Primary fuel rail nipple thingy
 

While I was cleaning the rail, this tiny piece of rubber fell out... looks like it would be part of a diaphragm or something.

Is this a show stopper for reassembly?

I guess thats what the PD looks like of S5 TII... I'm still new to this one...

I just put 40psi of air to it, and didn't feel it leaking... but who knows.

replace it, there is a screw in there to hold the diaphragm end to the housing, without it it will move freely and fail sooner than later. the screw/seal also prevent gas from leaking out once the diaphragm does fail.

replace the whole rail? (obviously?)

series 5 you have to replace the whole rail, or swap to a series 4 and the PD is replacable.

If you modify your s5 rail or get an s4 rail, you could do the banjo mod. Eliminates the pusasion damper.

the only mod I could think of to do to this one is to cut off that end, and weld on some plate... and I'm not doing that.

I think you would have to cut off the pd and tap it for a banjo bolt? Or weld on a fitting? If you can get it done, it sure would be the ridiculous price of almost 300$ for a new S5 primary fuel rail/pulsasion damper.

or just get aftermarket rails and 5/16 barb ends.

I would, but I just blew money on my MS2 and everything needed to run it... that will have to wait. In the mean time, I picked up an s4 rail today... just need a banjo bolt for it.