We just installed a shower and a toilet in our basement and tied all pluming to the underground main sewer line. We never had problems with flushing the toilet or with the bath tub before now. Now, when the second floor apartment flushes the toilet and also after showering, the basement toilet starts gurgling and the water backs up and comes out of the basement shower drain. I think everything is hooked up with the main vent that is vented to the roof. Can anyone tell me what is wrong? Thanks so much!