Hummm, I'm not sure what you are doing, but the 727 fuel system should not act like that.

This is the way the RW plane is designed to work...

The 727 has 3 tanks. The wing tanks are smaller than the center tank. The center tank has 4 boost pumps, the wing tanks have 2 pumps.

If the tanks are equal with fuel, the set up should be tank-to-engine.
All boost pumps ON, x-feed on 1 & 3 closed, x-feed on tank 2 open to keep the manifold under pressure.

Because tank 2 has four pumps the pressure is a bit higher and if you leave all the x-feed valves open the fuel could flow from tank 2 to all engines.

You cannot transfer fuel, only cross feed the fuel.

Example: If tank 1 has 7,000 pounds and tank 3 has 6,700 pounds you would do the following...

1. Engine continuous ignition on..................................ON
2. Boost pumps tank one on.......................................ON
3. X-feed valve tank one open....................................OPEN
4. X-feed valve tank three open..................................OPEN
5. X-feed valve tank two closed..................................CLOSED
6. Boost pumps in tank 3 off, one at a time - observe engine.
7. When tank 1 & 3 equal, all boost pumps on...............ON
8. X-feeds 1&3 closed, X-feed 2 open..........................CHECK
9. Ignition off.............................................................OFF

This is in checklist format because anytime the F/E does something with the fuel panel the crew must be involved to prevent engine failure.
In real life at low altitude and good engine fuel pumps, the engine will most likely suction feed from its tank, but you never know!  :o
At higher altitudes it may or may not, but are you feeling lucky?

I think there are some issues with the fuel system in the CS model along with a few other problems that CoolP (one of the forum members) is compiling a list so the next version can address these problems.

Lou