I can't find the answer to this in the manual?

It may not say this exactly in the manual, but the manual explains how the service and emergency lines work and what they do. If you understand that information, you should be able to deduce what would happen in the situation in the question. If you understand the material, it doesn't matter if you haven't seen this exact question before. For this, the answer is the middle one.

 

The way this was explained to me by a mechanic was the Tractor Protect valve will engage and all foot braking will be by the tractor brakes. If you are really loaded on ice or going downhill, you are in serious danger of HOT BRAKES or a jackknife situation. Trust me on this, one of the scariest things a driver can see is their trailer coming toward them in their California mirrors.

 

Nothing will happen right away.

Till you apply the brakes , then you will lose air pressure pretty quickly, and you’ll have no trailer brakes until the air gets low enough to trip the maxi brakes .......

the answer is the second one.