If a driver performs a service brake application and the tractor protection valve pops out, but the trailer brakes do not apply, what is the most likely cause?

1. The tractor's primary check valve is stuck open
2. The service (blue) and supply (red) lines are crossed, or the gladhand seals are leaking heavily
3. The trailer reservoirs are over-pressurized past 150 psi
4. The engine governor unloader lines are disconnected