Hydraulic disk brake stops working when hot; back to normal after cooling - is this expected?

Question

I have experienced a strange failure of my brake.

I was descending a steep smooth road, using only my front brake (my rear brake was broken; it was an unplanned bail from a route). I tried keeping the speed low, so i could jump off the bike at any moment.

After going this way for maybe 2 minutes, I stopped briefly to let some car pass (so released the brake for a few seconds). As I continued to descend, I pressed the brake, and surprisingly, its handle went all the way to the handlebar without doing any braking! So I immediately jumped off.

I didn't believe at first the bad luck I had, but then I pressed again and again on the brake (not while riding now), and gradually it returned to normal; after 1 minute it would give full braking force again.

So I wonder, is this an expected way a brake would fail while overheating? I heard that it would start to stink really bad before it became dangerous to ride, so I didn't worry about overheating.

The model of my brake is Hayes Nine.

Answer 1

The least you need to do is bleed the brakes to remove trapped air. It's best though if you completelly change the fluids and bleed. You can do that in an LBS, or yourself if you have a bleeding kit and know how to do it.

The reason for the behavior you described is that air is trapped in the system and makes things mushy and unpredictable. The fact that after a couple of lever presses it came back to "normal" is because hydraulic brakes are auto adjusting both on the pads end (when pads are worn) and on the lever end (when air is in the system).

Air bubbles exist in the system either because a proper bleed was not done in the first place or because the fluids absorbed moisture which vapourated and turned into air as soon as you overheated the fluids. Also note that as soon as the fluids absorb moisture (water) then the boiling point falls even more so having this issue is even easier.

Answer 2

All brakes work by converting kinetic energy into heat through friction.

In disk brakes, the heat builds up in both the rotors and calipers. Under heavy sustained braking, such as down a long steep hill, the calipers can get hot enough to start to boil the brake fluid.

Because gases are easily compressible, boiling brake fluid causes a loss of braking ability. This is called brake fade.

The good news. This will go away as soon as the brakes are allowed to cool down, and only happens on sustained heavy braking. 2 minutes constantly engaged is certainly a very long time to brake.

The bad new. This can only be avoided by using an individual brake less. On long downhills, switching between front and rear will allow each brake to cool in turn, and stop brake fade.

If the brakes do use something like DOT4 brake fluid (usually not for bicycles) the fluid will eventually wear out, and fade badly. This happens when water gets into the fluid, lowering the boiling point. Flushing the brakes with new fluid will help if the fluid has degraded.

This is true for all hydraulically activated brakes, on any vehicle anywhere.

Answer 3

Just to expand the answer for future reference. Sram have a good explanation of this:

Vapor fade is caused when brake fluid boils inside the brake caliper and hose. Boiling is where a drop in fluid pressure (which occurs when a fluid is heated) allows microscopic pockets of gas suspended in the fluid (bubbles) to expand, creating larger bubbles. Fluids under significant pressure require much higher temperatures to boil than less pressurized fluids. This means that the fluid can actually reach temperatures above its normal boiling point during hard braking (high pressure). However, as soon as the lever is released, the pressure immediately drops. With the fluid temperature still above its normal boiling point, this sudden loss of pressure causes the fluid to instantly boil, creating large bubbles in the brake system. With large bubbles now present in the system, pulling on the brake lever compresses the bubbles in addition to moving fluid, allowing the lever to bottom out against the handlebar before enough fluid pressure can be generated to actuate the slave pistons properly. This results in a loss of braking power.

And add this:

A brake system that has undergone vapor fade should be completely bled before riding again. Boiling brake fluid will cause a breakdown of the fluid composition and can exhibit lower boiling points in future brake applications.

Reference Hydraulic Disc Brakes Overview