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All the Parts Needed to Add Electric Over Hydraulic Disc Brakes to a Trailer

You may be considering adding electric over hydraulic disc brakes to a trailer that has no brakes or converting from hydraulic or electric drum to hydraulic disc brakes for improved stopping power and less maintenance. If so, you will need some basic information to build a system that will stop your trailer safely and dependably. The following article and tables provide information about your options and will help you find parts that will work together, including electric over hydraulic brake actuators. If you just want to find parts quickly, see our Parts Reference Table, below. To view an installation of hydraulic brakes on a trailer, see our video below.
Idler Hub
disc brakes

Why Go with Electric over Hydraulic Brakes

Going with electric over hydraulic brakes can give you a better braking system for your trailer than either straight electric or hydraulic surge brakes alone. Electric trailer braking systems provide fast response times and good control over the action of your trailer brakes, but they don't stop as hard as hydraulic brakes. They require the use of a brake controller that mounts inside your cab so that you can conveniently adjust the action of your brakes - how quickly they apply and how hard they stop, depending on trailer load and your preference. Hydraulic brakes can stop harder than electric brakes, but they are slower to apply. Conventional hydraulic brakes rely on a surge actuator that is built into the trailer coupler and that compresses when the tow vehicle slows and the trailer pushes against it. A delay occurs between the time the tow vehicle brakes are applied and the time the actuator applies the trailer's brakes. This delay is longer than that of electric brakes. Also, you have to lock out a typical surge actuator (which normally requires you to get out of your vehicle) so that you can back up the trailer. You don't have to do anything special to be able to back up a trailer that has electric brakes.With an electric over hydraulic brake setup, you get the best of both worlds - the great braking control that an electric brake controller provides and the stopping power of hydraulic brakes. Also, backing up will never be a problem. To set up electric over hydraulic brakes, you wire the tow vehicle for electric brakes and install hydraulic brake lines and brakes on the trailer. An electric over hydraulic brake actuator provides the key to going between the two types of systems. This type of actuator usually mounts on the trailer - it receives electric signals from the tow vehicle's brake controller and applies pressure to the trailer's hydraulic brake lines. With an electric over hydraulic braking system, you get the control and response time of electric brakes combined with the stopping power of hydraulic brakes. If you install electric over hydraulic brakes, you can choose between hydraulic drum or hydraulic disc brakes. Generally, disc brakes are more powerful than drum brakes and require less maintenance because they have fewer moving parts.
Electric Over Hydraulic Disc Trailer Brakes Parts Reference
brake mounting flange
Brake Mounting Flange
Spindle
Spindle
Disc Brake Assembly
Disc Brake Assembly
brake line kit
Brake Line Kit
electric over hydraulic actuator
Electric Over Hydraulic Actuator
brake line kit
Breakaway Kit
Brake Controller
Brake Controller
Trailer Brake Mounting Flanges
To operate the brakes, your trailer's axle must have a brake mounting flange attached behind the existing hub assembly on each side. Most axles have these, and if your trailer already has brakes (electric or hydraulic) it will. But if your trailer has no brake mounting flanges, you have two options:
  • You can replace the axle with a properly rated one that already has brake flanges welded in place. Ordering a new axle complete with hydraulic disc brakes may actually cost less than buying separate parts. However, to order a complete axle you will need to know the axle capacity, wheel diameter, wheel bolt pattern, spindle type (whether it is drop or straight) and type of brake assembly - in this case, hydraulic. You will also need to know the distance between your trailer's leaf springs, center to center, and between the hub faces.
  • You can purchase brake flanges and have a qualified welder weld them to the axle. When a brake mounting flange is welded to an axle, it must be welded square and concentric. This usually requires a jig to hold both parts in position. A trailer shop should have the equipment and personnel to weld brake mounting flanges to your axle.
Brake Mounting flange
Pictured is the most popular brake mounting flange. It's a square, 4-bolt pattern that is used on most axles up to and including 3,500 lbs. The next-most-popular flange is the 5-bolt design, which is commonly used on 5,200-lb, 6,000-lb and 7,000-lb axles. The pattern of the holes in the flange is standard, so all you need to note is the axle diameter and the number of holes in the mounting flange.
Disc Brake Hub and Rotor
disc brake rotor
hub and rotor
If you have a brake mounting flange on your axle or you will have one installed, you have two options. First, if you have idler hubs you can either replace them with an integral hub and rotor or install a disc brake kit that uses an over-the-hub rotor. An over-the-hub rotor kit makes it easy to upgrade an idler axle to a disc brake axle. Only the wheel bolt pattern is needed to select an over-the-hub disc brake kit. With over-the-hub disc brake kits, 14" and larger wheels are required, and fender clearance and wheel stud length should be considered because the wheel and tire combination is spaced 1/2" farther out due to the addition of the disc brake rotor.
The second and more popular choice is a disc brake kit with integral hub and rotor. If your trailer already has drum brakes and you're upgrading, this is definitely the way to go. You will have to measure your spindle or get the part numbers from the existing bearings and seal to make sure the hub/rotor assembly will work with the trailer spindle. Along with the the axle capacity, wheel diameter and wheel bolt pattern this will help to select the correct hydraulic disc brake kit for the trailer. If you cannot find part numbers for the bearings and seal, you can measure the spindle (see spindle diagram) at the inner and outer bearing surfaces as well as the seal contact surface to determine the correct bearing and seal parts needed to select a disc brake kit with hub and rotor.
Spindle
hub and rotor
Disc brake parts are manufactured with one of several different finishes - raw metal, e-coat, silver cadmium, Dacromet or stainless steel. Parts with different finishes will vary in corrosion resistance and price. The ability of brake parts to resist corrosion may be important to you if you use your trailer around water, especially salt water. Of the finishes, a raw finish will offer the least corrosion resistance. E-coat protects parts with a paint-like coating. The paint can get chipped or worn off, allowing rust to start. Both silver cadmium and Dacromet plating offer better alternatives for corrosion resistance and hold up very well. Stainless steel stands up the best to corrosion because it's virtually rustproof, but it's also much more expensive than the other kinds of finishes.
Brake Line Kits for Trailers
Caliper
disc accessories
disc brakes
Next, you will need a brake line kit to plumb the trailer from the disc brake calipers to the electric over hydraulic actuator. Brake line kits are selected by the number of axles on the trailer. And because of the way disc brakes work, they require flexible lines at the brake caliper, so the part of the brake line that connects to the caliper is usually made of rubber. This same flexibility in the brake line is needed for brake lines installed on torsion axles because of the amount of movement of the hubs on those kinds of axles.We have an electric over hydraulic disc brake kit available for trailers with 3,500-lb tandem axles, and it includes everything you need to switch your trailer over to disc brakes for improved performance and greater stopping power.
Electric Over Hydraulic Brake Actuator
actuator1
actuator2
Next, you will need to select an electric over hydraulic actuator that is compatible with disc brakes and that installs on the tongue of the trailer frame. Kits with plug-in connectors are available to make installation easier. Some electric over hydraulic actuators include breakaway kits. Most don't come with breakaway kits, but they will work with standard breakaway kits that are made for electric brakes.
Breakaway Kits
Breakaway kits apply the trailer's brakes if the trailer becomes uncoupled from the tow vehicle. These systems include a battery and wiring, and sometimes have a built-in charger. When choosing a breakaway kit, be sure that it meets the minimum size required by the actuator. To find out, check the actuator's user manual or check with the manufacturer to see if a minimum size is required.
breakaway
Vehicle-Mounted Electric Trailer Brake Controller and Wiring
breakaway
Finally, to operate your electric brakes, you will need to set up your tow vehicle with a brake controller and the proper wiring. A few actuators come with a portable or remote brake controller that communicates with the actuator by radio waves and plugs into a cigarette lighter or power plug in the cab of your tow vehicle. A remote brake controller lets you easily switch between different tow vehicles to pull the same trailer. You need to make sure that any brake controller that you are considering is compatible with electric over hydraulic actuators. At etrailer.com, we carry many popular brake controllers that are compatible with electric-over-hydraulic brake actuators. Our article, Trailer Brake Controller Information, explains brake controller similarities and differences, and our Brake Controller Application Guide will help you to determine all the parts needed for your vehicle.
Common Combinations of Trailer Brake Components
Typical Components of Integrated Hub and Rotor Hydraulic Disc Brakes Direct Mount to Spindle
2,000-lb Axle
1-3/4" Diameter
EZ Lube Option
Standard (NO EZ lube)
Brake Flange
4-34
Bearings
Inner- L44649
Outer- L44649
Seal
10-9
Disc Brake Kit
Single Axle- K2HR2E
Finish
E-Coat
Rotor Bolt Pattern
5 on 4-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
2,000-lb Axle
1-3/4" Diameter
EZ Lube Option
EZ Lube
Brake Flange
4-34
Bearings
Inner- L44649
Outer- L44649
Seal
RG06-020 (10-60)
Disc Brake Kit
Single Axle- K2HR2E
Finish
E-Coat
Rotor Bolt Pattern
5 on 4-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
3,500-lb Axle
2-3/8" Diameter
EZ Lube Option
Standard (NO EZ Lube)
Brake Flange
4-35
Bearings
Inner- L68149
Outer- L44649
Seal
58846
Disc Brake Kit
Single Axle- T4841400KIT
Double Axle- T4843700
Finish
Silver Cad
Rotor Bolt Pattern
5 on 4-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
Double Axle- T4830000
3,500-lb Axle
2-3/8" Diameter
EZ Lube Option
EZ Lube
Brake Flange
4-35
Bearings
Inner- L68149
Outer- L44649
Seal
RG06-050 (10-19)
Disc Brake Kit
Single Axle- T4841400KIT
Double Axle- T4843700
Finish
Silver Cad
Rotor Bolt Pattern
5 on 4-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
Double Axle- T4830000
5,200-lb and 6,000-lb Axle
3" Diameter
EZ Lube Option
Standard (NO EZ Lube)
Brake Flange
4-44-1
Bearings
Inner- 25580
Outer- 15123 or LM67048
Inner- 25580
Outer- 02475 or 14125A
Seal
GS-2125-DL (10-1) or GS-2250-DL
Disc Brake Kit
Single Axle- K2HR526D
Finish
Dacromet
Rotor Bolt Pattern
6 on 5-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
Double Axle- T4830000
5,200-lb and 6,000-lb Axle
3" Diameter
EZ Lube Option
EZ Lube
Brake Flange
4-44-1
Bearings
Inner- 25580
Outer- 15123 or LM67048
Inner- 25580
Outer- 02475 or 14125A
Seal
RG06-090 (10-10) or RG06-070 (10-36)
Disc Brake Kit
Single Axle- K2HR526D
Finish
Dacromet
Rotor Bolt Pattern
6 on 5-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
Double Axle- T4830000
7,000-lb Axle
3" Diameter
EZ Lube Option
Standard (NO EZ Lube)
Brake Flange
4-44-1
Bearings
Inner- 25580
Outer- 15123 or 14125A
Seal
GS-2125-DL (10-1) or GS-2250-DL
Disc Brake Kit
Single Axle- K2HR712
Finish
Raw Finish
Rotor Bolt Pattern
8 on 6-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
Double Axle- T4830000
7,000-lb Axle
3" Diameter
EZ Lube Option
EZ Lube
Brake Flange
4-44-1
Bearings
Inner- 25580
Outer- 15123 or 14125A
Seal
RG06-090 (10-10) or RG06-070 (10-36)
Disc Brake Kit
Single Axle- K2HR526D
Finish
Dacromet
Rotor Bolt Pattern
8 on 6-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
Double Axle- T4830000
Typical Components of Rotor Only Hydraulic Disc Brakes Mount over Idler Hub
3,500-lb Axle
2-3/8" Diameter
EZ Lube Option
Standard (NO EZ Lube)
Brake Flange
4-35
Disc Brake Kit
Single Axle- K2R35DS
Finish
Dacromet and stainless
Idler Hub Bolt Pattern
5 on 4-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
Double Axle- T4830000
3,500-lb Axle
2-3/8" Diameter
EZ Lube Option
EZ Lube
Brake Flange
4-35
Disc Brake Kit
Single Axle- K2R35DS
Finish
Dacromet and stainless
Idler Hub Bolt Pattern
5 on 4-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
Double Axle- T4830000
5,200-lb and 6,000-lb Axle
3" Diameter
EZ Lube Option
Standard (NO EZ Lube)
Brake Flange
4-44-1
Disc Brake Kit
Single Axle- K2R526E
Finish
E-Coat
Idler Hub Bolt Pattern
6 on 5-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
Double Axle- T4830000
5,200-lb and 6,000-lb Axle
3" Diameter
EZ Lube Option
EZ Lube
Brake Flange
4-44-1
Disc Brake Kit
Single Axle- K2R526E
Finish
E-Coat
Idler Hub Bolt Pattern
6 on 5-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
Double Axle- T4830000
7,000-lb Axle
3" Diameter
EZ Lube Option
Standard (NO EZ Lube)
Brake Flange
4-44-1
Disc Brake Kit
Single Axle- K2R712DAC
Finish
Dacromet
Idler Hub Bolt Pattern
5 on 4-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
Double Axle- T4830000
7,000-lb Axle
3" Diameter
EZ Lube Option
EZ Lube
Brake Flange
4-44-1
Disc Brake Kit
Single Axle- K2R712DAC
Finish
Dacromet
Idler Hub Bolt Pattern
5 on 4-1/2"
Electric over Hydraulic Actuator
T4813100
 HBA16
 K71-651
Breakaway Kit
50-80-313
Brake Line Kit
Single Axle- T4829900
Double Axle- T4830000
Updated by: Jacob JLast Updated: 10/31/2023
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