The break-away switch controls all of the brakes; pull the pin and all 4 wheels should lockup. While the break-away switch may be broken, it physically can not control only one of the axles' brakes.
So it sounds to me like a wiring issue. If you are comfortable with working with 12vdc wiring, the following should get you pointed in the right direction. Here is what is on my '99 Sunline and I would expect that yours is probably the same.
There is a single heavy duty wire coming from the front of the trailer along the driver's side frame member. It is blue in color and probably 12 or 10 ga. This is the +12vdc line controlled either by your brake controller or the break-away switch. By the front axle, this blue wire goes into a wire nut and there are two wires coming out of it - one to the front axle and one to the rear axle. This is the first thing that should be checked. Make sure that wire nut is good and all three wires are properly connected in it. If necessary, cut off the wire nut, re-strip the wires, coat them with dielectric grease and put on a new wire nut. Make sure it is tight, and wrap it and an inch or two of the wires with electrical tape.
Since that wire splits in two at the front axle, it's a good possibility that is your problem, however, there is more to check. Trace the hot line going to the axle from the above mentioned wire nut and make sure it's intact. It is very possible that there is a break in that wire. That wire also splits into two lines, one to the driver's side brake and one that runs through the axle itself to the passenger side brake. Since you said that the entire axle was dead, the problem is most likely to be before or at that split.
The ground for each axle is a short wire that comes up from the axle and terminates at a grounding connector which is screwed directly into the frame. This is another important thing to check. Make sure there is a good ground and that the ground wire is properly connected to the connector. If necessary, take it apart, clean everything, coat the bare wire and connector with dielectric grease and put it all back together. I usually put a wire brush on my electric drill, and clean the frame at this connection of rust and corrosion before re-assembling the ground connection.
I like to use the dielectric grease on any wiring that is on the outside of a vehicle. I coat the bases of light bulbs with it as well as any and all connections that I work on. It is pretty much impervious to water, so it protects the connections from corrosion and moisture.
If you have a voltmeter or test light, you can check the circuit all along the way. The probe on either will usually get into a wire nut without damaging the connection, and you can quickly determine what does and does not have juice. Test the grounding point on the wire side of the connection. If there is voltage there, the ground is bad.
My '99 has developed some rust on the frame and other exposed steel parts so I would expect yours to be in about the same condition. I've also found corrosion in some of the electric connections under the trailer.
There is one other possibility that I can think of, and that is a dead short within the front axle. This can be very difficult to locate and probably requires a technician to deal with it. But, since you didn't mention any blown fuses when you tested the brakes, I don't think it is a short.
I think that about covers it. If you go through all of this and don't find the problem, then I would think it's time to take it to a trailer shop.
Good luck and let us know what you find.
'12 F250 4x4 Super Duty PowerStroke 6.7 diesel
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