I'm installing a new sink in my kitchen. It has a large 9" deep bin and a small vegetable bin 7" deep. I'm going to install the garbage disposal to the vegetable sink. All of the pipes connect and flow out directly behind the larger sink. When I run the piping from the disposal to the main drain pipe, should I have a slight pitch downward or can it be level? I'm afraid junk from the disposal might back up if the pipe is level. What is considered the normal installation in this situation?

I was wondering if someone could help me out. I replaced all of my galvanized pipe with Pex and I have the hot water running into a manifold and branches off to all the fixtures in the house. The problem that I seem to be running into, is that I have a 70 gal hot water heater and when I go to fill my new tub that says it holds 55 gals to the overflow, if I just run the hot water, I run out of hot water about 4 inches below the overflow. I even installed a super low flow shower head,and the showers seem to go pretty fast. I even tried the bathroom sink as it got cold and it runs cold too. Any ideas? Thanks in advance,Joe