The first question should be are you hot packing you Tomatoes, or cold packing. I think you will find you have less liquid loss, and a superior final product if you hot pack your Tomatoes.

Steve

A common problem with using a pressure cooker is siphoning, usually because of fluctuating pressure. The problem with that can be reduced by using the weighted regulator instead of the guage(which I found almost impossible to control). Hot pack will help eliminate the air pockets that are inside of the tomatoes, but it can also be accomplished by taking the time, with a wooden or plastic spoon, gently pressing the tomatoes after the boiling water is poured over, working all the air bubbles out. So determine whether you are getting siphoning - even hot pack will take the level down in the jars if it's happening.

When you say "whole cools off" what do you mean? When pressure reaches zero and the pressure tab drops, take the lid off for 10 minutes and then remove the jars. Is that what you are doing?

Nope - taking the lid off as soon as the lid lock drops will contribute to siphoning - I just did that the other day. You take the WEIGHT off when the pressure drops to zero and the lid lock drops, THEN wait 10 minutes and remove the jars.

But raw pack, air bubbles, bands too tight, and changing pressure/temperature quickly will all do it.

Here is a link that might be useful: NCHFP Using pressure canners

I don't know how to post more than 1 clickable link

Here is a link that might be useful: NCHFP siphoning troubleshooting

Oh Wow!! Talk about information overload! Thanks for all the interest.
I use a weight of 15lbs. I wait until pressure up then start timer. After process time I wait until the pressure off, then remove the weight, then a further wait until the canner cools down.
I have tried fiddling with the heat to control the pressure to 10psi, but it's difficult to maintain. Not a good idea, much better to use the weight.
I cold pack whole tomatoes as we want whole rather than mush. Mush is easy as we put it through a food mill, then hot pack that after removing some liquid by boiling. We now use the freeze method of removing excess liquid, makes for a thicker sauce and requires less cooking down.
Have tried another canning today with whole toms again, noting all the comments, will see how we did soon.

One step I noticed missing is the 10 minute vent time.
The canner should steam for 10 minutes before you put the weight on.
Once it's rocking, THEN you start timing. Adjust the heat, VERY SLOWLY, to maintain a gentle rocking, not rattling the windows!
After the time is up, turn heat off. You can slide the canner off the burner if you wish, but not absolutely necessary.
When the pressure returns to zero and lock drops, remove the weight and "crack" the lid.
Wait 10 minutes.
Then you can remove the jars.

You don't say what your elevation is, but if the directions call for 10 lbs. pressure, that's what you should be doing.
If you're under 1000 feet elevation, processing at 15 lbs. is going to over process the food.....leading to mushier tomatoes. Not a safety issue, but quality.

Too high heat (rocker rattling loudly and quickly) can also cause some siphoning.

Let us know how it went with the last batch!

Deanna

Just like so many other things in life, pressure canning tricks us into developing some bad habits over the years. Little things that we don't even notice at first slip by us - using a pressure cooker rather than a canner, too much water in the canner, rings not screwed on right, running the heat up too high, impatience makes us skip steps, etc.

We could analyze your "game" to try to figure out where the breakdown is - it starts with the brand and size of your canner. But the easiest way is to send you to the link below and if you follow it step by step 1 time the odds are good you'll find out where the problem is.

Dave

Here is a link that might be useful: How to use a pressure canner

One other tip: Make sure you screw on the bands tightly enough. Not so tight that the lids can't move at all, but tight enough to keep the lids in place.

I had the same problem as you, and this turned out to be the problem.