Mountain Biking - Hydraulic brake bleeding.
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07-21-09, 08:09 PM
I figured this would be more appropriate here than mechanics, since everyone here (well, most..) know about mountain biking features.
My new avid juicy 7 needed bleeding. Note: needed. I did what I heard before; pumped the lever for a minute. Now it works fine! Couple questions -
a. Why does pumping the brake lever work as bleeding the brake?
b. Is "bleeding" the brake via pumping the lever a long lasting treatement? Will I go out tommorow to find that my front brake doesn't work up to it's potential?
Thanks for your help!
I'll give this a try...
Nothing but oil in the hose. Your lever pushes a piston which pushes the oil in your hose. The oil in turn pushes another piston at the opposite end that pushes the brake pads against your rotors. (A mechanical cable system is a "pull" - whereas a hydraulic system is a "push").
Lever Sag can be caused by one of two main things (there are others not related to your question).
1) Air bubbles in the system (bad news because there's all sorts of reasons why air shouldn't get in, in the first place). This will make your lever feel spongy and brakes less effective. The lever may even feel like it continues to slowly drop as you're applying constant pressure while braking.
2) Pad wear can cause the levers to sag in due course... because the pistons at the caliper end are poking a little further out by the time this occurs. Because the amount of oil in the hose remains the same, the piston at the lever end will be pulled-in a little further than usual. Although this is such a tiny amount, it's enough to be "felt." Sag caused by pad wear doesn't feel spongy and the braking power is not reduced, it's just the levers feel low when it engages.
By pumping the lever several times, the rapid motion of the piston causes it to suck a little bit more oil from the mini-reservoir (inside the lever body). You haven't technically "bled" the system, rather you just topped it up. There's now a tiny iddie biddie bit more oil in your hose to compensate for the pad wear... your lever returns to where it was when new. In your case where the brakes are brand new, the system probably just needed to be "primed" or "jolted" to it's optimum hydraulic condition. There's lots of little things that Mr. Avid sorted out for proper piston "retraction" that you and I don't need to know about.
Bleeding the system in hydraulic bike brakes is a little tricky because unlike cars and motorcycles, a bike's plunger piston only has a tiny reservoir, so most brands include a "kit" you need to attach to supply a steady flow of new oil needed while flushing out air-bubbles or air-pockets out of the hose line. Different brands have different ways of doing this operation.
The Fine Tune Adjusters
Some MTB brake systems like Avid, also have lever adjusters so that you can fine tune your lever height to exactly where you want it - you can raise it or lower it independently of the hydraulic condition.
If you apply what you thought I said and stuffed-up your bike as a result, you're own your own! :D
07-21-09, 09:03 PM
alright thanks for all the help! so will it stay as is or should i still get it bled?
It should be fine.
You don't need to do much to it (unless you can see leaks orwet marks) especially if the operation is normal.
With hydraulics the old adage holds very true:
If it ain't broken, don't fix it.
07-21-09, 09:39 PM
No, it does need to be bled. I think. at least it did earlier today. It would pull back all the way easily. Now since I pumped it, not so much. How long will it stay this way?
It's hard to say for sure because we can't "feel" or examine what you're describing.
There could be something genuinely amiss... or maybe what you're feeling could just be how Juicys feel when compared to the brakes you had prior. Some times if you over think something, you can experience all sorts of things going on.
One way to make sure is to find some friends who have the same brakes as yours and try them... or you can pop a visit to our LBS and try the "feel" of the same brakes on other bikes. If you're still suspicious after that, it might pay to have a mech look at it. If it's faulty and you're still under warranty, it shouldn't cost you anything for a replacement or repair (re-bleed), one would think...
07-21-09, 10:05 PM
Your best bet may be dropping by the LBS and have then give it quick once over.They'll know if you need to bleed them or not.
07-21-09, 10:34 PM
..Yes, I went to a LBS earlier and sure enough, yes. But then when I got home I pumped the brake lever and now it feels better. I still crave the answer to how long this effect lasts. They said I couldn't get it bled till saturday the 1st.
07-22-09, 12:19 PM
heres a little trick we use on the motorcycles
if you have a decently hard lever, but still think there might be air, zip tie your lever in to the bar(like you were pulling the brakes) and bounce the bike a little, then leave it over night. you open the system up and it should allow any air to move back to the top of the reservoir and out of the system
there is no telling how long your lever will last as hard how you have it now, the problem being there is a small ammount of air in the system, and you have most of it out, the biggest issue is when it gets hot, the air will expand giving you little to no braking power, but the likelyhood of this happening is pretty rare.
I still crave the answer to how long this effect lasts. They said I couldn't get it bled till saturday the 1st.
There isn't a specific answer...it varies.
If you're really worried, go buy an Avid bleed kit for $20 and some brake fluid and bleed them yourself. The Avid kit is simple and it's easy to bleed the Juicy/Code/Elixer brakes.
07-22-09, 01:38 PM
If you wanted to give it a try yourself, SRAM has some very good instructional vidoes on youtube.
here's the ones for Juicy 7s
07-22-09, 01:40 PM
One thing that i found adds to the poor brake feel is squeezing the brake when the bike is upright (rolling it around on one wheel). I had that happen once, bu my brakes have been fine since.
I never cut the hose from the factory though. (it is on the long side, but I'll cut it when I need to bleed it).
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