Test & Correct

Overadjustment

Readjust

Out-of-Round
Brake Drums

Machine or Replace

Incorrect Brake
Components

Replace

Loose, Bent or Broken
Brake Components

Replace

Faulty Breakaway
Switch

Repair or Replace

Loose Wheel
Bearing Adjustment

Adjust

Bent Spindle

Replace Axle

-20-

System voltage is measured at the magnets by connecting the
voltmeter to the two magnet lead wires at any brake. This may be
accomplished by using a pin probe inserted through the insulation
of the wires. The engine of the towing vehicle should be running
when checking the voltage so that a low battery will not affect the
readings.
Voltage in the system
should begin at 0 volts
and, as the controller
bar is slowly actuated,
should gradually increase
to about 12 volts. If
the controller does not
produce this voltage
control, consult your
controller manual.

VOLTMETER

The threshold voltage of a
controller is the voltage applied to the brakes when the controller
first turns on. Lower threshold voltage will provide for smoother
braking. If the threshold voltage is too high, the brakes may feel
grabby and harsh.

How to Measure Amperage
System amperage is the current flowing in the system when all the
magnets are energized. The amperage will vary in proportion to
the voltage. The engine of the tow vehicle should be running with
the trailer connected when checking the trailer braking system.
One place to measure system amperage is at the BLUE wire of
the controller, which is the output to the brakes. The BLUE wire
must be disconnected and the ammeter put in series into the line.
System amperage draw should be as noted in the following table.
Make sure your ammeter has sufficient capacity and note polarity
to prevent damaging your ammeter. With new ammeters that have
inductive pickup clamps, it is not necessary to cut the blue wire.

-21-

Braking Systems - Electric

How to Measure Voltage

Magnet Amperes Chart
Brake
Size

Amps/
Magnet

Two
Brakes

Four
Brakes

Six
Brakes

12¹⁄₄" x 3³⁄₈"

3.0

6.0

12.0

18.0

12¹⁄₄" x 4"

3.0

6.0

12.0

18.0

12¹⁄₄" x 5"

3.0

6.0

12.0

18.0

Braking Systems - Electric

If a resistor is used in the brake system, it must be set at zero or
bypassed completely to obtain the maximum amperage reading.
Individual amperage
draw can be measured
by inserting the ammeter
in the line at the magnet
you want to check.
Disconnect one of
AMMETER
the magnet lead wire
connectors and attach
the ammeter between the
two wires. Make sure that
the wires are properly
reconnected and sealed
after testing is completed.
The most common electrical problem is low or no voltage and
amperage at the brakes. Common causes of this condition are:
1.
2.
3.
4.
5.
6.

Poor electrical connections
Open circuits
Insufficient wire size
Broken wires
Blown fuses (fusing of brakes is not recommended)
Improperly functioning controllers or resistors

Another common electrical problem is shorted or partially shorted
circuits (indicated by abnormally high system amperage). Possible
causes are:
1. Shorted magnet coils
2. Defective controllers
3. Bare wires contacting a grounded object

-22-

All electrical troubleshooting procedures should start at
the controller. Most complaints regarding brake harshness
or malfunction are traceable to improperly adjusted or
non‑functioning controllers. See your controller manufacturer’s
data for proper adjustment and testing procedures. For best
results, all the connection points in the brake wiring should be
sealed to prevent corrosion. Loose or corroded connectors will
cause an increase in resistance which reduces the voltage
available for the brake magnets.

-23-

Braking Systems - Electric

Finding the cause of a short circuit in the system is done by
isolating one section at a time. If the high amperage reading drops
to zero by unplugging the trailer, then the short is in the trailer. If
the amperage reading remains high with all the brake magnets
disconnected, the short is in the trailer wiring.

Hydraulic Brakes
The hydraulic brakes on your trailer are much like those on your
automobile or light truck. The hydraulic fluid from a master cylinder
or actuation system is used to actuate the wheel cylinder, which
in turn applies force against the brake shoes and drum. The main
difference between automotive hydraulic brakes and hydraulic
trailer brakes is the trailers’ actuation system. These systems
respond to the braking signal from the tow vehicle and supply the
required brake fluid volume and pressure to the trailer brakes.

CAUTION

Braking Systems - Hydraulic

The maximum operating pressure for Dexter 12¹⁄₄"
hydraulic brakes is 1,000 PSI.

-24-

Duo-Servo
The duo-servo brake uses a dual piston wheel cylinder to apply
the brakes. This type of brake is typically used in an electric/
hydraulic, vacuum/hydraulic, or air/hydraulic system. A description
of operation of this brake is as follows:
When the brakes are applied, the double-acting wheel cylinder
moves the primary
Anchor Post
and secondary shoes
Hydraulic Wheel
Actuating Pin
Cylinder
towards the drum. The
Retractor
frictional force between
Spring
the brake drum and
Actuating Pin
lining attempts to turn
the primary shoe into
the secondary shoe.
The secondary shoe is Secondary
Shoe
forced onto the anchor
Primary Shoe
pin and from this point,
the secondary and
Adjuster Spring
Adjuster Assembly
primary shoes attempt
to “wrap around”. In essence, the brake has utilized frictional force
to help the applying force on both shoes.
If the brakes are applied while the vehicle is backing, the shoes
rotate in the direction of the drum rotation. This causes the
secondary shoe to leave the anchor and causes the primary shoe
to move against the anchor. Action of the brake is the same in
reverse as forward.

-25-

Braking Systems - Hydraulic

Hydraulic Drum Brake Operation

Uni-Servo
This type of hydraulic brake utilizes a single acting cylinder. Upon
actuation, the primary
Anchor Post
shoe is pressed
Hydraulic Wheel
Cylinder
against the brake
Retractor
drum, which causes
Spring
the shoe to move in the
Actuating Pin
direction of rotation.
This movement in turn
actuates the secondary
shoe through the
Secondary
Shoe
Primary Shoe
adjuster link assembly.
Braking in reverse
Adjuster Spring
is significantly less
Adjuster Assembly
effective than in the
forward direction. Uni‑servo brakes are normally used with surge
type actuators.

Braking Systems - Hydraulic

Self Adjusting Mechanism
for 12¼" Hydraulic Brakes
Forward self-adjust hydraulic brakes were introduced in March,
1997. This feature adjusts the brakes on both forward and reverse
stops. Brake adjustment occurs only when lining wear results in
enough gap between the shoes and the drum surface. This added
clearance will allow the adjuster mechanism to rotate the screw
assembly at the bottom of the brake. That action expands the
distance between the shoes and thus closes the gap to the drum
surface.

Hydraulic Parking Brake Option
The parking feature on Dexter hydraulic brakes is cable operated.
The cable end is attached to the internal parking brake lever to
actuate the brake. On Dexter 12¹⁄₄" brakes manufactured before
February 2002, the parking cable body mounts to a support plate
which is attached to the brake mounting flange. The cable end is
routed through the dust shield and the brake spider to attach to
the internal parking brake lever. For 12¹⁄₄" brakes produced after
February 2002, a short cable is installed directly into the backing
plate to provide a convenient means for the trailer manufacturer to
attach an appropriate operating system.
-26-

Park Lever

Park Cable

-27-

Braking Systems - Hydraulic

The internal parking brake lever of Dexter 12¹⁄₄" brakes transfers
the applied cable force through a cam mechanism. The cam
mechanism generates a spreading force between the primary and
secondary shoes. The shoes move toward the drum until contact
is made. Friction generated between the drum and lining contact
surface results in parking brake capability.

Hydraulic Disc Brakes

Braking Systems - Hydraulic

Dexter manufactures two types of disc brakes, the floating caliper
and the fixed caliper brake. With both styles, the disc brake uses
friction pads astride a ventilated rotor which is attached to the
wheel hub. When the brake is actuated, the pads are pressed
against the sides of the rotor causing drag to slow the rotating
disc. This action converts the kinetic energy (motion) into heat.
The heat is dissipated rapidly by the ventilated disc.
The floating caliper brake uses
piston(s) situated on one side of the
brake rotor. Hydraulic fluid pressure
pushes against the piston(s) to apply
the inboard brake pad. As the inboard
pad exerts force against the rotating
rotor surface, the caliper moves
laterally towards the trailer frame and
in turn applies an equivalent force to
the outboard brake pad against the
rotor surface. As the lining material
wears, the caliper will automatically
maintain the proper lining to rotor
clearance. The floating caliper design
is used on Dexter 10,000 and 12,000
lb. axle models.

5/5/04:
All text & arrows taken out for
the 9-15K service manual

Disc brake effectiveness is the same going either in a forward or
reverse direction. All Dexter disc brakes should be actuated with a
braking system that is capable of providing a maximum hydraulic
pressure of 1,600 psi.

-28-

To effectively operate your hydraulic trailer brakes, we recommend
the Dexter E/H electro/hydraulic actuator, controlled by the
Predator DX2® electronic brake controller. These high performance
hydraulic power modules will supply pressurized brake fluid to
your trailer brakes in proportion to the amount of braking effort
called for by the towing vehicles’ deceleration rate.

CAUTION
It is the responsibility of the end user to ensure that
their in-cab electronic controller is compatible with
the Dexter E/H actuator. Dexter attempts to provide
compatibility with most controllers available, but is
unable to anticipate design changes that might be
introduced by the various controller manufacturers.
The E/H 1000 will supply 1,000 psi for your drum brakes and the
E/H 1600 will generate 1,600 psi for maximum output for your
Dexter disc brakes. The sealed, weather tight housing contains the
electronics necessary to control the high pressure piston pump
and proportioning valve for smooth, efficient braking.

Vacuum/Hydraulic
The basic actuation system consists of a vacuum booster,
synchronizing valve, check valve and a plumbing kit which
includes all lines and fittings. A vacuum supply from the engine
manifold is routed to the front of the booster and the top chamber
of the synchronizing valve through flexible hoses. The rear of
the booster and the lower chamber of the synchronizing valve
are connected by a separate line. The vacuum from the engine
must pass through a normally closed check valve. The check
valve keeps gas vapor out of the system and ensures that the
highest vacuum available will be kept in the system. With the
engine running and with no brake pedal pressure, a vacuum exists
throughout the system. The synchronizing valve is connected to
the master cylinder hydraulic supply. When the brake pedal is
applied, the hydraulic pressure in the synchronizing valve forces a
poppet to open which allows atmospheric air to enter the bottom

-29-

Braking Systems - Hydraulic

Electric/Hydraulic Actuation Systems

chamber of the valve. Since this part of the valve is connected
to the rear of the vacuum booster, the vacuum on this side of
the booster chamber is lost. The vacuum on the front side of the
booster chamber is maintained and this atmospheric/vacuum
pressure differential causes a piston to move in the booster’s slave
cylinder. This piston applies the hydraulic pressure to the brakes
through the connecting hydraulic line. When the pedal is released,
the poppet in the synchronizing valve is closed and by internal
passages, the air in the rear of the booster chamber is removed.
This restores a vacuum which is equal to the vacuum in the front
part of the booster chamber. The pressure balance allows the
slave cylinder piston to be pulled back to its original position and
the hydraulic pressure to the brakes is released.

Braking Systems - Hydraulic

Air/Hydraulic
Air/hydraulic braking systems are commonly used when the tow
vehicle has a diesel engine which does not develop manifold
vacuum. The air/hydraulic tow vehicle has an air compressor
mounted on the engine to supply compressed air for the braking
system and other needs required by the tow vehicle trailer
combination. This air is routed to a booster chamber attached to
the trailer master cylinder. The air pressure multiplies hydraulic
output pressure which then sends fluid to the wheel cylinders.
The air over hydraulic systems are often controlled by a series
of control valves and servos to ensure proper braking under all
conditions.

-30-

Proper brake function is critical to the safe operation of any
vehicle. A properly installed vacuum/hydraulic, electric/hydraulic,
or air/hydraulic system should not require any special attention
with the exception of routine maintenance as defined by the
manufacturer. If problems occur, the entire tow vehicle/trailer
braking system should be analyzed by a qualified mechanic.
Typical problems in a hydraulic braking system are:
•
•
•
•
•
•

Air or vacuum leaks
Hydraulic system leaks
Air in brake lines
Water or other impurity in brake fluid
Rusted or corroded master or wheel cylinders
Actuation system malfunction

Please consult the following troubleshooting charts to determine
the causes and solutions for common problems found in trailer
braking systems.

-31-

Braking Systems - Hydraulic

Introduction to Troubleshooting

Troubleshooting
SYMPTOM

CAUSES

REMEDIES

Underadjustment

Adjust

Lack of Lubrication

Lubricate

Broken Brake
Components

Replace Components

Incorrect Brake
Components

Correct

Loose, Bent or Broken
Brake Components

Replace Components

Underadjustment

Adjust

Out-of-Round Drums

Machine or Replace

Incorrect Tire
Pressure

Inflate Evenly on Both
Sides to Req. Pressures

Unmatched Tires
on Same Axle

Match Tires on Axle

Restricted Brake
Lines or Hoses

Repair or Replace

Malfunctioning Cylinder
Assembly

Check for Stuck
or Sluggish Pistons

Defective or Damaged
Shoe and Lining

Install New Shoe and
Lining-Complete Axle

Noisy Brakes

Locking Brakes

Braking Systems - Hydraulic

Pulls to One Side

One Side
Out-of-Adjustment

Adjust

Improper Fluid

Replace Rubber Parts
Fill with DOT4 Fluid

Blocked Master
Cylinder

Open with Compressed
Air or Replace Cylinder