Bleeding Air From Forks

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Jan 22, 2001
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#1
I have noticed over the last few months a lot of people referring to bleeding the air from their forks. This is something I never do and had a look through my service manual ('95 YZ250) and there were no references to bleeding forks as part of your maintenance routine :think

I had my forks rebuilt just before I bought the bike and have never experienced any real changes other than the adjustments I've made to rebound & compression. Am I jeapordising the reliability of my forks from not doing this? I make sure to clean out the crud from the fork seals and have had no problems so far.
 

Jaybird

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#2
The fluid in your forks tends to want to oxidize. With that it releases gas. This gas will build up in the area above the fluid. Opening the little screws on the top of the forks, while the bike is on a stand will release this built up gas.
During this oxidation, another fine product that is produced is aluminum oxide. That happens to be what many grinding disks are made out of because of its great abrasion properties. Good idea to change your fork fluid periodically to get rid of the acidic fluid and any abrasives and/or aluminum particulate that may be in it.
Bleeding your forks during assembly and oil change is a differnt matter entirely. Hope I was of some help.
 
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#3
Thanks Jaybird,
I'll bleed them off tonight, I presume the air will escape nice and slowly? I have experience of bleeding off high pressure nitrogen! and have seen the results of incorrectly sealed plugs on high pressure(15,000psi) subsea tools (Oil Industry), 1/4" Lee plugs punching through 1/4" steel plate!!! Better safe than shot I suppose.

Could I replace the oil without having to dissasemble the forks? I presume the aluminium oxide is produced from the air corroding the inside of the forks?

I'd like to avoid stripping the forks out if I have to, I seen the bill for the last fork rebuild and it wasn't pretty.
 

Jaybird

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#4
I think the fluid itself is what is oxidizing. Becomming acidic. The pressure that you will release is negligible. You won't even know it came out, very low psi. No sooner than you open the release screw, it's released.
You don't have to completely disassemble your forks to change out oil. Different forks do have different procedures to follow though. Most will use some fluid to "flush" out the fork before final fill in hopes of getting rid of residue and particulate.
If it's been awhile since this has been done, or never....I suggest you have a technician do a full service on the forks.
 

Jeremy Wilkey

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#5
Jaybird,
I agree the oil is oxidizing, the AL parts in your forks are also a major contributor. When you serice a fork that has had oil in it to long the stench will about knock you over..

Regards,
Jer

:scream:
 
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#6
Jer / Jaybird,
I ble dthe forks out and your right I didn't notice much up a pressure release (Hiss etc) though It did smell a bit funny. In saying that tthough I was in the garage with my buddy and he lets off some pretty funky smells sometimes:( :(

HeeHee.:cool:
 

Gary B.

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#7
I was just about to post a question regarding those little brass screws on the top of my forks, when I stumbled onto this thread. Thanks! :D
 
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#8
Hello.
I've searched this site, and found all threads
related to fork bleeding, yet I could not find EXACTLY
how this is done on the 97 yz250.

I read that there's a little screw on top of the fork, but I have two.
Maybe I have some troubles with the terminology's, but one is
on top of a Nut, and the other is right under it.

Both being flat heads.
thanks.
 
Last edited:
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#11
Sorry to bring this up here, but...

Could you notice a difference in actual riding compared to not bleeding?
Also, I read that not bleeding could **** up the seals. Is that a fact?

Can you give me an example of how quick the seals blow if the forks aren't bled,
compared to forks that are regularly bled?
thanks.

I just did it btw, and the right one hissed a bunch, the left one, not so much.
 
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#12
Air pressure does not blow seals out....dirty seals with dirt in between the seal lips and pressure behind them blow the seals out.
You will also have dirty oil from all the dirt and water that has passed thru the "bridge" effect from you seals.
This effect also allows air into your forks.
The air build up will cause your forks to become very harsh and a pogo effect....not good
If you want this fixed, contact us, Rob