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Strange. Since you say the intake side is very strong, then the return has to match it. What it really sounds like is a leak or crack in the return line, but you would be losing a lot of water. If it were an air leak, you’d lose suction, not return. Same thing if filters were dirty or a valve stuck partially closed. So I guess what I’m saying here is that if your water level is steady, you’ll need to call a pool contractor in.

Sounds like a clog in the system.  I would disassemble and run some kind of snake through to clean the pipes.  If you have ever disassembled pipes that have been in use you know you will find a lot of garbage on the sides of the pipes.  Steel pipes can fill with rust.  Plastic and copper pipes can build up dirt and oils along the sides and in corners.  All this stuff is really nasty and can sometimes be cleaned with a round soft plastic brush at the end of a snake.

Providing your Guage is accurate (no debris where it screws in or just plane in-accurate) the most likely thing is your pump impeller has debris in it.  A wire coat hanger bent to fit into the pump basket area and into the inlet of the impeller can scape out any lodged debris.  Also, if you manual vacuum is on a hose that attaches to your skimmer, it will affect your flow.  It is like adding that much more plumbing to your suction line.  That, in turn, will reduce your operating pressure.  With out looking at it, these are all educated guesses.

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I’ve maintained pools like yours in the past for customers. Most of the times by the time I was hired they had completely let the pool go. Cleaning a pool that size can be a real chore given the depth that the deep end may have and the overall water volume.

No matter the size of the pool the good rule of thumb is to run the pool filter for about 8-10 hours a day. Based on what you may have for a filter and pump set up you want to run the pump long enough to turn over the entire volume of water in the pool at least once every day.

The GPM, or gallons per minute, should be printed on the filter or in the manual. Lets say for instance as a nice round number your filter system can do 100 gpm at it’s perfect working condition. So in an hour it can turn 6000 gallons.

Depending on the depth of your pool your approximately 30000 gallons and your filter when in actual use and depending on the other variables would probably at best have a gpm of 70 since lots of factors slowly bring down a filters gpm when they are in actual use. With a pool as large as yours you’ll probably factor out closer to the 10 hour a day mark.

As far as cleaners the best one would be a matter of opinion. I’ve used a lot of different robotic and other style of cleaners. I would recommended one of the robotic pool cleaners for a pool your size. They are a little expensive but worth the cost since they really do a lot of the everyday labor jobs in owning a pool for you. My favorite has been the Tiger shark by Aquavac. It is fast, powerful, durable, and easy to use and maintain. Especially where you have a lot of trees around you want to make sure you get those leaves out of the pool everyday to avoid water problems. Another highly recommendable automatic pool cleaner is the Dolphin Pool Cleaner – another excellent choice.