Then do you have the factory brake controller on the dash as below?
From your 2011 E450 owners manual:
Electric brakes and manual, automatic or surge-type trailer brakes are safe if installed properly and adjusted to the manufacturer’s specifications. The trailer brakes must meet local and Federal regulations.
WARNING: Do not connect a trailer’s hydraulic brake system directly to your vehicle’s brake system. Your vehicle may not have enough braking power and your chances of having a collision greatly increase.
The braking system of the tow vehicle is rated for operation at the GVWR not GCWR.
Integrated trailer brake controller (if equipped) Your vehicle may be equipped with a fully integrated electronic trailer brake controller (TBC). When used properly, the TBC helps ensure smooth and effective trailer braking by powering the trailer’s electric brakes with a proportional output based on the towing vehicle’s brake pressure.
WARNING: The Ford TBC has only been verified to be compatible with trailers having electric-actuated drum brakes (one to four axles) and not hydraulic surge or electric-over-hydraulic types. It is the responsibility of the customer to ensure that the trailer brakes are adjusted appropriately, functioning normally and all electric connections are properly made. Failure to do so may result in loss of vehicle control, crash or serious injury.
The TBC user interface consists of the following:
(GAIN adjustment buttons) Pressing these buttons adjusts the TBC’s power output to the trailer brakes (in 0.5 increments). The GAIN setting can be increased to a maximum of 10.0 or decreased to a minimum of 0 (no trailer braking). Pressing and holding a button raises or lowers the setting continuously. The gain setting displays in the message center as follows: TBC GAIN = XX.X.
2. Manual control lever
: Slide the control lever to the left to activate power to the trailer’s electric brakes independent of the tow vehicle’s brakes (see the following Procedure for adjusting GAIN section for instructions on proper use of this feature). If the manual control is activated while the brake is also applied, the greater of the two inputs determines the power sent to the trailer brakes.
• Stop lamps
: Activating the TBC manual control lever illuminates both the trailer brake lamps and the tow vehicle brake lamps except the center high-mount stop lamp (presuming proper trailer electrical connection). Pressing the vehicle brake pedal also illuminates both trailer and vehicle brake lamps./