FAQ - Subject - Solenoid valves used for trailer hydraulic brake systems.
Solenoid valves are used on hydraulic trailer brake systems to lock the brakes out while backing up.

[1] What is the difference between a blocking solenoid and a bleed type solenoid valve?

***Blocking solenoid valves are inline [ normally open valves] that close when 12 VDC is applied.
This locks out the potential for any further brake pressure to be induced into the brake tubing system
while the solenoid is electrically activated.
This type of valve does not require a return line back to the master cylinder reservoir.
One potentially negative feature of this type of valve is that if brake fluid pressure was trapped between
the master cylinder and the wheel brake devices before the electrical activation of the solenoid,
this brake pressure will remain trapped and may inhibit the trailer from being backed up.

******* BLEED SOLENOID valves are typically 3 way [ 3 port] valves. Three way bleed solenoid
Valves allow free passage of brake fluid thru the valve during normal braking operations,
when not electrically activated.
The third port of the valve is connected to a return line back to the master cylinder reservoir.
This port is closed when the valve is not electrically activated.
When electrically activated, this third port opens allowing release of pressure back to the reservoir
& atmospheric pressure [ 0 PSI gage pressure].
Because this bleed valve releases all brake line pressure, it is an excellent choice for applications
where backing uphill are necessary. However, it does require a return port machined in the
master cylinder reservoir.
****** 2 way bleed SOV - It is possible to install a 2 way normally closed valve into the system
by installing a TEE in the brake line tubing & using one port of the SOV to connect to the
return line back to the reservoir. Again when electrically activated this valve opens & pressure
is released.

If the master cylinder in your
brake coupler is made from a composite
material & is not metal then you will need
to provide a ground wire connection
directly to the brass body of the solenoid valve.

Solenoid valves do not operate well
if there is a poor ground or a poor
connection of the 12VDC power to the
valve.