Originally Posted by
Jon Wehrenberg
This is a place for Peter to jump in and help. It is my understanding the assembly that attaches to your bus receives air pressure only when the service brakes are supplied. Its purpose is to function as a pass through device when everything is working perfectly. When the toad breaks away however, it will break the air line from the bus to the toad so the bus mounted device functions like a propane tank valve and shuts off air flow when it senses the line to the toad is now broken. That preserves the bus braking system and is a safety device.
I am presuming your broken fitting was between this device and the bus braking system.
The device on the toad serves to create vacuum via pressure venturi and it is that vacuum that keeps your power brake booster in a negative pressure condition to provide power assist when the brakes are applied. Additionally that device has a small internal air pressure storage tank to apply the toad brakes in the even of a breakaway. When the toad is connected to the bus and all is operating normally, your air brake pressure on the bus is also supplied to the toad to operate the toad brake pedal via the small air cylinder attached to the brake pedal.
If the system, as I described it is correctly installed it is unrelated to the bus air system pressures and it may be just a coincidence you are experiencing problems reaching normal pressure. At the pressures you describe you will not even be able to lift the front of your bus, and the rear will be marginal.