|   Juicy update #3. Easy bleed. | Tscheezy
Jan 8, 2004 10:04 AM | | Hey people. We just got a solid dump of snow (Wah!) so I am off my bike for a bit. I had wanted to thread the rear brake hose through the seat tube support on my Turner but had to undo the hose from the lever to do this since neither the caliper or lever would fit through the hole. As long as I was taking the hose apart, I figured I would do a bleed just for kicks to see what that was like. I was pleasantly surprised.
The bleed kit is pretty expensive for what you get, imo. I got if for $26, and it comes with two small (25cc) syringes, a bottle of DOT 5.1 fluid (enough for 5-6 bleeds of a single brake unit), some barbed fittings and crush olives, and two slick little adapters for screwing the syringes into the calipers and reservoirs. The adapters are basically a plastic plug which screws into the syringe with a short run of surgical tubing with a pinch clamp for stopping flow, and finally a threaded brass adapter with a knurled ring for screwing into the brake itself. The only part that seemed "custom" in the box was the little brass adapters and plastic bits, which could not have cost more than a few $$ for Avid to produce, so the price seemed kinda steep for the kit overall.
The lever/reservoir unit has a semi-clear rubbery-plastic bladder that acts as the fluid expansion chamber. You slide a cover off to view it. My brakes came with a small air pocket in each reservoir bladder, but even after turning the bike upside down a few times and having been shipped in that state, no air ever seemed to escape into the hose system as the levers always provided a firm braking platform. I was curious how hard it would be to evacuate all the air in the system though.
Anyway, to start the process, you draw some DOT fluid into one syringe, clamp the hose on the syringe adapter off, then pull a vacuum on the fluid by pulling the plunger back. The dissolved gasses in the fluid "boil" out, and you have a de-gassed fluid. You screw the adapter into the caliper and then do the same again, drawing any bubbles out of the caliper and boiling dissolved gasses out as well.
Then you screw the other, empty, syringe into the lever reservoir and push fluid from the caliper syringe into the empty lever syringe through the entire system, replacing all the fluid in the caliper, lines, and lever. Bubbles being transported up to the lever assembly are allowed to escape into the clear reservoir so you can see if any emerge. The plunger on the lever's syringe gets pushed out during this process to accept the used fluid being displaced. No spills, no mess, since the whole system is still effectively sealed.
The last step is to rotate the lever up at a 45 degree angle and push a few more CCs through the system from the caliper syringe to displace whatever air had finally settled in the reservoir. Removing both syringes and putting the plugs back in and rotating the levers back down revealed no air pockets under the clear plastic window on the reservoir. Pretty cool.
The brakes have a few hundred miles on them so far, and the fluid coming out of the front brake was almost totally clear but the fluid from the rear brake was a little bit turbid with a sort of black, inky suspension/discoloration. Not sure what this was or if that is normal.
In general, the system seems very well thought-out and even details like ease of bleed have been addressed. Other than my issues with the rotors, I am still extremely impressed with the brakes overall.
Take it easy, people.
tscheezy |
| Re: "dissolved gasses in the fluid"... | AndyH
Jan 8, 2004 1:21 PM | | I'm a little confused about this statement:
Anyway, to start the process, you draw some DOT fluid into one syringe, clamp the hose on the syringe adapter off, then pull a vacuum on the fluid by pulling the plunger back. The dissolved gasses in the fluid "boil" out, and you have a de-gassed fluid. You screw the adapter into the caliper and then do the same again, drawing any bubbles out of the caliper and boiling dissolved gasses out as well.
What kind of "dissolved gases" exist in fresh DOT brake fluid? And how on earth would they "boil" out by pulling on the syringe. Is this Avid "marketing-speak" or is this your description? |
| I'm not a real scientist... | Fist of Ham
Jan 8, 2004 1:57 PM | | ...but I did read the bleeding instructions on the Avid site. From what I vaguely remember in physics class is that if you reduce pressure on just about any liquid, it will boil. I think boiling is just a term used to describe changing from a liquid to a gas. Most of us are familiar with boiling water, but other liquids boil as well. Now, I could be totally off base here, it has been a while since I have given that subject much thought. But if you "pull a vacuum" on the liquid, it will reduce the pressure... when there is no pressure, that is a vacuum... and all it takes to boil a liquid is to reduce the pressure on it. Or, you add heat, that will increase the pressure of the liquid and when it has more pressure than the surrounding material, it will boil. Somebody could probably explain that a helluva lot better than me.
My question is... same as you... what gasses are present in brake fluid?
Q2: Assuming gasses are bad in brake fluid, why have I not seen any other manufacturer's brake bleeding instructions mention anything about "boiling them out"?
Q3: I have not ever heard of "boiling them out" in reference to bleeding automotive brake systems either.
Marketing hype, or is this a real issue? I'm curious myself.
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| Exactly, I've never read or heard about this in an automotive... | AndyH
Jan 8, 2004 3:38 PM | | ...application, including the "weekend-racer" types (track events, autocross, etc.) whom I would assume would want to extract the last bit of performance out of their brake fluids. |
| I'm not a real scientist... | Fat Elvis
Jan 8, 2004 5:51 PM | | The bubbling is just air that is dissolved in the fluid. It's just like when you heat up a pot of water. Before it reaches the boiling pt (100 C), it forms bubbles on the sides of the pot, usually around 50-60 C. That's just air dissolved in the water. On the other hand, if you pull a vacuum on the water, the air bubbles will also form. The boiling point of the brake fluid is way too high for it to start boiling under the vacuum you can pull with a plastic syringe. |
| You are pulling a vacuum using the syringe plunger | Tscheezy
Jan 8, 2004 2:04 PM | | This is an official step in Avid's protocol, and while they don't explain why you do it, here is what I am going to guess is the reason (drawing heavily on long-gone-by science classes: be very afraid!).
Gasses dissolve in liquids, or, they are "in solution" (e.g. fish "breathing" water to obtain oxygen dissolved in the water). Lowering the pressure on the fluid will cause these gasses to come out of solution and form bubbles (e.g. a diver suffering "the bends" from Nitrogen bubbles in their body when ascending to shallower depths and lower pressures). Unless DOT fluid is stored under a vacuum, it will naturally have some gas dissolved in it. The colder the liquid, the more gas it can hold in solution (e.g. a cold beer has a lower head-space pressure and foams less than a warm beer). To keep these gases from forming bubbles at unwanted moments, like when the brake fluid warms up, you can evacuate the gas ahead of time by pulling a vacuum on it, and then storing the fluid in a container with no access to air (a sealed system with no head-space). In this case, the brake system itself.
Now my big disclaimer: I have no idea if the train of thought above derailed before leaving the station or if any of this is actually applicable to hydraulic disc brakes, but I just followed the directions and then rationalized the steps as well as I could. Avid talks about pulling a vacuum until no more bubbles evolve, I called it "boiling" because that is what it looks like. You can boil liquids at lower temperatures by lowering the pressure on them, but in this case we are not volatilizing the DOT fluid itself.
See ya'll
tscheezy |
| hmmm... | alibi
Jan 8, 2004 2:32 PM | | Unless you provide some headspace inside the syringe, you are not degassing the fluid. You can pull back on the plunger of a fluid-filled capped syringe and see bubbles form all day, but if there is no region of lower pressure, i.e. an air space, for the gas to "boil" into, the gasses will simply go back into solution when the plunger returns to it's start position and pressure returns to ambient.
Tscheezy, your explanation is bang-on (not so many cells have gone over the dam as you might think), but I would add that the headspace inside the syringe is mandatory. Maybe that was in your initial post and I missed it. |
| Good point... | Tscheezy
Jan 8, 2004 3:20 PM | | Not the one about me still having some brain cells left, of course. ;^)
Anyway, I still think it does work, and specifically based on a gas-bubble surface-area basis. You can get gas to diffuse into the liquid at a certain rate based on the surface area for exchange you provide, and how quickly the dissolved gas can diffuse away from the gas-liquid interface (the gradient). What I think you are doing is causing many tiny bubbles to evolve (huge surface area), which then coalesce into larger bubbles ( same gas volume, but less surface area), which then merge into an air pocket in the syringe (still the same volume, but with the lowest surface area, and your head space). Now left to it's own devices, the gas would be reabsorbed by the fluid over time, but you get done with the bleed long before it has a chance to reenter solution, because the gas now has to rediffuse from the main bubble in the syringe all the way through the de-gassed fluid from where it came. If you were to shake the system vigorously and cause the main bubble to disperse into many smaller bubbles and thus increase the gas/liquid's surface area dramatically, it would reenter the solution quickly.
As an example, I can carbonate my homebrew in about 3 days by letting the CO2 dissolve into the beer passively though the head space in the keg, OR I can shake it and have it carbonate in about 15 minutes because of the foam having the biggest gas/liquid interface possible.
I'm gonna stop, because I'm over-taxing my remaining cells.
See ya,
tscheezy |
| re: Quality of Avid's instructions? | CrashTheDOG
Jan 8, 2004 3:19 PM | | Tscheezy,
As I mentioned in the Turner forum, installing my Juicy's when they arrive from Larry will be my first hands on experince with hydraulic brakes. How complete are the instructions I'll recieve with the brakes? Are they fairly self explanitory?
I've downloaded, read, printed, read and reread the "Disc Brake FAQ's" to try and minimize the learning curve on any issues I might have. I'm hoping that by installing them myself I'll have one more part on my bike that I'll be able to service and maintain.
If you can think of any issues that aren't addressed in the instructions, or some hints on installing them on the 5 Spot and Z1 SL (does the front line fit in the SL's holder) I'd appreciate any help I can get. |
| The brakes themselves have very few instructions with them. | Tscheezy
Jan 8, 2004 3:36 PM | | The bleed kit, on the other hand, has pretty detailed and easy to follow instructions. Then again, bleeding is relatively complicated compared to bolting a set of brakes on.
It really is pretty much a no-brainer. I did not use the SL's hose guide, because I like to route the hose up the outside or inside of the fork leg so I can send the hose arcing across the M-arch. This makes a smooth bend up and back to the lever, and the hose does not rub anywhere. Look at some of my pics and you will see my routing. For the rear, I ran the hose along the provided guides on the Spot and around (not through) the seat tube support to begin with. Other than that, follow the instructions which do come with the brakes and you will probably do fine. There really aren't any "gotchas". Just don't squeeze the brake lever with no rotor or plastic spacer (provided) in the caliper, or you will press the pads together and have to pry them back apart.
If you hit a snag, post up, but it really isn't very hard.
tscheezy |
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