Low Brake Fluid Switch
Master cylinders in many vehicles are equipped with
a low brake fluid switch. This switch is usually installed in
the reservoir cap, Figure 9-16, or in the side of the reser-
voir, Figure 9-17. This switch consists of a float and a set
of contacts. The switch contacts have electrical power
when the ignition is on. The contacts are attached to the
float, and close when the float falls below a certain point.
The switch electrical circuit is powered through a dash-
board-mounted fluid level warning light. When the fluid
level becomes too low, the float falls in the reservoir, caus-
ing the contacts to close. Closing the contacts grounds the
circuit and illuminates the dashboard warning light.
Hill Holder
Drivers often have difficulty coordinating the clutch,
brake and accelerator pedals to start moving on a steep
hill. To make starting easier, some vehicles have a hill
holder that uses the brake hydraulic system. The main part
of the hill holder is a pressure hold valve. The valve is
installed in the brake line between the master cylinder and
the left front and right rear wheel brakes. The valve has an
internal ball that can be moved by both gravity and a
mechanical connection from the clutch pedal. Under the
right conditions, the ball will seal off the wheel brakes
from the master cylinder.
When the vehicle is driven on a road with no grade
or a downhill grade, the ball remains away from its seat
Chapter 9 Hydraulic Valves, Switches, Lines, and Hoses 157
Figure 9-14. A cross-sectional view of a two unit combination
valve.The left half is a metering valve, and the right half is a pres-
sure differential valve. Study the construction. (DaimlerChrysler)
V Valve
stemt
O-ring
Spring
Sleevev
Switch assembly
From master
y cylinder seconda ry
outlet
Outlet to
height sensing
valve bypass
Differential
piston
O-ring
valve
Outletttl O
to height
sensing
valve
O tl t t Ou tlet to
front brakes
From master cylinder
primary outlet
Figure 9-15. Cross-sectional view of a three-way combination
valve. Note that this valve incorporates a warning switch,
proportioning and metering valve in one unit. Study the
construction. (Raybestos)
Rear R
outlet o
Rear inlet Re ar i nlet
Brake light
warning switch
Front
inlet in le t
Left
front
outle et
Bleeder e
rod
Met tering
valve alve
Right front
outlet
Proportioning
valve
Pressure
differential
valve
Figure 9-16. Cutaway views of float level sensors that are a
part of the reservoir cap. A—Switch contacts are open, circuitt
is ok. B—Switch contacts are closed.Warning light will come on
if the sensor is functioning correctly. This will alert the driver to
a low fluid level in the reservoir. (Honda)
Reservoir cap
A A
B
Switch contacts
closed
Switch contacts
open
Float
Magnet
Figure 9-17. This master cylinder has a brake fluid level switch
located in the side of the reservoir.
Brake fluid
level switch
Previous Page Next Page