Final answer:
A higher than normal air pressure in a pneumatic system suggests a likely problem with the defective governor circuit, as it regulates the operation of the air compressor.
Step-by-step explanation:
When air pressure is higher than normal in a pneumatic system, the most likely problem is a defective governor circuit. The governor controls the air pressure by regulating the air compressor's operation. If it's not functioning correctly, it could fail to signal the air compressor to stop, leading to an overbuild of pressure. However, if an un-loader valve is stuck open, it would lead to a loss of pressure rather than an increase. Excess signals from the supply tank or an air compressor over capacity would not typically cause an increase in pressure in the system.