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

Parts for Adding Hydraulic Disc Trailer Brakes
Part	Brake Mounting Flange	Spindle	Disc Brake Assembly	Hydraulic Disc Brake Actuator	Brake Line Kit
Consider	Axle diameter	Bearing and seal dimensions Seal contact surface (B) - Inner bearing (C) - Outer bearing (D) - Axle capacity -	Bearing spindle seal Wheel bolt pattern	Select Hydraulic surge brake actuator	Type of axle Number of axles

Trailer Brake Mounting Flanges

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.

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
Axle Size	EZ Lube Option	Brake Flange	Bearings	Seal	Disc Brake Kit	Finish	Rotor Bolt Pattern	Surge Actuator	Brake Line Kit
2,000-lb Axle 1-3/4" Diameter	Standard	4-34	Inner - L44649 Outer - L44649	10-9	Single axle - K2HR2E	E-Coat	5 on 4-1/2"	T4715400 T4715420	Single axle - T4829900
	EZ Lube	4-34	Inner - L44649 Outer - L44649	RG06-020	Single axle - K2HR2E	E-Coat	5 on 4-1/2"	T4715400 T4715420	Single axle - T4829900
3,500-lb Axle 2-3/8" Diameter	Standard	4-35	Inner - L68149 Outer - L44649	58846	Single axle - T4843100 Double axle - T4843700	Silver Cad	5 on 4-1/2"	T4839100	Single axle - T4829900 Double axle - T4830000
	EZ Lube	4-35	Inner - L68149 Outer - L44649	10-19	Single axle - K2HR35D Double axle - T4843700	Silver Cad	5 on 4-1/2"	T4839100	Single axle - T4829900 Double axle - T4830000
5,200-lb and 6,000-lb Axle 3" Diameter	Standard	4-44-1	Inner - 25580 Outer - 15123 or LM67048	GS-2125-DL or GS-2250-DL	Single axle - K2HR526D	Dacromet	6 on 5-1/2"	T4750800 T4750600 DM8104311	Single axle - T4829900 Double axle - T4830000
			Inner - 25580 Outer - 02475 or 14125A	GS-2125-DL or GS-2250-DL	Single axle - K2HR526D	Dacromet	6 on 5-1/2"	T4750800 T4750600 DM8104311	Single axle - T4829900 Double axle - T4830000
	EZ Lube	4-44-1	Inner - 25580 Outer - 15123 or LM67048	RG06-090 or RG06-070	Single axle - K2HR526D	Dacromet	6 on 5-1/2"	T4750800 T4750600 DM8104311	Single axle - T4829900 Double axle - T4830000
			Inner - 25580 Outer - 02475 or 14125A	RG06-090 or RG06-070	Single axle - K2HR526D	Dacromet	6 on 5-1/2"	T4750800 T4750600 DM8104311	Single axle - T4829900 Double axle - T4830000
7,000-lb Axle 3" Diameter	Standard	4-44-1	Inner - 25580 Outer - 15123 or 14125A	GS-2125-DL or GS-2250-DL	Single axle - K2HR712	Raw finish	8 on 6-1/2"	DM8204311 T4750720	Single axle - T4829900 Double axle - T4830000
	EZ Lube	4-44-1	Inner - 25580 Outer - 15123 or 14125A	RG06-070 or RG06-090	Single axle - K2HR526D	Dacromet	8 on 6-1/2"	DM8204311 T4750720	Single axle - T4829900 Double axle - T4830000

Typical Components of Rotor Only Hydraulic Disc Brakes Mount over Idler Hub
Axle Size	EZ Lube Option	Brake Flange	Disc Brake Kit	Finish	Idler Hub Bolt Pattern	Surge Actuator	Brake Line Kit
3,500-lb Axle 2-3/8" Diameter	Standard	4-35	Single axle - K2R35DS	Dacromet and stainless	5 on 4-1/2"	T4839100	Single axle - T4829900 Double axle - T4830000
	EZ Lube	4-35	Single axle - K2R35DS	Dacromet and stainless	5 on 4-1/2"	T4839100	Single axle - T4829900 Double axle - T4830000
5,200-lb and 6,000-lb Axle 3" Diameter	Standard	4-44-1	Single axle - K2R526E	E-Coat	6 on 5-1/2"	T4750800 T4750600 DM8104311	Single axle - T4829900 Double axle - T4830000
			Single axle - K2R526E	E-Coat	6 on 5-1/2"	T4750800 T4750600 DM8104311	Single axle - T4829900 Double axle - T4830000
	EZ Lube	4-44-1	Single axle - K2R526E	E-Coat	6 on 5-1/2"	T4750800 T4750600 DM8104311	Single axle - T4829900 Double axle - T4830000
			Single axle - K2R526E	E-Coat	6 on 5-1/2"	T4750800 T4750600 DM8104311	Single axle - T4829900 Double axle - T4830000
7,000-lb Axle 3" Diameter	Standard	4-44-1	Single axle - K2R712DAC	Dacromet	5 on 4-1/2"	DM8204311 T4750720	Single axle - T4829900 Double axle - T4830000
	EZ Lube	4-44-1	Single axle - K2R712DAC	Dacromet	5 on 4-1/2"	DM8204311 T4750720	Single axle - T4829900 Double axle - T4830000

Updated by: Raymond P.

Last updated: 7/6/18