Fuel draining unevenly in modular tanker

[AbacusWizard]

So I recently designed a modular tanker for long-distance transport of colossal quantities of fuel. The engine module (at the front; it's a pull design rather than a push design) has 16 atomic engines and each fuel module has four large Kerbodyne rocket fuel tanks, assembled in orbit with large docking clamps because it's (in my opinion) way too heavy to launch the whole thing at once.

Close-up of fuel module:

And here's the fully assembled tanker:

The general design of each fuel module's central spindle is like so:

[dock] [sAS wheel]* [adapter] [girder] [batteries] [probe core]** [monoprop tank] [girder] [adapter] [sAS wheel]* [dock]

* radially linked to each SAS wheel: 4 fuel feed lines; from the aft wheel to the tanks and from the tanks to the fore wheel

** radially attached to the probe core: 4 RTGs, at the end of each of which is a cubic octagonal strut, attached to which is a large Kerbodyne fuel tank

The intention here is for fuel to drain from the aft-most tanks first, so that when they are empty I can detach and abandon that module and the remaining modules will still be full of fuel. I assumed that the arrangement of fuel feed lines would mean that the engine module would try to drain fuel from fuel module #1, which would in turn drain fuel from fuel module #2, which would in turn drain fuel from fuel module #3; thus only fuel module #3 would actually lose fuel. I used girders as structural elements because they disallow fuel crossfeed, which I assumed meant that fuel could only flow through the fuel lines.

SO HERE'S THE PROBLEM:

Fuel isn't draining only from module #3 as I had intended. However, it's also not draining evenly from everything! It's draining evenly from all four tanks of module #3, and from THREE of the four tanks in each of modules #1 and #2! Any idea why this might be, or what I could do in the design phase to make it work like I want it to? At the moment I'm coping by manually transferring fuel out of the aft-most module whenever the total fuel load is low enough to make a difference, but it'd be nice to avoid that next time.

[Baron_von_Fluffy]

I don't know why it would cause this but one easy solution to avoid transferring fuel would be to disable resource transfer for them. Tedious but better than transferring fuel.

edit: (unless, ofcourse, it starts draining lopsidedly even like that - docking port bug maybe? you could try and bypass that with kerbal attachment system's fuel pipes)

[Kryxal]

There's no center tank, is there? Try adding a small central tank, to be fed by the four outer tanks, and see if it makes a difference. Worst case, if it's just draining fuel from too MANY places, you can switch all the tanks off that you don't want active till you jettison the previous set.

[Laie]

* radially linked to each SAS wheel: 4 fuel feed lines; from the aft wheel to the tanks and from the tanks to the fore wheel

Here's your problem. Fun stuff often happens if you build fuel piping that goes in a circle. Or, to put it differently: there may be as man lines leading into a tank as you like; but several lines leading out of one structure often doesn't work as intended. Also, even if you can make it work as intended on the launchpad, circular piping often ceases to function after (un)docking.

I don't think there will be as setup that does what you want. Locking tanks may be your only option.

[Virtualgenius]

Use the crossfeed enabled mod then use goodspeed pump mod and it will auto balance all tanks or you can turn it off

http://forum.kerbalspaceprogram.com/threads/76499-0-90-CrossFeedEnabler-v3-2-Dec-20-14

http://forum.kerbalspaceprogram.com/threads/67163-0-23-5-Goodspeed-Automatic-Fuel-Pump-v2-14-1