After it's flushed the water level is very low. This just started, it's an older toilet, no obvious leaks. Searched here and found no answer. Thanks in advance for any help.

After it's flushed the water level is very low. This just started, it's an older toilet, no obvious leaks. Searched here and found no answer. Thanks in advance for any help.
I have put up two ballcocks,(see images) make very sure the small tube that runs from the ballcock to the white overflow tube is connected. This fills the bowl as the tank's filling. Don't shove it down too far as it could start to siphon. Regards, Tom

I know where Tom is headed but I'm going to take a different approach since I always wind up just re-saying what Tom posts. You may have a partially blocked vent that is allowing the toilet trap to get siphoned. Do you get kind of a chugging at the end of the flush? Go with Tom but keep this in the back of your mind for later.

Thanks for the quick replies. I have the bottom type and I guess it's the vent because it looks fine. Some bubbles did come out of the tube #4 on your diagram. Just how does one unblock a vent?

Rod with a 1/2" snake from roof to basement floor plus 20' . Shove your rod in about a foot and pull back 6" , keep doing this until your reach the basement or an obstruction and go back and forth again and again in an obstruction. Finish with a bucket of hot water and the vent should be better.