I have an existing bath/basin set up. The basin used to drain poorly and I noticed that there was a negative fall on the pipe. So I cut a small amount off the pipe down from the u-bend to the 'horizontal' waste pipe to give it a slight fall. Now the bath drains very slowly and when I drain the basin water often comes up into the bath from the waste. If I disconnect the pipe below the basin u-bend the bath drains excellently. If I disconnect the basin above it's u-bend the bath doesn't drain. Any ideas? I can post a photo/drawing of the set up later if that would help but I'll have to take the bath panel off. Thanks