You should only have one pipe going to the tank (or more likely cesspool), the displacment of any air in the tank is insugnificant and even it was sugnificant it would mearly go right back up the pipe your "afluent" came in from and right up through your "stink pipe" in the roof (this is why traps are so important at each fixture in the home or those stinky gases might come right into your kitchen sink
Now... 1951... this is not a good sign im sorry to say... whats the problem your having (toilets backing up? sinks not draining? lawn stinking like sewer? visable afluent someplace outside?), You can try to roto rooter or flush or chemicals but my guess is that will gain you nothing long term. You need to dig (you wont have any info on the system you have at town hall) so you should get a machine their if you know someone who has a lil terimite or w/e.
I would think you have clay pipe to brick tank/cesspool and area or tank has run its life cycle... two choices... 1 legal, 1 probably not legal... #1 install a new septic which requires pumping and then caving in old and also requires an engineered design for approval. #2 use the friends terimite find a new location near the old and dig a 5' diamiter hole 8-10' deep and build a quick cesspool (super easy and cheap with cinder blocks), here are some good info links.http://www.perkulator-cesspool.com/pre- ... system.htm
This link is broken, either the page no longer exists or there is some other issue like a typo.
I will keep my fingers crossed for you bro! GL