Plumbing Delta technical questions - failure mode

[lakedude]

What do the cylindrical metal parts do in a Delta shower cartridge?

Background:

Faucet leaks, installed new seat springs and rubber piston like washers. Still leaks. Delta sends me a new cartridge for free. I start poking around with the old one trying to determine the failure mode.

Got the basic water flow figured out. Water enters rear and up front a rotating metal plate with 2 odd shaped holes starts, stops and regulates the flow.

This is all well and good but in the rear half the cartridge there are 2 concentric cylinders with several holes in each. My first guess was that these cylinders are for mixing but they are before the hot and cold would mix, near as I can tell.

What do these parts do?
What is the typical failure mode?

The cartridge looks pretty good inside, like it should still work.

http://img441.imageshack.us/img441/2480/deltainternal.jpg

 

[Saffron]

Did Delta specify what that cartridge is used for?

From the picture I would guess it is used for air-rating and/or pressurizing the water? Kind of like the metal screen insert on a sink faucet. Another idea is maybe it's for feedback when you turn off the water flow, the water will flow reverse down your plumbing. This is the help drain the upper faucet and plumbing so when it gets freezing cold it won't burst your pipes.

Just a shot in the dark as I really don't know.

As far as Failure Mode is concerned I have no clue what that is in a shower head.

 

[Tulips]

Best guess, The spool and sleeve in the Pressure-Balanced Cartridge work together, automatically adjusting the flow of incoming hot and cold water to maintain a consistent temperature based on pressure variances