Just installed sink, shower, and toilet in basement. Have grinder pump to lift waste to septic. Toilet overflows when I connect the p-trap to the sink. When connected and I fill sink and drain the sink the toilet water is moving. When the p-trap is disconnected toilet works normal. Drain for the sink is 20" above finished floor, bottom of p-trap is about 15" above floor. Top of vent pipe is about 6.5' above the drain pipe. What is my problem and how do I fix it?
Thank You