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.

Braking Systems - Electric

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.

-20-

Brake
Size

Amps/
Magnet

Two
Brakes

Four
Brakes

Six
Brakes

Magnet
Ohms

7" x 1 1/4"
10" x 1 1/2"
10" x 2 1/4"
12" x 2"
12 1/4" x 2 1/2"
12 1/4" x 3 3/8"

2.5
3.0
3.0
3.0
3.0
3.0

5.0
6.0
6.0
6.0
6.0
6.0

10.0
12.0
12.0
12.0
12.0
12.0

15.0
18.0
18.0
18.0
18.0
18.0

3.9
3.2
3.2
3.2
3.2
3.2

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 the magnet
lead wire connectors and attach
the ammeter between the two
AMMETER
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

-21-

Braking Systems - Electric

Magnet Amperes Chart

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.

Braking Systems - Electric

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.

-22-

Dexter offers several varieties of hydraulic trailer brakes. Your vehicle may be
equipped with drum brakes or disc 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.
In the following pages you will find a more detailed description of the hydraulic
brakes and actuation system used on your trailer.

CAUTION
The maximum operating pressure for Dexter brakes:
• 7" diameter drum brakes
maximum operating pressure is 750 psi
• 10" diameter and larger drum brakes
maximum operating pressure is 1,000 psi
• Hydraulic disc brakes (all sizes)
maximum operating pressure is 1,600 psi

-23-

Braking Systems - Hydraulic

Hydraulic Brakes

Hydraulic Drum Brake Operation
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:

Braking Systems - Hydraulic

When the brakes are applied, the double-acting wheel cylinder moves the
primary and secondary shoes towards the drum. The frictional force between
the brake drum and lining attempts to turn the primary shoe into the secondary
shoe. The secondary shoe is forced onto the anchor pin and from this point, the
secondary and primary
Anchor Post
shoes attempt to “wrap
Hydraulic Wheel
Retractor
around”. In essence,
Cylinder
Springs
the brake has utilized
frictional force to help
Backing
the applying force on
Actuating Pin
Plate
both shoes.
Hold Down
If the brakes are
Spring
Secondary
applied while the
Shoe
vehicle is backing, the
shoes rotate in the
Primary Shoe
Adjuster Spring
direction of the drum
Adjuster Assembly
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.

Uni-Servo
This type of hydraulic brake utilizes a single acting cylinder. Upon actuation, the
primary shoe is pressed against the brake drum, which causes the shoe to move
in the direction of rotation. This movement in turn actuates the secondary shoe
through the adjuster assembly. Braking in reverse is significantly less effective
than in the forward direction.

-24-

Self Adjusting Mechanism
for 12¼" Hydraulic Brakes
Forward self-adjust hydraulic brakes were introduced in March of 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
(Not available on all sizes)
The parking feature on Dexter hydraulic brakes is cable operated. On the 10"
and 12" brakes, the parking cable body is mounted to the brake backing plate.
The cable end is attached to the internal parking brake lever to actuate the
brake. On Dexter 12 1/4" brakes manufactured before February of 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 12 1/4" brakes
produced after February of 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.
-25-

Braking Systems - Hydraulic

Another variation is called a “free backing” brake which is commonly used
on trailers with a surge hitch system. When backing with a surge brake hitch,
normal brakes are applied through the surge mechanism and if there is more
brake force on the trailer than the tow vehicle can override, no backing is
possible. The free
Anchor Post
backing brake was
Hydraulic Wheel
Retractor
developed to allow
Cylinder
Springs
backing in this
application. This brake
has a primary shoe on Backing
Actuating Pin
Plate
a pivot which allows
normal application in
Hold Down
the forward direction,
Spring
Secondary
but allows the primary Shoe
shoe to rotate away
Primary Shoe
from the drum surface
Adjuster Spring
when backing.
Adjuster Assembly

The internal parking brake lever of 10" and 12" Dexter brakes, which is mounted
to the secondary shoe, transfers applied cable force through a parking strut
which is attached to the primary shoe. This transferred load 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.
The internal parking brake lever of Dexter 12 1/4" 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.

Braking Systems - Hydraulic

Park Lever

Park Cable

-26-

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 3,500 lb., 10,000
lb. and 12,000 lb. axle models.
The fixed caliper method uses pistons situated on both sides of the rotor. During
actuation, hydraulic pressure pushes against the pistons to apply the inboard
and outboard brake pads equally to decelerate the rotating rotor. The caliper is
fixed and stays stationary during brake
actuation and brake adjustment. Brake
pad to rotor clearance is maintained
as lining wear occurs via the brake
piston and internal caliper seal. The
fixed caliper design is used on the
Dexter 6,000, 7,000 and 8,000 lb. axle
models.
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.

-27-

Braking Systems - Hydraulic

Hydraulic Disc Brakes

Actuation Systems
To effectively operate your hydraulic trailer brakes we recommend the Dexter
electrically controlled hydraulic brake actuation device (also referred to as E/H
brake 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 brake
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.

Trailer
Plug
Blue
Black
White

+12 Volt 40 Amp
Circuit From
Tow Vehicle
Ground From
Tow Vehicle

Orange/Black
Charger
Minimum
9 Amp Hour
Breakaway
Battery

Blue

Black

BreakAway
Switch

White

WHITE

Lanyard

Light Blue/Yellow

From Tow
Vehicle Controller
(Typically Blue)

BLACK

Braking Systems - Hydraulic

Electrical Schematic

Yellow
Connect
To The
Trailer
Ground
Circuit

-28-

Note: Either an onboard or breakaway
battery may be used.

Hydraulic
Line

Troubleshooting Guide
Brakes are slow to respond
1. Re-bleed the trailer brakes and actuator.
2. If the trailer is equipped with drum brakes, readjust the drum brakes to
the brake manufacture’s recommended running clearance.
3. Slow response can be caused by trailer wiring that is too small.
4. For trailers where the E/H brake actuator is located less than 10 feet
from the tow vehicle, 12 gage wire is recommended for the black and
white wires between the tow vehicle and the E/H brake actuator. All other
wires should be a minimum of 16 gauge.
5. For trailers where the E/H brake actuator is located more than 10 feet
from the tow vehicle, 10 gage wire is recommended for the black and
white wires between the tow vehicle and the E/H brake actuator. All other
wires should be a minimum of 16 gauge.
6. Slow response can be caused by improper adjustment of the brake
controller. On inertia-based electronic brake controls, adjust the
pendulum (inertia sensor) to a more aggressive setting and/or increase
the gain setting.
Unit will not run when the ignition is on and the brake pedal is depressed
1. Verify that the trailer and tow vehicle are wired as detailed on the
electrical schematic.
2. With the ignition switch on and the brakes not applied, you should
have 12-13 volts between the black and white wires on the E/H brake
actuator.
3. Clean and replace the ground between the trailer and the E/H brake
actuator.
4. Test operation of the unit using the breakaway test procedure.

-29-

Braking Systems - Hydraulic

Scan to view
E/H Actuator Installation video

Breakaway test procedure - do not leave the breakaway switch pulled for
more than two minutes during any of the steps outlined below
1. Pull the breakaway switch on the trailer.
2. If the unit runs and builds pressure, that indicates the actuator is
functioning properly. The problem most likely is a defective electronic
brake controller in the tow vehicle or defective wiring between the tow
vehicle and the E/H brake actuator.
3. If the unit runs but will not build pressure, the problem most likely is
a defective solenoid valve in the E/H brake actuator and the actuator
should be replaced.
4. If the unit still does not run after the breakaway battery is fully charged,
verify that the voltage between the white wire and yellow wire is at least
12 volts.
5. If the voltage is less than 12 volts, either the breakaway switch or the
breakaway wiring is defective.
6. If the voltage is greater than 12 volts, the E/H brake actuator should be
replaced.

Braking Systems - Hydraulic

Trailer brakes too aggressive
1. Reduce the gain setting on the in-cab electronic brake controller.

-30-

Drum Brake Adjustment - Manual
Most Dexter 12 1/4" hydraulic brakes have a self adjusting feature.
If manual adjusting is required, use the following procedure:
Brakes should be adjusted (1) after the first 200 miles of operation when the
brake shoes and drums have “seated,” (2) at 3,000 mile intervals, (3) or as
use and performance requires. The brakes should be adjusted in the following
manner:
1. Jack up trailer and secure on adequate capacity jack stands. Follow
trailer manufacturer’s recommendations for lifting and supporting the
unit. Make sure the wheel and drum rotates freely.

!

CAUTION

Do not lift or support the trailer on any part of the axle or suspension
system. Never go under any trailer unless it is properly supported on
jack stands which have been rated for the load. Improperly supported
vehicles can fall unexpectedly and cause serious injury or death.
2. Remove the adjusting hole cover from the adjusting slot on the bottom of
the brake backing plate.
3. With a screwdriver or standard adjusting tool, rotate the star wheel of the
adjuster assembly to expand the brake shoes. Adjust the brake shoes
out until the pressure of the linings against the drum makes the wheel
very difficult to turn.
Note: For drop spindle axles, a modified adjusting tool may be
necessary.
4. Then rotate the star wheel in the opposite direction until the wheel turns
freely with a slight lining drag.
5. Replace the adjusting hole cover and lower the wheel to the ground.
6. Repeat the above procedure on all brakes. For best results, the brakes
should all be set at the same clearance.
Most of the brake components are very similar to those used in electric brakes,
and maintenance is comparable for the hub and drum, shoes and linings, and
bearings. Specific maintenance activities are as follows:
-31-

Braking Systems - Hydraulic

General Maintenance ‑ Hydraulic Brakes

Wheel Cylinders
Inspect for leaks and smooth operation. Clean with brake cleaner and flush with
fresh brake fluid. Hone or replace as necessary.

Brake Lines
Check for cracks, kinks, or blockage. Flush with fresh brake fluid. Bleed system
to remove all air. Replace as necessary.

Shoes and Linings
A simple visual inspection of your brake linings will tell if they are usable.
Replacement is necessary if the lining is worn (to within 1/16" or less),
contaminated with grease or oil, or abnormally scored or gouged. Hairline heat
cracks are normal in bonded linings and should not be cause for concern. When
replacement is necessary, it is important to replace both shoes on each brake
and both brakes of the same axle. This will help retain the “balance” of your
brakes.

Braking Systems - Hydraulic