the Braking Systems
Some moving objects are easy to stop. Some are not!
A lack of stopping ability caused problems on early cars and is still a problem on badly
maintained modern vehicles.
So this webpage is all about brakes.
We will look at how they work, how to maintain them and at some of the brake problems you may come
This part of site is an attempt to organize and add to the web resources for the Rover SD1
to form a cohesive and easily usable guide for those of us without easy access to
expert repair and/or advice. It is not offered in any way as a definitive source
and we take no responsibility for any errors that may exist.
Webmaster Rene Winters
A brake works by causing friction between a non-rotating pad or shoe and a disc or drum which revolves
with the road wheel. This friction produces the force required to slow the vehicle.
It also converts the energy of the moving vehicle into heat which disperses into the air around the brakes.
Obviously it is important that this heat which builds up is dispersed as quickly as possible, as excessive
heat can cause braking problems.
A modern braking system uses the hydraulic principle. Because liquid is incompressible pressure applied at
the piston of one cylinder will be equally transmitted to a similarly sized piston of another cylinder.
If the second piston has a larger surface area than the first, the force exerted will be proportionally
greater than that applied at the first piston. However, the distance moved by the second piston will be
By connecting several cylinders to the main cylinder, force can be distributed equally from the main
piston to the other pistons.
On a basic car braking system pressure is applied at the main cylinder, or master cylinder as it is called.
The brake pedal pivots at point A, and the position of the master cylinder push rod B relative to the
position of the pedal provides a mechanical advantage. This enables a greater force to be applied at the
master cylinder piston.
This force is transferred as hydraulic pressure from the master cylinder to each wheel cylinder when the
brake pedal is depressed. The pressure will be maintained until the pedal is released.
At the road wheels the pressure applied is turned into braking effort by drums or discs.
We will look at the drum brake first.
Two types of drum brake may be used; a twin leading shoe design or a leading-trailing shoe arrangement.
The twin leading shoe type is normally fitted at the front wheels; it uses two pistons, one for each shoe.
Application of the brake pedal will cause the pistons to move out and force their respective brake shoes
into contact with the drum. When the car moves forward the drum revolves, and its turning movement will
assist in pulling the brake shoes even more tightly into contact with the drum when the brake pedal is
This twin leading shoe design becomes much less effective when the car is driven in reverse. Both shoes
are repelled by the turning movement of the drum and much greater pedal effort is required by the driver
to stop the car. For this reason a leading trailing shoe arrangement is often used.
This is a leading trailing shoe design and it is normally found at the rear wheels. It uses a single wheel
Whichever way the drum is turning one shoe will be drawn into the drum and the other repelled. Therefore
this type of brake will be equally effective in either direction, and its design is also adaptable for
Drum brakes on all four wheels are still to be found on some Mini and Marina models.
On the majority of BL cars, however, drum brakes on the front wheels have been replaced by discs
and Jaguar Daimler vehicles use disc brakes on all four wheels.
The basic disc brake design is very simple, and extremely effective.
The disc is attached to and rotates with the wheel hub. It is straddled by a caliper which is bolted to the
stub axle flange.
The caliper contains two pistons. Between each piston and the disc in the centre is a friction pad.
When the brake pedal is depressed, hydraulic pressure moves the pistons, and clamps the disc between the
pads with equal and opposite force. Heat generated through friction causes the disc to expand against the
pads. Because the expansion rate of the disc cannot overcome hydraulic pressure applied to the pads, the
disc is pressed further against the pads thereby increasing its efficiency.
This arrangement is equally effective which-ever way the wheel is being turned; and the design also allows
good heat dissipation from the exposed brake disc.
Unequal braking could cause a driver to experience some hair raising moments; so we go to great trouble
in designing our brakes to give maximum adhesion at all four wheels.
There is a weight transfer forwards under braking of approximately 70%. Due to certain characteristics
designed into the vehicle to give good handling and road holding, there is an un-equal weight distribution
front to rear, which varies from model to model.
So the braking effort has to be proportioned front to rear. There are a number of ways to achieve this.
For example it can be done by fitting larger brakes at the front or by using larger wheel cylinders by
using disc brakes at the front and drums at the rear by employing a valve in the hydraulic system to reduce
pressure to the rear wheels; or by a combination of these things.
On heavier high performance cars a greater braking effort is obtained by fitting larger calipers containing
four pistons, two on each side. Thus, increased pressure can be applied to larger pads.
On cars fitted with rear disc brakes, the con-ventional handbrake system is replaced by an additional pair
of disc pads. In the design shown here they are applied mechanically and are held against the disc by
tension in the handbrake linkage.
Now that we have seen the basic system, let us examine some of the extra features fitted to our brakes
for additional safety and ease of driving.
A weakness of the basic hydraulic system we have been looking at is that leakage at any point will put the
whole system out of action.
All our cars, therefore, are now being fitted with a dual braking system.
Here two independent hydraulic circuits are employed. They may be split front and rear, or diagonally.
An essential part of this system is a tandem master cylinder.
It contains two master cylinders placed one behind the other in the same bore, each with its own fluid
In normal operation the first master cylinder piston generates pressure in one hydraulic line. It also
applies fluid pressure to the second piston which in turn generates pressure in the other line.
If there is a leak in the first hydraulic line, application of the brake pedal will cause the first piston
to move up its bore until it contacts the second piston. This second piston will then generate its normal
pressure in the second line only, and half the brakes will operate.
If the leak is in the second line, initial movement of the first piston will force the second piston to the
end of its bore. Pressure will then build up in the normal way in the first line.
A brake servo is fitted to the majority of current production models.
It boosts the force exerted in the master cylinder when the brake pedal is depressed.
There are several designs of servo but they all have the same effect. In this type,
manifold vacuum is applied to the servo at point A, and occupies both sides of the
diaphragm whenever the brakes are not being applied.
When the footbrake is applied, a valve allows air to enter the chamber on the right hand
side of the diaphragm. This applies atmospheric pressure, approximately 14 pounds per
square inch or 1 kilogram per square centimetre, to the whole surface of the piston.
The resultant difference in air pressure between the two sides of the piston will move
the diaphragm to the left. This move-ment will boost the driver's pedal effort and greatly increase
hydraulic pressure to the wheel cylinders.
We have already mentioned that there are also a number of devices that can be fitted in the line to reduce
pressure to the rear wheels. The simplest is this pressure regulating valve fitted to Minis with single
Light application of the brakes will allow fluid pressure to pass through the valve without actuating it.
However, when the pressure is increased it will force the valve seal to seat at point A. Thus any further
pressure will now be applied to the front brakes only.
The Triumph Dolomite Sprint is fitted with a load conscious reducing valve; it senses the vehicle weight
on the rear wheels as the rear of the car rises under braking. This takes the weight off the rear wheels
and reduces adhesion between wheels and road surface. The valve will adjust braking pressure to the rear
wheel cylinders accordingly.
The Austin Maxi uses a pressure reducing valve.
Unlike the Mini pressure regulating valve, the pressure reducing valve fitted to the Maxi does not cut
off pressure to the rear wheels at a predetermined point; it only reduces it.
Other models, such as the Jaguar, Rover and TR7 use a failure sensitive pressure reducing valve. This unit
not only reduces pressure to the rear wheels under braking, but will also direct full pressure to the
front or rear in the event of a failure in one of the lines.
Finally in this section a word about the pressure differential warning actuator, or "Pudwuh" fitted on
cars with a dual line system.
It is a device which is fitted between the two lines. If one line develops a fault, the pressure
differential created when the brake pedal is depressed will cause the shuttle to move, and the switch to
A light will then come on to warn the driver of the fault. On some cars the same light is used to indicate
a low brake fluid level. Illumination of this light, for whatever reason, must be checked immediately.
On some cars, such as this Princess, the switch above the light can be operated to ensure that the bulb
in the warning light is working. Bulb failure would give no indication of a braking problem.
Two other types of switch can be found on certain BL cars; the first is a level sensitive switch. Under
normal conditions a float in the master cylinder reservoir keeps the warning light switch in the reservoir
cap open. If the fluid level drops below a predetermined amount the float will also drop, allowing the
switch to close. Current will now flow and the warning light in the car will operate.
The second type is thermal sensitive. With this type, current is prevented from flowing through the switch
by the cooling action of the surrounding brake fluid. When the fluid level drops, however, the cooling
medium is lost. The switch then becomes warm thereby allowing contact to be made, and the circuit to the
warning light is completed.
The warning light may be checked by the switch already mentioned, or by pressing the top of the master
cylinder cap, or remiving it as necessary. If the fluid level is satisfactory, the car can still be
driven at a much reduced speed but the fault must be found and rectified as soon as possible.
Some 'Pudwuhs' reset automatically when the fault is cured. Others do not, so check with the Repair