Parts Needed to Add Hydraulic Disc Brakes to a Trailer

Whether you are adding hydraulic disc brakes to a trailer that has no existing brakes or changing to disc brakes from electric or hydraulic drum brakes, you need some basic information before you start building a system that will stop your trailer safely and dependably. The following article and tables explain your options and will help you find parts that work together. If you just want to find parts quickly, see the table below, "Parts for Adding Hydraulic Disc Trailer Brakes."

Why Choose Hydraulic Disc Brakes?

Hydraulic disc brakes can provide a safe, secure braking system with strong stopping power for your trailer. And because a hydraulic braking system lacks electrical parts, a trailer with hydraulic brakes can be safely backed into water. This makes hydraulic brakes perfect for boat trailers. In addition, when compared to hydraulic drum brakes, disc brakes stop harder and require less maintenance. That's because they have fewer moving parts.

Parts Reference Table

Part

What to Consider

Brake Mounting Flange

Axle Diameter

Spindle

Bearing and seal dimensions
Seal contact surface (B)
Inner bearing (C)
Outer bearing (D)
Axle capacity

Disc Brake Assembly

Bearing spindle seal
Wheel bolt pattern

Hydraulic Disc Brake Actuator

Select
Hydraulic surge brake actuator

Brake Line Kit

Type of axle
Number of axles

Trailer Brake Mounting Flange

To operate the brakes, a brake mounting flange must be attached to your trailer's axle behind the existing hub assembly on each side. Most axles have these flanges, 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.

1. You can replace the existing axle with a properly rated axle on which brake flanges have already been welded in place. Ordering a new axle that comes complete with hydraulic disc brakes may actually cost less than buying separate parts. However, before you can order a complete axle you will need to know the axle capacity, wheel diameter, wheel bolt pattern, spindle type (that 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.

2. You can purchase brake flanges and have them welded to the axle by a qualified welder. When a brake mounting flange is attached 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.

Pictured (above, right) is the most popular brake mounting flange. It's a square, four-bolt pattern that is used on most axles up to and including 3,500 pounds. The next-most-popular flange is the five-bolt design, which is commonly used on 5,200, 6,000 and 7,000 pound 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

If you have a brake mounting flange on your axle or plan to have one installed, you have two options:

Over-the-Hub Disc Brake Rotor

Disc Brake Kit

1. 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. Before you can select an over-the-hub disc brake kit, you have to know the wheel bolt pattern.

With over-the-hub disc brake kits, your trailer wheels must be 14 inches or larger. You will also need to consider fender clearance and wheel stud length because the wheel-and-tire combination is spaced 1/2 inch farther out - a result of adding the disc brake rotor.

2. The 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. To make sure that the hub-and-rotor assembly you're looking at will work with your trailer spindle, you will have to either measure your spindle or get the part numbers from the existing bearings and seal. Along with the axle capacity, wheel diameter and wheel bolt pattern, knowing either this spindle measurement or the part numbers will help you select the correct hydraulic disc brake kit for your trailer.

If you cannot find part numbers for the bearings and seal, you can measure the spindle (see spindle diagram) at the inner bearing surface (C), the outer bearing surface (D) and the seal contact surface (B). Once you have these measurements, you can look up the part numbers of the bearings and seal that you need in order to select the right disc brake kit.

Disc brake parts are manufactured with one of several different finishes - raw metal, e-coat, silver cadmium, or Dacromet or they are made entirely of stainless steel. Parts vary in price and in the degree of resistance to corrosion, depending on the finish. For example, the ability of brake parts to resist corrosion would be important if you typically use your trailer around water, especially salt water. Of the different finishes, "raw" offers the least resistance to corrosion. Better than a raw finish is e-coating, which fights rust and corrosion with a paint-like coating. But a downside to e-coating is that the paint can wear off or get chipped, allowing rust to start. Both silver cadmium and Dacromet plating offer better alternatives for corrosion resistance than either a raw finish or e-coat, and they hold up very well. But parts made of stainless steel stand up best to corrosion because they are reliably rustproof. The only downside here is that stainless parts are much more expensive than parts having any of the other finishes.

Brake Line Kits for Trailers

Next, you will need a brake line kit to plumb the trailer from the disc brake calipers to the hydraulic surge actuator. Brake line kits are selected according to the number of axles on the trailer. 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 is needed for brake lines installed on torsion axles because the hubs on those axles move so much during operation.

Hydraulic Disc Brake Actuator

Another item that your trailer will need is a brake actuator, a device that applies pressure to the brake lines to activate the brakes. The actuator is bolted or welded to the trailer at the tongue and connects to the ball on the hitch of the tow vehicle. The actuator works by compression. When the tow vehicle slows, the trailer pushes against the actuator, causing it to apply the brakes.

The brake actuator that you select must meet or exceed your trailer's GTWR. The actuator also has to match the size of the hitch ball on the tow vehicle. You can select manual or electric reverse lockout options if you would like to have these features in your actuator. Reverse lockouts ensure that you can back your trailer. Because actuators work by compression, they will apply your trailer's brakes when you back up. The lockouts let you override the actuator, preventing it from applying the brakes.

Finally, you should select a brake actuator that is designed specifically for disc brakes. Hydraulic disc brakes operate at a higher pressure (1,500 psi) than drum brakes (1,000 psi), so you have to make sure that the actuator you want to use is compatible with disc brakes.

Common Combinations of Trailer Brake Components

Typical Components of Integrated Hub and Rotor Hydraulic Disc Brakes
Direct Mount to Spindle

EZ Lube Option

Standard

EZ Lube

Brake Flange

4-34

Bearings

Inner- L44649
Outer- L44649

Seal

10-9

RG06-020

Disc Brake Kit

Single Axle- K2HR2E

Finish

E-Coat

Rotor Bolt Pattern

5 on 4-1/2"

Surge Actuator

T4715400
T4715420

Brake Line Kit

Single Axle- T4829900

EZ Lube Option

Standard

EZ Lube

Brake Flange

4-35

Bearings

Inner- L68149
Outer- L44649

Seal

58846

RG06-050

Disc Brake Kit

Single Axle- T4843100
Double Axle- T4843700

Single Axle- K2HR35D
Double Axle- T4843700

Finish

Silver Cad

Rotor Bolt Pattern

5 on 4-1/2"

Surge Actuator

T4839100

Brake Line Kit

Single Axle- T4829900
Double Axle- T4830000

EZ Lube Option

Standard

EZ Lube

Brake Flange

4-44-1

Bearings

Inner- 25580
Outer- 15123 or LM67048

Inner- 25580
Outer- 02475 or 14125A

Inner- 25580
Outer- 15123 or LM67048

Inner- 25580
Outer- 02475 or 14125A

Seal

GS-2125DL
or GS-2250DL

RG06-090
or RG06-070

Disc Brake Kit

Single Axle- K2HR526D

Finish

Dacromet

Rotor Bolt Pattern

6 on 5-1/2"

Surge Actuator

T4750800
T4750600
DM8104311

Brake Line Kit

Single Axle- T4829900
Double Axle- T4830000

EZ Lube Option

Standard

EZ Lube

Brake Flange

4-44-1

Bearings

Inner- 25580
Outer- 15123 or 14125A

Seal

GS-2125DL
or GS-2250DL

RG06-090
or RG06-070

Disc Brake Kit

Single Axle- K2HR712

Single Axle- K2HR526D

Finish

Raw Finish

Dacromet

Rotor Bolt Pattern

8 on 6-1/2"

Surge Actuator

DM8204311
T4750720

Brake Line Kit

Single Axle- T4829900
Double Axle- T4830000

Typical Components of Integrated Hub and Rotor Hydraulic Disc Brakes
Mount over Idler Hub

EZ Lube Option

Standard

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"

Surge Actuator

T4839100

Brake Line Kit

Single Axle- T4829900
Double Axle- T4830000

EZ Lube Option

Standard

EZ Lube

Brake Flange

4-44-1

Disk Brake Kit

Single Axle - K2R526E

Finish

E-Coat

Idler Hub Bolt Pattern

6 on 5-1/2"

Surge Actuator

T4750800
T4750600
DM8104311

Brake Line Kit

Single Axle- T4829900
Double Axle- T4830000

EZ Lube Option

Standard

EZ Lube

Brake Flange

4-44-1

Disc Brake Kit

Single Axle - K2R712DAC

Finish

Dacromet

Idler Hub Bolt Pattern

5 on 4-1/2"

Surge Actuator

DM8204311
T4750720

Brake Line Kit

Single Axle- T4829900
Double Axle- T4830000

Written by: Raymond P

Updated by: Jacob J

Last updated: 1/24/2024

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