This one is very common and becomming more so as these mechanical oil pressure guages age. When the gauge starts to leak behind the instrument cluster it may take a while for you to notice. (ie - dark black engine oil dripping on your foot). The leak usually starts out as a slow seepage and if you catch it early enough it won't make a such mess. Watch for signs of oil dampness on the underdash panel and in and around the bottom of the instrument cluster. I have found no easy way to fix a leaking pressure guage. You could spend a lot of time messing with it and never get the leak stopped permanently. It some cases it would even require soldering and guess how fun that is when you have oil inside the unit. Just plugging off the guage is a dangerous proposition as you won't know what your oil pressure is and finding a suitable fitting to close the end of the line is nearly impossible.
The best way to fix the problem is to find a good used cluster from a parts car. All 1977 to 1985 models are not the same so be sure to purchase the right year for your car. I often have good used units in stock. Email if you need one. See my Instrument Cluster manual if you need instructions to remove and replace the cluster.
