The way CTIS works is the PCU send pressure thru the dump valves, thru the hubs to the wheel valves. The wheel valves are remote control valves. As soon as the truck side goes above ~5 PSI, they open and connect tires to the dump valves.
The dump valves are the key to how things are controlled. They are remote pressure regulators. Whatever pressure is put on the input, they mimmic on the output. you put 100 psi on the input, it flows thru to the output until the output = 100PSI. If you lower the input to 50PSI, air flows back from the output port and exits the vent port on the dump valve until the output = the 50PSI applied to the input.
When CTIS activates, it seals the system(closes control valve on PCU) and gives a brief shot of supply air. This air goes to the dump valve inputs and is mimmiced on the output which feeds down thru the hubs to the wheel valves. When the valves see 5PSI, they open, tire pressure feeds back into the system and the whole system stabilizes at tire pressure(tires will equalize to the same pressure). This is how CTIS can read tire pressure with the single sensor on the PCU.. Want to inflate, turn on supply pressure(supply solenoid on PCU) and air flows from the PCU out thru the dump valves and inflates the tires.
Want to deflate? Lower the PCU to something just over 5PSI to keep the wheel valves open and the dump valves will mimmic this pressure by dumping tire pressure back thru the dump valve vents. The PCU does this by connecting the manifold to a 6 PSI relief valve using the dump solenoid. This relief acts like a pressure regulator and holds the truck side/dump valve inputs ~6PSI and they then dump tire air very quickly…
want to stop everything, open the control solenoid and vent the PCU to zero. The dumps will mimmic this to their outputs which will cause the wheel valves to close/seal.
in your case, you have a real bad leak that is lowering the system pressure below tire pressure but not low enough to close the wheel valves and they will feed that leak until they exhaust all their air…
you can disconnect each hub CTIS line in turn and cap it with a plugged fitting to determine which hub seal is leaking. Then its spindle overhaul time to fix the bad seal…
