Technical FAQ: Air in hydraulic brakes, proper bike storage, and more

This week, Lennard Zinn answers questions pertaining to air in a hydraulic road brake system, storing bikes upside down, and more.

Hydraulic brake pressure

Dear Lennard,
I had a Force CX1 hydraulic (rear) brake that I just could not get enough pressure in. After a bleed, the lever would hit the handlebar with a good squeeze. That lever and brake were warrantied for a separate issue, and I got a brand new set from SRAM. Since I have internal routing on my cyclocross bike, I unfortunately had to immediately re-bleed. And I’m having the same issue! It must be me! I’m following standard bleed protocol (via your book and SRAM’s YouTube page). I have SRAM/Avid’s full professional bleed kit. (I have not had this issue with the front brake, oddly.)

I looked for solutions in your book but found nothing. However, I think my edition is from 2013, and road hydraulics have changed since then, I’d guess. What could this be? Why would the pistons keep recessing fully if they aren’t contacting anything?
— David

Dear David,
You may have the right bleed kit and may have followed the bleed instructions, but you have air in that brake. I have those same brakes on one of my bikes, and they not only are tight, but they are also my standard of comparison for road disc brake performance.

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Did you take apart the lever and open that connection? If so, that’s probably the source of your problems. The banjo fitting from the caliper is probably too big to fit through the holes in your frame, but if you can avoid cutting the hose and can fit that through the frame, that is the way to go. And if you have to cut the hose or open a connection to fit the hose through the frame, the banjo fitting at the caliper can be opened. Unlike many banjo fittings, it is not a crimped fitting; it has an inline hose connection that can be unscrewed there, and a new olive and barbed nipple can be installed at that end instead of at the lever end.

I’m trying to steer you to the caliper, because to connect a hose at a SRAM hydraulic road brake, the lever must be disassembled. The lever end of the hose also terminates in a banjo fitting, but once it’s connected again to the master cylinder, you can’t pressure-test the system until you’ve reassembled the lever. And if there is a leak at the lever, you have to take it all apart and start over. In case it’s not clear the circle of frustration you can be on, I’ll reiterate: you won’t know if you got it right or wrong until the lever and its connection are completely reassembled.

If you did manage to avoid these lever pitfalls and only opened it at the caliper, my guess is that you either have a leaking caliper fitting or, perhaps when you bled it, the hose wasn’t trending up from caliper to lever the entire way. If the caliper was higher than the lever, for instance, or even if there was a loop in the hose that was higher than the lever, some air could have avoided going out into the lever syringe.

First, I would wrap a tissue around the connection at the caliper and another one around the entire lever body after peeling the rubber hood back. Then I’d pump the brake repeatedly to see if any DOT fluid leaks out on the tissues. If none does, then you don’t have to disassemble and reassemble anything; you just have to bleed the system carefully and completely.
― Lennard

Storing bikes upside down

Dear Lennard,
I have a question that no one seems to have an answer for. I store my bikes hanging upside down, but I am wondering whether this is a good idea for hydraulic braked bikes, such as my gravel and mountain bike. Does it compromise the braking if the fluid in the reservoir drains (or does it?) as the bike hangs upside down? I have noticed a spongy feel to the brakes after I take them down. It usually returns to normal after a few squeezes, but it sometimes takes quite a while to feel right.
— Frank

Dear Frank,
It’s not a great idea to hang those bikes upside down because it can allow air into the caliper, or at least into the hose. It’s not that the reservoir drains, but rather that air floats to the top. Air bubbles that were previously above the fluid in the lever reservoir and hence out of harm’s way and not causing any problems, can, when upside down, work their way through the metering hole into the master cylinder, and from there into the hose and eventually to the caliper.

It’s often not a big deal, because after you pump the lever a few times, the air bubbles may work their way back up into the reservoir. However, air bubbles can get trapped in some of the corners in the caliper; then the full brake performance will not have returned after a few pulls of the lever.

Some brakes have a flexible bladder in the brake lever, like in one of those open baby bottles with a plastic bag for a bladder. When the baby sucks on the nipple with the bottle standing upright, the baby’s removal of liquid just decreases the volume in the plastic bag while eliminating all of the air. Similarly, if a bladder-type brake is bled properly, there will be absolutely no air in the bladder or anywhere else in the system, and it won’t matter what position the bike is stored in.

But if the brake has a fixed-volume reservoir, you can have no braking after pulling it down from its upside-down storage position. You can have a bit of air at the top of the reservoir without compromising performance, but if any of that air got in the cylinders or anywhere between them while it was hanging upside down, you no longer have a continuous column of non-compressible fluid throughout the system. The spongy feel or absence of braking is the result of compressing the gas and shrinking the volume of the bubbles, as opposed to what is supposed to happen — namely, the slave piston receiving the full push the master piston applied to the opposite end of the column of brake fluid.
― Lennard

Sealants in road tubulars

Dear Lennard,
My question is regarding Vittoria Pit Stop and sealants on road tubular tires in general. I have a front tire that I’ve used two cans of Vittoria Pit Stop on, and I was wondering how many times a tubular tire can/should handle sealant treatments.
— Angelo

Answer from Vittoria:

When using traditional sealants in tubulars, it is important to remember that there is an inner-tube inside of the tubular. Therefore, sometimes these punctures are not visible. That said, you should only have to use the recommended amount once, assuming the puncture is within the size range that is possible to seal.

If your tubular is not sealing, then it is fair to assume that the puncture is too large to seal and the tubular must be replaced.

Lastly, it is important to replace the sealant in the tubular every six months or so, as the sealant will tend to lose effectiveness over time.
— Ken Avery
Vittoria Vice President, Marketing and Product

Follow up: Quick-link opening

Dear Lennard,
I just want to say that Wippermann does not describe its Connex Link as one-time only. Read here
 and here.
— Oleksandr

Follow up: Pedal-thread direction on unicycles

Dear Lennard,
Sorry to keep piling on the pedal thread discussion, but I am confident that the pedal thread directions are to keep the pedals from loosening from normal riding. I ride unicycles, which have the same pedal threading as bicycles (left is left-hand thread). I once rode my unicycle backwards by accident (right pedal on left side). Despite pedals being tightened properly, I got about a mile and one pedal fell off and the other was loose. This never happened when riding the unicycle normally.

Another way this can be tested is to take the front (captain) crankset of a tandem crankset, flip it around, and mount it on a normal bicycle. This will result in pedal thread directions being opposite from normal. I imagine the result will be the same as I experienced from riding my unicycle backwards.
— Charles

Dear Lennard,
In regards to why left pedals are reverse (left-hand) threaded, it is instructive to also look at the directions of the bottom bracket cup threading. Because all the forces on the pedals are transferred through the cranks to the BB, the precession effect of these forces will also manifest themselves on bottom bracket threads. However, because the threads are on the stationary cup rather than the rotating spindle, the precession forces will tend to want to rotate the BB cup in the opposite direction compared to the pedal spindle. This is why the right cup is left-hand threaded on English, ISO, and other BB cub standards, because precession forces will tend to unscrew right-hand threads on the right cup.

The primary exception is the Italian threading standard, which uses right hand threads on both cups. But it should be noted that Italian-threaded BBs are prone to the right cup unscrewing in use, unless tightened very, very firmly. Cartridge style Italian threaded BBs, which have no shoulder on the left cup, are sometimes known to have the entire cartridge migrate toward the right, with the right cup screwing outwards (loosening) and the left cup screwing inwards (tightening), which is due to the precession forces.
— Mark