Tech FAQ: Bleeding Avid Elixirs and DOT v. Mineral Oil

Dear Lennard,
After about a year of use, my rear Avid Elixir needed bleeding. Doing the full monty looks easy enough, but being lazy, I just bled both levers, which made everything feel good as new. So one question is how often do I really need to bleed the calipers and lines; doing a full fluid replacement? I’ve heard once a year, but is that really necessary?

I wondered because you mention in your latest book that the fluid in the caliper can get contaminated. How much of a concern is this? You also mention lubricating the caliper pistons for some older Shimano brakes with sticky pistons, and I was wondering if that’s something I should do every once in a while for my Elixirs. It’s certainly would be easy to pop out the pads and drop in a little oil.

I was also curious where the air in the reservoir comes from. The system seems tight as a pizza — no leaking fluid that I can see. Is there just always residual air in the fluid that is forced out over time? I’m guessing this might be the case because over the year the rear brake clearly accumulated more air in the reservoir than did the front.

As well, are there any advantage/disadvantages to brakes that use DOT fluid vs. those that use mineral oil? When a friend was buying a bike recently, the sales guy was heavily advocating DOT style brakes, but I have several friends with Shimano systems that love them. One hasn’t touched his XTR brakes in seven years — just putting in new pads as necessary, and they still seem to work just fine.
– Steve

Dear Steve,

Lots of questions here.

1.   In general, if it feels tight and works great, that is a good guide telling you that further maintenance is not a necessity.

2.   The time period totally depends on how, where and when you ride. Obviously, more extreme weather (high heat with long, steep descents), mud, rain, etc. will shorten the maintenance interval, as will longer rides on terrain requiring more braking.

3.   Contamination of the fluid certainly can happen with hard use in extreme conditions lots of heat or lots of water-borne grit both tend to facilitate dirt getting past the piston seals in the calipers. High-quality brakes used under moderate conditions actually require very little maintenance, and the fluid may be totally clean still after a couple of years of riding.

4.   If the brakes are retracting okay, there’s no need to lubricate around the caliper pistons; it will just attract dirt for no good reason.

5.   All brakes will eventually allow a bit of air to come out of fluid that appears air-free. That said, Avid brakes are the only disc-brake system that can be easily vacuum-bled by the consumer. This increases the service interval by eliminating more air held within the fluid than simple gravity bleeding methods can do. Pulling a vacuum on brake fluid allows many air bubbles to come out of solution that would otherwise not be discovered or bled out.

6.   I don’t think that there are any inherent advantages of DOT brake systems over mineral oil ones. In both cases, the brakes can be designed very well, and lots of research on brake fluids of both types has resulted in fluids with high-temperature performance characteristics, among others.
– Lennard

Dear Lennard,

I am confused over my laid back seat post on my Scott Scale mountain bike. It is the first one I have ever had. What are the advantages and what are the drawbacks? What if any difference in set up?
– Dale

Dear Dale,

I’m not sure what you’re wondering about. If you mean the setback on the seatpost itself, most of the Scale models come with a standard setback to the seatpost of about an inch.

And if you mean the angle of the seat tube itself, that too is not very laid back except in comparison with full suspension bikes. Depending on size, all Scales have either a 73-degree or 73.5-degree seat angle, which is a normal angle for road bikes. Full-suspension bikes need a steeper seat angle to keep the rider from wallowing down over the back wheel when climbing with the rear suspension compressed. But a hardtail generally works well with a road-bike-type position of the saddle relative to the pedals.

So just set up the seat height and setback like you would a road bike position. Of course, there are a million theories on this. I offer one method that you can find either in my road bike maintenance book or in my cycling primer, but I urge you to seek out other opinions as well.
– Lennard

Dear Lennard,

Will the new Shimano 10 speed “Dyna-Sys” XT cassette (11-32, 11-34) and XT rear derailleur work with Shimano Ultegra 6700 and Dura Ace 7900 road shift levers?
– Dale

Dear Dale,

(Editor’s Note: This is a revised answer to Dale’s question.) No. The cable pull is different. As Shimano Multi Service Technical Representative Nick Murdick puts it, “The Dyna-Sys rear derailleur uses a different cable-pull ratio then anything else we make as well, road or mountain, so it can only be used with a Dyna-Sys shifter.

“I imagine people will try to match the Dyna-Sys cassette with 9-speed mountain rear derailleurs, which do use similar cable pull ratios but they are not exactly the same so there will be some drop off in shifting performance.

“The change in cable pull ratio for the Dyna-Sys system comes from the evolution of the modern mountain bike. When we designed the previous standard for cable pull people were riding steel hard tails. Now, with full-length housing, moving suspension pivots and people riding in harsher conditions then ever, the whole system needed to evolve as well. The shifter now has more leverage to pull a contaminated cable around corners and through dirty housing. It is also better matched to the rear derailleur so the light effort to pull cable and get to the next bigger gear is more consistent across the cassette.”

That said, the older XT / XTR rear derailleurs should work OK. It’s better to use the narrower upper pulley with Shimano’s “center-on” side play from a Shimano 10-speed road rear derailleur (7800/7900) than the 9-speed MTB upper pulley. The cable stroke on a Shimano 9-speed MTB rear derailleur is technically not a perfect match to Shimano road STI levers because the MTB cable stroke was designed to work for both High- and Low-Normal rear derailleurs. But for 9-speed conversion, it is an acceptable conversion, and it should be for 10-speed as well with that jockey wheel change.

– Lennard

Readers can send brief technical questions directly to Lennard Zinn.

Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

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