Back aisles has abs sensors but the rear doesn't.

It's fine. Leave it the way it is

 

You're legal with 1 axle having ABS sensors, the valve will still control all 4 positions.

 

The system you have is designed to work with only sensors on 1 axle. During an ABS event or when a wheel locks the system dumps air from all four wheel positions.

Some trucks are the same and only has sensors on 1 drive axle.
They are named as 2S 4M in the case of your trailer or 4S 6M stands for 4 sensors and 6modulator’s or ABS valves and so on depending on the system.

With the advancement of Traction control and stability control most all newer equipment with have sensors on all wheel positions.

 

Pretty standard setup.

A relative of mine bought a brand new tridem trailer about 20 years back. Bit foggy on the details but I think it was a 4S 6M system and one of the axles would chew through brakes faster than the others. Ended up having to convert to 6S 6M to balance out the brake wear.

 

Must be a spring ride Trailer. I have an air ride with a 2S4M system. Sensors are on the rear axle only. The reason being, on a spring ride, the front axle supposedly will lock up first. Air ride rear axle will lock up first. Other axle automatically follows. I like the system. Less sensors, wires, and cheaper valve. Only drawback is not getting an abs light if for some reason a wheel end is running loose, on the axle w/o sensors. Something that should be checked occasionally anyways.

 

Much better description than mine!