Braking efficiency can show up in a variety of ways, including overheating, brake fade, brake pad degradation and more. Two serious brake efficiency issues are vapour lock and spongy pedal feel.
Vapour lock is one of the most dangerous losses of braking efficiency and is connected to the brake fluid. As significant heat is generated when braking, brake fluid is specifically designed to resist such high temperatures. Being hygroscopic, brake fluid absorbs water. In a hydraulic brake system, it gradually absorbs moisture, reducing the brake fluid’s boiling point.
When the moisture levels reach a certain level, the heat can cause the brake fluid to boil in extreme conditions, which creates gas bubbles. Gasses are more compressible than liquids, resulting in a lack of pressure on the brake pedal, which leads to a total loss of brake power. This is known as vapour lock.
Avoid vapour lock
1. Choose the right brake fluid
The brake fluid you decide to use is of utmost importance, as it will affect your entire braking system. Vehicle manufacturers will recommend specific types best suited to their vehicles. It is very important that you adhere to this.
A quality brake fluid should:
- give solid pedal during braking
- have a high boiling point
- keep its viscosity within closely defined parameters
- provide the correct level of lubricity
- have anti-corrosive properties
All these important attributes are found in Ferodo brake fluids, exceeding the requirements across the entire range.
2. Test the brake fluid
Vehicle manufacturers stipulate their minimum recommendations, however the correct time to change the fluid shouldn’t be based on mileage or your vehicle’s age. One surefire way to know when it’s time to change your brake fluid is by attempting to boil it.
Conductivity testers don’t boil the fluid. They give you an estimation of the water content. Theoretically, conductivity increases with moisture content, but these testers easily confuse new fluids and contaminated fluids.
Ferodo has developed its own high-quality brake fluid tester to let you accurately measure brake fluid. In cases where accurate water content is necessary, Ferodo can determine it through the use of industry-standard techniques, such as the Karl Fisher method for water content.
3. Change the brake fluid
To change the brake fluid, you must bleed the system. It is important though to first carry out a master cylinder leak pass test; this is done by attaching a pressure gauge to the nearest caliper brake pipe. The system must be pressurised to 50 bar for 45 seconds. There shouldn’t be more than a 4 bar pressure loss during this time. If the pressure drops further than this, it is indicative of a master cylinder leak pass, which will require your attention.
Vehicles with front and rear disc brakes
- It is important to bleed the brakes in a set order. Start with the brake caliper furthest from the master cylinder.
- With all bleed nipples closed, attach a bleed pipe to the first caliper and then loosen the nipple.
- Slowly and firmly stroke the brake pedal continuously until the brake fluid in the bleed pipe is visibly clean and clear of any air bubbles.
- Press the pedal down completely and tighten the bleed nipple before removing the bleed pipe.
- Repeat this procedure on the opposite rear caliper.
- Once you’ve done the rear the calipers, check the front calipers to ensure that they are also functioning properly and free of air bubbles by repeating the process.
Vehicles with drum brakes
- When bleeding vehicles with drum brakes, always do so in a set order. Start with the wheel point that is the furthest from the cylinder.
- Before starting, ensure that the correct manual adjustment is carried out on the clearance between the brake shoes and brake drum. Consult the manufacturer’s specifications.
- Before loosening any bleed nipples, fit a bleed pipe to the bleed nipple of the wheel cylinder furthest from the master cylinder. Now loosen this bleed nipple.
- Slowly and firmly stroke the brake pedal until the brake fluid in the bleed pipe is visibly clean and clear of any air bubbles.
- With the pedal pressed down all the way, tighten the bleed nipple and remove the bleed pipe.
- Repeat this process on the opposite rear wheel cylinder.
- After finishing the rear wheel cylinders, repeat the process on the front calipers, starting with the furthest and finishing with the closest to the master cylinder.
Spongy pedal feel
This effect is recognised by a sluggish brake which feels much softer than it should, leading to excessive force being needed to slow the vehicle. This can often be cause by faulty brake hoses, which lead the brake fluid leaks, as well as ballooning of the hose itself.
How to avoid faulty brake hoses
- Ageing – Check your brake hoses regularly for any chaffing, cuts or general deterioration.
- Fitment – Make sure that it’s not twisted or bent and that the correct tightening torque is applied. There should also not be contact between hoses and suspension components.
- Quality – It is important to choose high quality hoses that offer high resistance, good flexibility and low moisture permeability.