When flushing the toilet the water goes down a little slow and the flap seems to close off the flow a little soon. Once the tank is filled then the water level in the bowl is close to as high as it should be. For the next few minutes the level in the bowl will go down until it is about ¼ of where it should be. During this process there will be a groaning sound.

It began about six months ago. I replaced all of the tank parts. Had the septic tank pumped out, and added 10 feet of length to a vent. Then when I was headed to buy a new toilet it suddenly started working correctly. It worked fine for about 3 months and now it is back to the above problem.

The toilet sits on top of the main stack. There is no large vent coming off the stack. It would take major remodeling to fix that. The kitchen sink is vented with drain pipe. Coming off the washer in the basement there is another vent (drain pipe size) about 10 feet from the septic tank. This is the one that I lengthened six months ago.

The septic system was installed about 20 years ago. The ground above the drainfield is covered with blackberries. I have sprayed them and plan on attacking them the necessary hardware. Could the blackberries be an issue? Should I use septic tank root killer? (Roebic K-77 Root Killer)

What else can I do?