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In the Fox RP23 shock I had, the propedal was not at all propedally... With no air in the positive chamber, I could compress the shock by hand and feel the propedal switch had no effect on shock. I considered sending it off to Push for a rebuild. That certainly would have been the easiest thing to do. Finally, I decided to try and fix it myself. I figured the worst that would happen is I just put everything back together ("as-is") and
then send it off to Push. You know, make them earn their money.
Much reading of the interwebz suggested the IFP chamber had lost pressure. This shock (as well as many others in the Fox lineup) requires a nitrogen needle, a slotted 4mm hex wrench, and a replacement rubber puck to re-inflate the IFP chamber. I wanted to spend as little money and time as possible on this experiment. So, I set out to find another route.
Over on emptybeer, there was a sharp-eyed fellow who realized a fuel rail pressure relief valve from Ford V8 engines had the same threading as the 4mm IFP set screw. Woah!
Since I didn't feel like slumming around the local pick-a-part salvage yards, I started googling Ford fuel rail diagrams and determined the part was a Motorcraft - EFI Auxiliary Valve (CM3461). I went out and bought one at my local auto-parts store for $7.99.
Fox doesn't publish a manual for servicing the IFP chamber. More searching and I found the exploded diagrams for my RP23. It showed all of the torque specs and the IFP depth necessary.
Armed with a diagram, I'm invincible!
I removed the IFP port set screw and rubber puck. Drilled a 1/16" hole in the puck, reinserted it, and screwed in the CM3461 on top where the set screw was previously.
You should note that the schrader valve on the CM3461 is long enough that the high pressure pump doesn't engage with the valve core. Still works fine, though.
After some playing around, I determined that if I screw in the CM3461 finger tight, and then give it another 1/2 turn with a wrench, I could pump air into the IFP port without any leaks. And, if I turned the CM3461 another 1/2 ~ 3/4 turn, the puck would compress enough that I couldn't pump any air into the IFP chamber. This was a nice discovery as it gave me a 2nd level of protection from air leaking out of the IFP chamber (compressed rubber puck and the valve core).
I set the IFP to the proper depth (2.25" in my case) and filed the reservoir with 5wt fork oil. Then I carefully screwed on the cap and filled the IFP chamber to ~400psi. Although pure nitrogen is called for, I used a custom blend (78% nitrogen, 21% oxygen, 1% other) in my shock.
Regular old air works fine in the RockShox IFP chambers, I'm sure it'll work fine in here also.
Once pressurized, I compressed the shock several times (open, and in each of the propedal modes) to try and dislodge any air trapped in the shim stack.
Then I depressurized the IFP chamber (do
not cycle the shock with the IFP chamber depressurized, you'll screw up the IFP depth) and removed the set screw and ball bearing on the cap to access the reservoir.
Using a hypodermic needle, I injected additional 5wt fluid in until all of air was out.
Replace the ball and set screw (don't cycle that shock). Remove the CM3461. Re-install the sleeve. Re-install the CM3461.
Re-pressurize to 400psi.
Now you can cycle that shock. Feels good, doesn't it?
Before disengaging the pump from the valve, use a wrench and give it another 1/2 ~ 3/4 turn to compress the puck and help lock in the air. Then, remove the pump from the valve.
All done!
Cycling the shock by hand (with no positive air pressure), I'm amazed at how well it works. With propedal open the shock is silky smooth on the compress and rebound stroke (rebound actually works, also). When set to the 1st level of propedal there's a noticeable "shelf" that you must press past before the shock resumes normal compression. And, at the 3rd level of propedal I can't compress it by hand.
Close-up of the CM3461 installed on the RP23.
I happened to have a nice cover (labeled "- Air", from my SID rebuild kits) to put on the new valve.
For a total expense of $7.99 (not including the small amount of fork fluid) this worked out very well and it didn't take much time.
I'll report back in a few months with how it's holding up.
If it looks like it's fixed for the long term, I'll probably invest in a real inflation needle and some o-ring cord to make the rubber pucks.
In case anyone else is interested in this modification, I found (and downloaded) the diagrams for several other models of Fox shocks. I also found a spreadsheet (looks like it's from Fox) that gives the IFP depth and psi required for a ton of different Fox part numbers. Let me know and I'll look up whatever you need or try to put the spreadsheet online somewhere.
Feel free to ask if you have any questions. Oh, and you can click on any image above to get a full size view.
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