About-brakes.com - welcome on our site. You can find here a lot information about brakes, brake pads and brake linings.
Power Brakes and master cylinders.
Power brakes (also known as power assisted brakes) are designed to use the power of the engine and/or battery to enhance your braking power. Whilst you can generate a fair amount of force using your foot, using systems from elsewhere in the car to help you apply even more force means that you get more powerful brakes as a result.
The four most common types of power brakes are: vacuum suspended; air suspended; hydraulic booster, and electrohydraulic booster. Most cars use vacuum suspended units (vacuum boosters). In this type of system, when you press the brake pedal, the push rod to the master cylinder opens a vacuum control valve. This allows vacuum pressure (normally from the intake manifold) to "suck" on a diaphragm inside the vacuum assist unit. This extra vacuum suction helps you to produce more force at the pedal end of the brake system.
You'll notice in the image below (which I scurrilously had to modify from someone else's site - if that's you, I apologise), that the master cylinder has two brake circuits and two master pistons. These circuits are separate and are typically connected to the front-left and rear-right wheel on the first circuit, and the front-right and rear-left wheels on the secondary circuit. This means that if one circuit fails, the second one will still work and it will still apply braking force to the front and rear of the car.
Hydraulic booster systems usually utilise pressure from the power steering system to augment pressure on the master brake cylinder.
Electrohydraulic booster systems use an electric motor to pressurize the hydraulic system downwind of the brake pedal which has the effect of amplifying the internal pressure in the whole system.The advantage to this system is that as long as you have battery power, you have power brakes even if the engine fails. With vacuum-assist brakes, no engine means no assistance.
If you're curious about how power brakes work, go out to your car and with the engine off, step on the brakes. They'll have a slightly solid, almost wooden feel to them. Turn the engine on and do it again and you'll notice a lot less back-pressure on the pedal. This is the power assist which is making it easier for you to depress the pedal.
One last thing about brake master cylinders : they cost an absolute bomb to replace. If you find yours is leaking, patching it up is not an option. Brand new master cylinders can go for around $1500 without labour costs. Remanufactured ones come in slightly cheaper at around $900. Bear that in mind when your 20 year old beater develops a leak - it's probably cheaper to buy another used car than to replace the master cylinder.
