I have a 2005 expedition and a travel trailer with electric brakes. My question is when i hook up the trailer my brake pedal seems to depress as if it was leaking fluid, does that mean there's an electrical problem somewhere in the trucks system?
It doesn't go to the floor but it does go lower than without the hookup.

 

the trucks hydraulic brakes and the trailers electric brakes are two completely separate systems. I'm wondering if the electric brakes are working at all? If the truck's hydraulic brakes are doing all the work you will have to apply the brake pedal harder...hence the deeper pedal.

hit the manual apply button on your brake controller when moving; you should feel the trailer brakes slow down the truck and trailer.

 

That happens sitting in the driveway and not moving so it can't be what you suggest. Haven't even had the trailer out on the road yet.

 

It would seem if the vehicle's brakes function normally without the trailer hooked up, there may be a problem either in the wiring to the trailer or in the trailer itself.
Whatever it is , it seems to be affecting the ABS system.
Might be a good idea to check if there is a good separate ground to the trailer and is it working as it should.

 

who wired up the brake controller?

pull the abs fuse; if the problem is still there, you know its not the abs pump dumping pressure.