I have an '04. My rear passenger side is sagging, even only after a few hours parking (no leaking at all). Just took it to the dealer. They told me the valve is bad, needs to be replaced. Cost is ~$3000. So they contact Mercury Insurance (my Ext Warranty), and Mercury told me I have to pay to flush the system first, see if that fix the problem, before they authorize the valve replacement.
Dealer wants $1200 for the flush. Is that reasonable?

I had mine flushed in late 06. It was covered under warranty but I was told it would have cost about $900. It didn't solve the problem though and eventually they have to replace the valve block. It's been good since.

They flushed mine last month. I had the same problem.

First, the driver's side was sagging. They replaced something. Then the passenger side started sagging. The dealer flushed it at this point, but that did not solve the issue. They eventually replaced the block, which they had to order from Germany and it was for an 09 model so they also had to replace the hoses (different lengths).