We have owned our house for a little under a year and have had issues with the toilet from the beginning. The old toilet would clog very easy and we had to snake it often. We installed a new toilet about six months ago. It also gets clogged easy, but about a month ago we had a very tough clog that I was able to get unstuck. Since then, the water in the bowl will siphon all the way out within about 30 minutes. We've had a plumber out a couple times and they snaked the toilet, and I believe they also snaked the plumbing out to the septic tank. The water still siphons out, and they said it was either the toilet, or our plumbing in the basement was done incorrectly. I read some forums on here, and snaked the vent on the roof with a 50' snake, but that has not helped. There is no leak in the basement under the toilet either.
I attached a picture of the plumbing in the basement. The plumbers said that the horizontal pipe that the toilet drains into should then drain into that vertical pipe at a 90 degree angle instead of the 45 degree angle. They said the way it is now will not create enough suction and that is why we are having problems. Is this possible? Any other ideas?