoir backflowing while checking

The transmission fluid is stored in the transmission and does not have a separate reservoir Could it be you power steering reservoir?

i am not sure if this will help, as i am not sure of how you are checking tranny fluid but here goes. I have always been told by my dad and mechanics that the ideal way to check tranny fluid on older cars is to start engine, let it get to operating temp and then set emergency brake, chaulk the tires and place in nuetral. after that stand to one side of car, not in front (don't want law suits) and then check dipstick after removing once, cleaning and then checking again (making sure you are on level ground). this supposedly gives the most accurate readiing. I am guessing since once in gear, fluid runs through so many lines and tranny is fairly complex that you want to make sure fluid level is good and present while in gear. makes sense to me. as for reservoire, i have not seen that either but i will check if i have on my 88 eldo. best of luck, i hope that this is right and it works for you. (not sure on newer cars, haven't had to ask or check)...