As we noted earlier, a too-tight cable prevents complete retraction of the piston, which blocks the passageway connecting the master cylinder's two reservoirs: the working reservoir on the wet side of the piston and the expansion chamber. Typically, a too-tight cable is a problem when the fluid heats up, expands and causes the brake to rub, or even lock up.
The opposite, however, can also happen. A too-tight cable can also keep the working part of the hydraulic system from being replenished while the system is cooling and the brake fluid is contracting. In this case contracting fluid creates a vacuum, sucking the piston into the lower half of the remote master cylinder. The primary indicator for an in-vacuum system is an extremely limp brake lever, and braking only at the very end of the pull. Attempting to readjust the cable or pads with the piston sucked downward will be totally ineffective.
Under what circumstances will the in- vacuum problem occur? Most often, this occurs just after you've descended a hill. While the fluid is still hot, you might either over-tighten the cable or, more likely, park the bike in such a way as to depress the brake lever (against a fence or another bike). Another possibility is when a rider uses some sort of strap to lash down the brake lever, thinking the disc will make an excellent parking brake. In any of the above cases, as the brake cools and the fluid contracts (a couple of minutes is all it will take) the piston within the master cylinder is sucked into vacuum. The least common (though still possible) way we've seen for a brake to go into vacuum is if the cable is over-tightened on a warm afternoon and then you try to use the bike the following morning when the temperature is still cool.
Prevention Strategies
1) Never over-tighten the cable.
2) Never lash down the brake lever. The disc is not a parking brake.
3) Do not attach the disc brake cable to a shift lever - the disc is not a drag brake.
Two Step Cure
Because an in-vacuum system can't restore itself until the piston is retracted, and the piston can't be retracted while the vacuum exists, the system must be opened to release the vacuum. Because you don't want air to enter the system, you'll need to: 1) Loosen the cable. 2) Fill the larger clear plastic bleed hose with 2 to 3 inches of DOT fluid. 3) Attach the bleed hose on top of the caliper's bleed valve (the steel nipple with the rubber cap). 4) Use an 8 mm open-end wrench to open and then immediately close the valve with one-quarter counterclockwise and clockwise turns. The system will inhale about an inch of brake fluid, which allows the spring at the face of the piston to push the piston back into its normal, retracted position.
At this point, the brake will work fine, but there is a little bit too much fluid in the expansion chamber, which means the next time the brake heats up, there won't be enough room in the expansion chamber to accommodate the expanding brake fluid. To cure this second problem, which will cause a hot brake to drag, you'll need to remove the slotted vent screw at the top of the master cylinder, drop the bleed kit's plastic spacer (or a common one-quarter-inch ball bearing) into the recess, and then tighten the slotted vent screw back down which will pressurize the system. If you then open and close the bleed valve the excess fluid will dribble out. Don't forget to remove the spacer or ball bearing and wipe away all excess fluid.
A Typical In-Vacuum Scenario:
At the bottom of Five-Mile Hill Luke and his friends stopped to have a Coke at a convenience store. While parking his tandem, Luke inadvertently depressed the brake lever by leaning it up against another bike. Three minutes later he returned to discover a limp brake lever. To confirm his fears, Luke loosened the cable, pulled back the master cylinder's boot and plunger to find that the "dry" side of the piston, normally visible, had disappeared. As the brake had cooled, contracting brake fluid had sucked the piston down into the lower end of the master cylinder.
Calmly, Luke walked back inside the store, retrieved a plastic soda straw, and stood in the parking lot until someone drove in. At this point, he asked to "borrow" a couple of inches of brake fluid. Because Luke realized 99.9% of all cars, trucks, and motorcycles use DOT 3 or 4; he could get all the fluid he needed from an under-hood brake fluid reservoir. He dipped the straw two inches into the reservoir and placed his thumb over the top of the straw to extract the fluid. Holding the straw away from his body, he walked back over to his bicycle, jammed the straw onto the caliper bleed valve, released his thumb from the top of the straw and used his multi-tool to open and close the valve. As the valve was opened, the master cylinder spring pushed the piston back into its normal retracted position. As quick as the valve could be opened and closed, most of the fluid was sucked out of the bottom of the straw. The system had been restored.
Days later, on the eve of the next ride, Luke figured out how to remove the excess fluid.
To optimize the fluid level Luke removed the master cylinder's slotted vent screw, inserted the 6mm white plastic spacer from his Formula bleed kit (he could have instead used a one-quarter inch ball bearing) into the chamber, and tightened the vent screw back into place. In doing this, the spacer was forced down against the reservoir's float, which in turn pressurized the hydraulic system and applied the brake. Luke then opened and closed the caliper bleed valve, which allowed the excess fluid to dribble into the rag he was holding. Because he remembered to remove the spacer from the reservoir, Luke's system again had the proper 6mm of space for fluid expansion.
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