For surge braked trailers, a reversing valve is recommended to facilitate backing the trailer. This is necessary because disc brakes function equally well in either direction, unlike uni-servo drum brakes.

Note: Uni-servo drum brakes need very little hydraulic pressure to activate them while going forward, but a tremendous amount of pressure is required to operate the brakes in reverse. Therefore, many users simply overpower drum brakes when backing up.

Most surge brake actuators require the installation of a reversing solenoid by the trailer manufacturer. These solenoids are energized by connecting them to the tow vehicle's back-up light circuit. The solenoids come in two types:

1. Normally Open - This solenoid valve is installed "in-line," normally at or near the master cylinder.

a. Advantages:

i. Quick and easy to install.

b. Disadvantages:

i. If trailer is stopped in a downhill attitude, some positive pressure may remain in the brake line. When the back-up light is energized, the solenoid valve merely "traps" the pressurized brake fluid, which may prohibit backing up the trailer.

ii. If trailer hits any significant object (such as a curb) while in the process of backing up, the entire force of this impact is transferred to the master cylinder piston (seal). A seal could be ruptured in this manner.

1. Normally Closed - This solenoid valve is typically plumbed from a "Tee" that is screwed into the master cylinder outlet or, alternatively, the solenoid valve may have three ports. From the valve, a "return line" must be installed to dump the brake fluid back into the master cylinder reservoir.

a. Advantages:

i. Eliminates the problems listed in b(i). and b(ii). above.

ii. Some surge brake actuators may be purchased with a normally closed valve installed by the factory.

b. Disadvantages:

i. Requires a few additional fittings and a short length of brake line.

ii. Master cylinders in some surge brake actuators are not "plumbed" to accept a return line.

NOTES:

1. Solenoid valves may be one wire (grounded case) or two wires (ungrounded case). On a two wire model, generally either wire can be grounded and either wire may be connected to 12 vdc (i.e., polarity is not a problem).

2. Solenoid valves should have an "in" and "out" marked on the housing. This configuration must be observed when piping a solenoid valve (i.e., it will not work properly otherwise).

3. Older hydraulic drum brake systems required a 10 - 12 psi residual pressure be maintained in order to keep the wheel cylinder piston seals flared. To accomplish this, many surge brake actuators have installed a small diaphragm type check valve just inside of the master cylinder output port.

Disc brake systems must not be subjected to a 10 - 12 psi residual brake line pressure (the brakes will overheat). Therefore, the diaphragm (if installed) must be perforated with an ice pick or awl prior to installing and operating a disc brake system.