I have a ford cargo 0813 . The air compressor for the brakes does not cut o?

When i start up my ford cargo 0813 (1988) the compressor can be heard pumping up the cylinders as it should . I understand at approximately 8.5 bar the compressor should knock off but it continues right up to 12 bar on both gauges and still doesnt stop . I believe that a relief value must be stuck . Am i right , where is it or how to i mend this problem please ?