Minta - What do you use to thicken your pie fillings? How much headspace do you use? I canned a few batches a while back and use Clear Jel (special thickener that stands up to the heat of canning) You ended up with a thick canned pie filling like you buy from the store - ready to use right out of the jar - BUT the headspace called for was too little - I had major siphoning - jars still sealed though. If I did again I would leave at least 1.5" to 2" headspace - which seems crazy!
I use clear jel too (not the instant clear jel) . I leave about 1 inch of head space. A little trick to avoid the major siphoning from the lids, turn off the water a few minutes earlier and leave the jars in the water for about 20 minutes or so before taking them out. This helps to stabilize the pressure inside the jars which reduces the leakage.