I bought a house knowing the plumbing was a problem. I ripped out ALL the waste pipes (drain pipe) and replaced with new, all PVC. Every thing works fine except the drain in the tub/shower. It is a slow drainer so I went back under the house and looked it over and I had put in an "S" trap in line so I cut out all the new pipe I had just put in and while I have all the pipe leaving about 6" on both ends. I put a 5 gal bucket under the drain end and had the wife run the shower without the shower head (more water) full bore and it drains it out as fast as we could put it in. I put a "P" trap back in line figureing it will drain faster and it doesn't. So, to review it is all new plumbing, it is all on a slight angle to the 4" waste line (no flat zones) and not pluged any where as I've checked both ends when I cut out the pipe to replace the "S" with a "P" and they are open. Any ideas? I will admit this to help speed up the process there is no vent in the system any where. Could this be the problem, no vent? Why does every thing else drain OK except the tub which is the last drain in the system by the way.