| Author | Message | ||
     Reepicheep |
I replaced my front tire, and when I put it all back together, I noticed that the brake lever was now *really* taught, like the best brake bleed I have ever had. I never removed the caliper, so I don't think I released a bubble that way, but in the process of wiggling everything off I did push the pistons way back (such that it took quite a few pumps of the brake lever on reassembly to get it to engage the first time). So here is my question.... Was this just an hallucination based on riding my KLR-250 (where Kawasaki added insult to injury by putting the KLR-650 rear brake on the KLR-250 front)? Or was there a bubble trapped up in the caliper somewhere, and when I retracted the pistons, I released it to go up the brake lines, where it worked its way out? Is that possible? If so, would an "easy between bleeds tweak" to a daily rider be to do a periodic application of a clean scraper to force the pistons to retract, leave them that way over night, then pump them back up in the morning? It would be a 5 minute job with no disassembly, easy enough to do once a month or so. | ||
| Fdl3 |
I cannot answer your question. I can only corroborate your experience. | ||
| Mtg |
It looks like you answered your own question: it was probably a bubble trapped in the caliper. I wouldn't recommend pushing the pistons in, then pumping the lever as an easy bleeding technique. The fluid down in the caliper gets really hot and turns brown while junk in the lines seem to collect down there. Just do a quick brake bleed and you'll keep the fluid in the caliper fresher while flushing out the junk (and water) that accumulates over time. | ||
| Henrik |
Some people mention having good luck firming up their brakes by strapping down the front brake lever with a bungie cord overnight. The idea is that the pressure would push out any bubbles in the system. I personally haven't tried it, but have a hard time logically explaining to myself how that would work. But then, if it works I guess it works. But that could have been the mechanism that firmed up your brakes; dislodging a bubble that then somehow escaped the system. Realistically though, I'm thinking it's just wishful thinking  Sorry. Henrik | ||
| Dentfixer |
Pushing in the caliper pistons accomplishes 2 things. 1. It will free them up from the dust and stuff that accumulates around the piston and seals. 2. This will also allow the pistons to be re-lubricated (from the brake fluid) all the up their surface. If your pistons are even slightly sticking it will show itself as a "spongy" brake feel. This is most likely what occurred on your bike. Any bubbles is from poor bleeding or overheating to the boiling point. I believe these instances would be quite a bit more noticeable than the slightly sticky pistons in the above example. | ||
| Alchemy |
Expressing my complete ignorance of the subject I wonder if the added pressure over an extended time would move small bubbles into solution or dissolve them resulting in a better feel immediately afterward. Negative pressure would bring them out of solution but that would happen at another time and maybe at a better place in the system. Make any sense? | ||
| Reepicheep |
The first plausible explanation I heard for how "leaving the lever squooze overnight" helps was: 1) The pressure makes any air bubbles in the system physically smaller (which it would, the air is compressible and the liquid is not). 2) A smaller bubble is more likely to work it's way up and out of any bubble traps in your system then a larger bubble is. I can also imagine that if the bubble is up against some sort of sealing surface (caliper piston or bleeder bolt), air under pressure over night would probably "leak out", where fluid under pressure would not. To be honest, if I had to "bet" on one of these mechanisms, it would be this last one. Somebody call Mythbusters. I expect I just had a bubble up in there, in the caliper piston (where I can bleed all day long and not get it out), and when I pushed the pistons back it was displaced out and up into the brake lines, where it worked its way up and out of the system at the master cylinder. Alchemy's description makes sense as well... if air could indeed go into solution in brake fluid (can it? Doesn't that kind of defeat the purpose of brake fluid?), then from a statistical standpoint, it would always be a net gain. Trapped bubbles being forced into solution will come back out of solution "somewhere else", so some of what was trapped won't be trapped again. After enough cycles, most trapped air would be gone. Lubricating the pistons makes sense as well. Fun stuff to think about. | ||
| Bookyoh |
I have had similar experiences after removing the wheel and caliper for a tire change. On my M2, the level travel reduced so much that the brake could be applied enough at low speed and not activate the brake light switch. I found that the brake rotor had shifted to one extreme so that it slightly overhung the carrier. I lightly lubricated the carrier "buttons" with WD-40 and was able to push the rotor axially. After reassembly, the brake lever travel was back to normal. I just had a new tire put on my S3T this week and have the same brake lever travel issue that I had on the M2. I'll take the pads out, push in the pistons, and try and move the rotor tomorrow. Mark | ||
| Dmhines |
I just had this same exact thing happen today. Took wheels down to Cycle Gear for some new tires ... Put everything back together and Whoa!!!! The front brake is super sensitive and has 0 play in it ... Prior to changing the tire it was somewhat spongy but predicable and would be fully engaged with about an inch or so of pull. I need to get used to it ... I prefer the brake lever feel I had before ... wish there was a way to get it back ... | ||
| Id073897 |
The caliper piston's o-rings apparently tend to stick and therefore pull the pistons back into the caliper a bit too far. This gives more play at the brake lever because you have to move the pistons a longer way until the brake pad touches the rotor. This is a well known problem for (e.g.) Triumph bikes. Regards, Gunter | ||
| Bombardier |
A Note on WD-40 and brake pads/rotors. This stuff was developed for the Atlas Space Rocket programme and does not burn off the pads/rotors. My bike was in the rain when I was quite ill and as I had no strength to move it I sprayed it all quite liberally with the trusted WD-40. After I got better my first corner at the end of the street changed into a straight followed by a gutter hop as the brakes did not grip at all. Tried to burn it off the pads by riding to work(50km) with the front brake dragging - no good. The only fix was new pads. All on K100 BMW by the way. |
