Brake fluid vapour lock

This is a term used when the brake fluid in a vehicle brake system is overheat and loses efficiency.

First we will explain what brake fluids usually consist of.

There are three main types of brake fluids, Mineral brake fluids, Glycol brake fluids and silicone brake fluids. The GLYCOL brake fluid is most common and used in 99.5% of modern vehicle brake systems. There can be up to 2 pints of brake fluid used in a vehicle brake system and this is piped around the vehicle using copper or rubber hoses to feed the fluid from the operator to the brake system. Operating the brake forces the fluid into the master cylinder and then brake calipers or wheel cylinders to actuate the brake.

Back to your physics class there are two types of fluids, compressible and incompressible.

Incompressible fluids are liquids, compressible fluids are gases so it is obvious that incompressible fluids are what we need in a brake system to transfer the operators instructions firmly to the brake actuating components. Any compressibility is highly undesirable.

During the life of a vehicle or even a drum of brake fluid sitting on the floor of a workshop things happen to Glycol fluids because they are what we call “Hygroscopic”....they absorb water even through the walls of the (would you believe) slightly pervious rubber brake hoses and open top on cans or vehicle master cylinders.

This water vapour drawn into the fluid will of course boil at somewhere around 100 degrees (or a little more under Pressure) so any water content in a brake fluid is bad news. It also causes system internal parts to corrode.

Over a 2 year period glycol fluids exposed to the air will absorb up to 13% of their weight in water.

This is MUCH improved in recent years with master cylinder sealing caps etc but the problem still exists that water gets in there eventually.

Coming back then to the point of this article on vapour lock (or vapour lock). This is the conversion of water in the brake fluid to steam when the brakes are heated significantly which causes compressibility in the brake system. This vapour lock leads to increased pedal travel and can result in a significant loss of brake effect.

This is not to be confused with dynamic brake fade.