slow draining tub, help!

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.

Whoops! You committed the big sin when you didn't vent the fixture.

vent

There has to be a vent in the system, as you get farther from it the tub has less and less chance that it can pull air; you need a vent, sorry.

Looks like I get to go under the house and put in a vent. Thanks, I guess I just never realized that a vent would affect how fast or slow a drain would drain. Thanks!

EACH fixture needs a vent. A vent keeps the other drain traps in the system from getting sucked dry when another one is draining. A dry drain allows flies, bugs, rats, mice, sewer gasses, etc. to come up through the now dry trap (although some rats don't seem to give a rat's xxx about a little water!). An S-trap won't pass code and has its own problems, you need P-traps for everything except the toilet which has its own trap built-in.

We all agree he needs a vent, but why in this situation should the lack of a vent affect the tub's drain rate? He says he's going directly into the "waste line", whatever that might mean, which is presumably vented via the stack at some point. I think he's got a problem downstream from where he added the P trap. (Full disclosure: I've been wrong before.)