Gear

Tech FAQ: Better Brake Bleeding

Yes, even tech meister Lennard Zinn has fouled up disc brake bleeds. He tells how not to…

Dear Lennard,
I have your latest
Zinn and the Art of Mountain Bike Maintenance book and love it. I use it as my “Bible.”

I’ve used the Avid Elixir CR brakes for about a year now. They seem to need bleeding much more than they should. I have probably had to bleed about five times so far since purchase. I read in some forums that DOT brake fluid, once opened in its container, can absorb moisture from the air and should be discarded shortly after. I use Motul DOT 5.1 for my bleeds and have had this bottle (which is still half full) for over a year now.

I thought I did the most perfect bleed for my front brakes two weeks ago and I have recently found them a bit spongy again.  Assuming it was a very good bleed, do you think the idea of old DOT fluid being less effective has merit? Could I have a problem with the caliper or hose for this to happen? Any other ideas?
-Jeff

Dear Jeff,
First off, let me say that SRAM Customer Service in Chicago is generally known for handling consumer questions like this pretty well.

That said, I will address a few specific points here. First, yes, DOT fluid does indeed absorb moisture from the air.You can test this by filling a glass with that DOT fluid of yours (which is now junk, since you’ve had it so long) and leave it overnight. Unless you live in the desert, that glass will be overflowing in the morning, since the DOT fluid will have absorbed so much water from the air overnight.

The water that is most certainly in your can of DOT fluid won’t necessarily shorten your bleed interval, but it will take your brake fluid’s heat capacity. In other words, the water decreases the boiling point of your brake fluid, and you could find on a long, hot downhill that you suddenly don’t have brakes as a result. If your brake fluid boils, it of course goes from the liquid state to the gaseous state. While liquids are essentially non-compressible, gases are certainly not (or we wouldn’t be rolling on pneumatic tires!). So, pulling your brake lever will only increase the gas pressure in the lines, but it won’t have the direct effect of pushing your brake pads against the rotor very hard — certainly not as hard as with only liquid in the lines.

I’m also pretty sure that letting the fluid boil in your lines results in air getting into the system more easily. I theorize that this is because the gas tends to blow past the piston seals and then suck air back in on the return. Boiling also will certainly take any air that’s in the lines already out of solution, where it can be trapped up in your lever and decrease your braking even when cold.

There is, of course, also the possibility that the seals around your pistons leak, or that seals at hose junctions are leaking.

As for the bleeding problems, I would not doubt if part of your problem is poor seals in your bleed kit. I know from unfortunate personal experience that if you don’t rinse that bleed kit out completely with water after every time you use it, the seals will become compromised. And then you won’t pull the kind of vacuum or push the kind of pressures you’re supposed to.

We have screwed up a number of these bleed kits here at Zinn Cycles (since we build 29ers for really tall people, we constantly have to replace the rear brake hose with a longer one to get it to reach and consequently do a lot of brake bleeding, and we sell lots of SRAM and Avid brakes). One way we’ve screwed up the bleed kits is by using the same kit to bleed both Avid brakes and RockShox Reverb seatposts; the fluids are not compatible and the RockShox fluid will swell the seals in the bleed kit. But even if you don’t commit this faux pas, the seals in the syringes in the bleed kit will deteriorate over time if they are not rinsed after each use. Seems like the ideal way is to rinse them first with rubbing alcohol (what you use right away if you ever drip brake fluid onto your frame’s paint job) and then with water.

We have had some success with just buying a couple more syringes at hardware stores and attaching the bleed kit fittings to them with heat, wire and zip ties. But we’ve also sometimes found that the plastic plunger in these syringes sometimes can’t take the vacuum and, when pulling a hard vacuum to draw air bubbles out of the system, the plastic knob will break off of the end of the piston to which the rubber seal attaches.

Long answer, but I’d get a new bottle of Avid DOT fluid and a new Avid bleed kit and see how you do.
-Lennard

NEED MORE BRAKE BLEED INFO? Read this Tech FAQ column by LZ. Or maybe you like to watch? Here’s LZ doing a brake bleed video in his shop

Dear Lennard,
I was surprised to read that one can’t use Shimano 10-speed road shifters with shifters with a Shimano MTB rear derailleur. Last summer was the third time I’d ridden the hilly D2R2 in New England. The first time I rode a compact front with my standard 12-26 rear and wanted something lower. The second time I used a triple up front with a 9-speed cassette. The shifting was handled nicely by an XT mid-cage derailleur (RD-M739). I didn’t really need my lowest gears, and felt like I was carrying extra stuff around with me for some 120 miles. Last summer I used an Apex 11-32 cassette, Ritchey compact 34-50 crankset, my trusty old M739 derailleur, and 10-speed DA shifters. Performed flawlessly.

Maybe the old derailleurs work better with the road shifters.
-Thom

Dear Thom,
That is exactly right; the old derailleurs do work better with the road shifters.

I thought I made it clear that 9-speed Shimano MTB rear derailleurs are compatible with Shimano 9-speed or10-speed shifters, both road and mountain. That’s why those two setups you describe worked.

But 10-speed Shimano (Dyna-Sys) MTB rear derailleurs are only compatible with Shimano 10-speed MTB shifters. They are not compatible with any Shimano road shifters — neither10-speed nor 9-speed.
-Lennard