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I did a lower leg service of my 2011 Revelation RLTi's over the week end and thought it would be a good idea to service the air springs while the lowers were off.
When apart it all looked very clean and well lubed so I just cleaned all the parts and relubed as per the manual.
Now everything thing is back together it would seem the air is able to pass from the negative to positive air piston at will, and vice versa leaving my 140 mm fork sitting around the 100mm mark!
There really isn’t too many parts in there to muddle up and I didn’t remove any o rings which all looked as new so nothing should be scratched so what could be the problem?
The forks didn’t have this problem before and I now therefore admit to being a numpty!
Short of send them to LOCO for a coil conversion any ideas?!
Are you inflating them positive then negative chambers from being full empty and extented?
hmmmm I don't think they were extended when I first inflated them, positive first. The air pushed the fork out....
put 100psi in the poistive with neg. chamber empty, and try another shock pump too to see if it's playing up too.
Ok thanks for the advice Loco, I'll give it a whirl tonight when I'm home, allthough with my luck at them moment I may be phoning you to book them in yet!
I've got a similar problem with my RLT Revs in that if I set the positive and negative pressures up, after a few weeks left alone the travel reduces with only about 100 mm of stanchion showing.
This would indicate that either pressure in the negative chamber is increasing or pressure in the positive chamber is decreasing. Now pressure does not increase spontaneously and it would not pass from positive to negative chamber if there was a leak between them ( it would equalise) so it must be the positive chamber losing air. This is confirmed when I put a shock pump on the positive valve and it shows zero pressure.
Since the negative chamber is one side of the positive chamber and we've already concluded there's no leak between them, the only other route the air from the positive chamber could be taking is through the top cap, so that's the next thing I'm going to look at.
The top caps are just standard shraeder valves. I'm pretty sure you can unscrew (eg with tweezers) and replace with one out of an old tube. Worth a try as a first call.
Ok, if I let all the air out of the forks, negative first then positive I can't pull the fork out to extend it, some sort of vacuum being created. Even holding the negative valve in doesn't help so I can't try loco's advice above. If the positive is 100 psi, I can only get to about 50 psi in the negative before the fork starts getting sucked into its travel?? Tried 2 shock pumps to check..
The top caps are just standard shraeder valves. I'm pretty sure you can unscrew (eg with tweezers) and replace with one out of an old tube. Worth a try as a first call.Superficial - Member
I was thinking more of the seal round the screw thread but now you've mentioned it I'll investigate the valve too (I have a tool for removing Schrader cores). Thanks Superficial.
This would indicate that either pressure in the negative chamber is increasing or pressure in the positive chamber is decreasing
It's an air leak BETWEEN the two chambers. As the fork compresses in use, on each stroke, the +ve chamber pressure increases and the -ve chamber pressure decreases. If you have a leak a tiny amount of the high pressure air in the +ve chamber leaks through to the lower pressure -ve. As time goes on the effective pressure difference between the chambers changes, the pressures try to equalise, and a 'higher' -ve pressure will drag the fork down. A quick lube service might fix it, failing that new seals. But don't just leave it, if it's a lack of lube you might do damage
Now pressure does not increase spontaneously and it would not pass from positive to negative chamber if there was a leak between them
That is EXACTLY what IS happening. Believe me. 🙂
EDIT
Just re-read your post. Are you actually USING these forks? That's not clear.
If not, yes, it's either the seal in the top cap or the valve that's leaking. Both easy fixes.
Ok so we have a leak between the two chambers, best to change the seals and inspect the other components for damage at the same time.
Join the club! Coil for me next time...
Yep there is the option of going, dual air (fixed), uturn coil or Dual positon coil although there are a few other parts needed too.
Join the club! Coil for me next time...
yeah. Because it's a lot cheaper and easier to buy and fit a coil than it is to use a shock pump or replace an O ring...! 🙄
Yes, but they're more reliable that particular iteration of the uturn system 😀
Loco, is it new seal time for me also?
out of curiosity wouldn’t a coil make the fork more linear in compression?
Ideally Id like it to use the first part of the travel to soak up the chatter and then firm up to push against
Yes I'd say it'd be the best course of action as if it's leaking it'd suggest the seal isn't sealing and re oiling or greaseing it is a bit of a bodge and will only be tempory.
You need to check for other damage too.
Springs are a bit more linear by nature, but the compression adjust should allow you to dial the fork in to work haow you want. 😀
This generation of Dual Air forks seem to suffer a lot from pos/neg migration.
I changed the o-rings in mine with a kit from ebay. The ones included in there were noticeably tighter fitting than the OE rockshox ones and completely cured the issue. They will hold pressure for 6 months plus, when before I had to top up the positive about once every two weeks or so.
Over on MTBR people have been using quadrings instead of o-rings when rebuilding for the same reason. I also used fox float oil rather than grease as it seems to lubricate the seal better.
Holy thread resurrection!
Similarish problem. I seem to be losing pressure from the negative chamber but it seems to be leaking into the space between stachions and sliders. I know this because went I pumped the negative up to 135psi and pushed down on them, it blew the seal up the stanchion.
So, is this the same problem/cure or something a bit different?
I was just going to post a similar question.
My son has some rev race u turns, they have been serviced twice and they work pretty well but they continue to lose travel and when I let the air out to reset them the negative valve spews lubricant out. Can they be cured?
Seal replacement usually curses this, but wcheck inside of the stantion for wear/marks especially lengthways scores as these will allow air to leak past the seals.
Cheers, I'm about to take the air spring apart. There's a little dirt around the wiper seals but the oil in the lowers looked fine. I'd be very disappointed if there's any much inside the air spring and I'd love to know how it got in there.
They're a week under being one year old. I'm now wondering if a home cure is the best way forward or box them off to Merlin. I'm happy to do the work, just wondering if I can get seals for less than the cost of postage!
when I let the air out to reset them the negative valve spews lubricant out
A small amount is normal. The lubrication oil naturally settles at the bottom due to gravity. Turning bike/forks upside down when changing neg air pressure should fix this.
Is everyone talking about 2011 Revs here, and will Black Box ti ones be prone to the same issues, potentially? Only asking because I'm recently running these forks. Very happy with them so far, tho I did need to top up the air as they 'sank' about 15mm over a couple of months of hard use. Would this be expected? And what service intervals are you guys recommending, please? I'm a bit of a technophobe, so any tips gratefully received. Ta v much.
Any Rock shox dual air fork could potentially have similar problems to this thread.
Service intervals are in the technical manual available on SRAM's website.
But I tend to give mine a lower leg service twice a year a full service once a year.
If it makes any difference, mine (the wife's) are 2012.
As mine blew the wiper seal up the stanchion, I'd imagine it's one of the two seals on the negative piston that's the problem. Does anyone happen to know the sizes of these? I imagine the one on the positive piston is the same as the larger one of the two on the negative piston.