Sounds like a weak ground connection somewhere on the trailer or tow vehicle.
Check for a bad ground circuit at the rear tial lights.
The Bendix OEM knobs for those valves are yellow and red. Yellow is for the brake control valve, which releases the brakes on the power unit. The red is for the trailer supply valve, which provides air to the trailer, both to release the trailer brakes, to inflate the air ride suspension, and to actuate the brakes when the brake pedal is depressed.
The master cylinder seals are worn out. Have it replaced soon, very soon!
Often this type of behavior is caused by a bad ground for the lighting circuit. Check the ground circuit for the brake lights.
First you need to understand that most, if not all, vehicle [autos, trucks, and trailers] rear lighting systems have both tail light bulbs and brake light bulbs, OR a single bulb on each side [right and left] that each have two filaments [what glows when electricity is applied]. The tail lights are supposed to be on when the headlights, or parking lights, in the towing vehicle are turned on. The separate brake light bulb, or brake light filament in the dual filament bulbs is only supposed to turn on when the brake pedal is depressed, and the towing vehicle brake lights are on. I suspect that what you are calling brake lights are actually the tail lights. To check this out, run some tests. Have an assistant sit in the vehicle and do the following on your command: 1. Apply brakes by pressing on the brake pedal. The brake lights should turn on. 2. With foot OFF of the brake, turn on the headlights. The trailer tail lights should turn on. 3. With the headlights on, have the assistant depress the break pedal. Even though the tail lights are already on, the brake lights should come on also, causing the trailer lights to brighten noticeably 4. While you're checking, have the assistant turn the ignition key to on, and one at a time test the left and right turn signals. Again, I think what you think are brake lights, are actually the tail ligts.
that sounds like i ground in the bake by the lights or if it has a trailer plug i would start their
Fuse, wiring, bulbs
Most trailer brake controllers require 3 or 4 "inputs" and have 1 output. Ground, any point on the vehicle metal frame. Power, any fused connection that provides power when the key is on. some controllers don't use a power connection. You'll need to refer to the owners manual that came with the controller. Brake light, a signal from the brakes indicating that the brake pedal has been depressed. The output goes to the trailer plug pin that controls your trailer brakes.
There is a connection issue between the trailer brake controller and the trailer.
It sounds to me like you have wiring problems.
The 2001 Dodge Dakota, with a towing package, has a trailer brake built in. The trailer brake is part of the towing package.
Possible ground issue at the taillights.