The modern brake hydraulic system contains various
valves which control hydraulic pressure to parts of the
brake system. One valve is the metering valve, used to
prevent application of the front brakes until the rear brakes
are partially applied. Another valve is the proportioning
valve which controls pressure to the rear wheels during
hard braking. A variation of the proportioning valve con-
trols pressure according to the weight placed on the vehi-
cle. The pressure differential valve warns the driver if one
side of the hydraulic system looses pressure. All of these
valves may be combined into one unit called a combina-
tion valve. Combination valves can contain two or three
Electrical brake switches are also used in various
ways. In addition to the switch attached to the pressure dif-
ferential valve, switches also control a dashboard light that
warns the driver when the fluid level in the master cylin-
der reservoir is low. Another switch is installed on the
brake pedal linkage to illuminate the brake lights when the
brake pedal is depressed.
Hydraulic pressure from the master cylinder reaches
the wheel brakes by way of rigid steel lines or flexible rub-
ber hoses. Steel line hydraulic units that do not move in
relation to each other, while hoses connect the moveable
wheel assemblies with the frame or body lines.
Brake fittings are threaded connectors which connect
brake lines and hoses. Fittings can be standard or metric
pipe threads. Double-lap or ISO flares are used to ensure a
solid connection between steel lines and fittings with no
leaks. The two types of flares are the double-lap and the
ISO flare. One of these two types must be used in brake
system flares.
Review Questions—Chapter 9
Please do not write in this text. Write your answers on
a separate sheet of paper.
1. A ______ responds to changes in vehicle load.
2. ______ may contain 2-3 valves and an electrical
3. The low brake fluid switch is installed on the master
cylinder ______.
4. How does the low brake fluid switch respond to a low
fluid level?
5. The rear brake light switch is installed on the ______
6. Rigid brake lines are made of ______.
7. Brake lines are usually coated with another metal or
Teflon to reduce ______.
8. Define a tubing flare.
9. What are the two types of brake tubing flares?
10. Flexible hoses are firmly mounted at ______.
ASE Certification-Type Questions
1. Technician A says that the metering valve keeps the
rear wheels from locking up. Technician B says that
the proportioning valve keeps the front wheels from
applying too quickly. Who is right?
(A) A only.
(B) B only.
(C) Both A & B.
(D) Neither A nor B.
2. When is the metering valve closed?
(A) When no pressure is applied to it.
(B) When heavy pressure is applied to it.
(C) When the driver releases the brake pedal.
(D) When the front disc pads are applied.
3. The load-sensing proportioning works because a load
placed in the vehicle trunk ______.
(A) compresses the frame bushings
(B) compresses the shock absorbers
(C) compresses the rear springs
(D) None of the above.
4. Technician A says that during light braking, the pro-
portioning valve is open. Technician B says that dur-
ing light braking, the residual pressure valve is closed.
Who is right?
(A) A only.
(B) B only.
(C) Both A & B.
(D) Neither A nor B.
5. A properly functioning residual pressure valve keeps
the _______ seals from collapsing.
(A) caliper
(B) wheel cylinder
(C) master cylinder
(D) Both A & B.
6. Under which of the following conditions will the
pressure differential valve illuminate the dashboard
(A) The engine is first started.
(B) The metering valve sticks closed.
(C) A severe leak develops in a rear wheel line.
(D) A front hose swells shut.
162 Auto Brakes
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