Sullivan Diesel Compressor not making air

I have a Sullivan 375 Diesel powered air compressor. On start up the engine is running high RPM almost as if it is in loading mode but there is zero air pressure. Normally on start up the compressor will show around 40 PSI on gauge at idle. Once the load/run button is switched the air  pressure will jump up to 110/120 psi. Just wondering if the coupler between the engine and compressor could be broke causing the high RPM's and zero air. Is there an easy way to check this?