I have researched this and have not found a solid answer on this question. I installed a Walbro 255 on an 89 S5 TII and have purchased an aftermarket FPR. I need to know if I should hook the vacuum line up on the new FPR to the LIM at the spot of the stock FPR. If I do, should the stock FPR be capped off on the fuel rail or does it stay hooked up as well? Does the return line from the secondary rail to the tank stay in place? Thanks,

 

I would look into taking the stock FPR out and replacing it with a Banjo bolt...why?I don't know how the car would act with One FPR hooked up and One disconnected.
Usually People will redo the Fuel lines and eliminate anything on the rails to Put in the FPR.
It hooks up Vacuum wise to the Nipple on the lower manifold that faces the firewall(inside Face of the Lower manifold).
Look up a thread with the words" Stainless Steal Fuel lines".(NOTE: that is the way the poster Spelt the Title)..it has Superb info on Parallel fuel systems and lots of Pics and Links to another forum that even gives you part numbers to assemble the System.

 

Thanks a ton. I will follow the link and see how to do it right. That is exactly what I was looking for.

 

i would use s4 t2 fuel rails since you can just unscrew the fpr...pretty sure the s5's are all one piece.

 

yes remove the factory FPR, the diaphragm will mess stuff up. the aftermarket FPR's usually have a built in pulsation dampener