VDO Oil Pressure Gauge

Hi, I have recently fitted VDO Gauges to my mk1 1.8 Gti cabriolet. One problem. When I release the hand brake the oil pressure reading flys off the gauge yet when I engage the handbrake the gauge reads normal. I have chased all of my wiring back to their origins and everything is fine. I can only think that there is a short somewhere but where I dont know. I have even run separate live and earth feeds to the gauge but still the same problem. Any ideas guys?