I bought it 2nd hand. I think the relief valve (which is under the Filter right?) got clogged with oil, and is supposed to auto relive air?
I think it's a really old model though it's stats hold up well for a modern model...
Aztek AC1

ON OFFER THE AZTEK AC1 COMPRESSOR PROVIDES A CONSTANT SUPPLY OF PULSE FREE AIR WHICH CAN BE REGULATED TO THE DESIRED PRESSURE AND IS VIBRATION FREE AND WILL DELIVER UP TO 80 PSI (6 BAR) QUIETLY YOU WOULD HARDLY KNOW IT WAS RUNNING IT HAS A AIR PRESSURE REGULATOR AND GAUGE A MOISTURE TRAP ON AIR OUTLET 30 db/1m AIR RESERVOIR- 1 LITRE 135 WATT MOTOR 17 LITRE / FREE AIR OUTPUT (0.7 c.f.m) CONTINUOUS RUNNING WITH PRESSURE SWITCH 14 INCH BSP OUTLET THREAD, HEIGHT 360MM, LENGTH 390MM, AND WIDTH 195MM ITS OVER 17KG SO POSTAGE COST ARE HIGH (£25 UK ONLY OR CAN BE COLLECTED FROM ME IN ORMSKIRK LANCS) IT HAS BEEN USED BUT ITS IN A LOVELY CONDITION ON OFFER WITH NO RESERVE
Thanks For your help guys