Air Dam on check valve

[Scotty]

Recently after a ejection pump change out a few months ago, the check valve doesn't want to release due to apparent air pressure coming from the main waste pipe. The pit is 4 ft from the main cleanout which is the last foot before it disappears into the street. With the cap of the cleanout off, there's a nice blast of (fresh?) air coming out. Suspecting a old valve, I had changed it out and mounted it horizonally according to the recommendations of the manufacturer in addition to an air bleeder hole slightly above the base of the connection and rodding the cleanout to approximatly fifty feet. The problem hadn't gotten too much better resulting in the property owner calling for the street dept to check things out. He had taken it upon himself to remove the cover to 'looksee' and got a crappy perspective for it. Well the city said there was grease in there. Considering all the used condoms that came out with the aforementioned perspective, unless the manager of the carryout restaurant has something going on in her apartment above the store, there's dysfunction's with the city. At this point the owner has the check valve totally off. I've insisted that he reinstall it as soon as possible for the obvious reasons that city code requires it.
The Questions is this: There's a 45 degree angle from the horizontal to the waste stack about 6 inches long. Would extending that length and shortening that of the horizontal 2 inch pipe containing the check valve increase the pressure enough to overcome the air holding the new check valve closed? Or might I suggest getting the city engineer in there and ask why in the wide world of sports after 30 years is this happening? Glad to hear any and all ideas.

[Scotty]

After doing some rodding at the aforementioned restaurant, I found that a whole section of four inch pipe was clogged with grease, (nope, no grease trap for the back sink!). And sombody in their infinate wisdom had made a loop basically allowing the flow from that source to flow into the main line via a two inch. Somehow, there's a pipe leading out the back which I got about 13 feet into before deciding, once again wet, smelly and with attitude, time for a pro with a pro machine. The little motorized unit I was using wasn't making appreciable headway. The tape just kept wanting to twist and what I got off the tip amounted to impacted fecal matter or clay. I'll hear by the end of today if there's a break or if the plumber can punch it through. Meanwhile, as the four inch pipe has been cleaned out, continous flow has been restored (it was spilling over upstairs when the sink was drained) to the back sink and the air dam problem by the ejection pump doesn't seem to be an issue. I'll still get some pics but my lithium ion batteries that I got last summer for the camera need changing.
I've found a better word for getto plumbing, it's called Chicago.