I bought a 77 diesel last yr and was told it ran and worked well with no problems. it does run ok but after after a cpl hours brushogging would lose oil pressure. this was after I put on 2 new filters..new oil and a new oil pressure gauge. so I brought it back home and let it cool down and the next time same thing. I took out the oil bypass spring and plunger and stretched the spring and it worked for a little longer and did the same losing of pressure after a cpl hours run time. i figured it might be a weak spring that after heating up was losing tension,so today I went and got a stiffer spring and cut it to length and installed it instead of shimming the old one. I started the tractor and had good oil pressure for about a minute of run time and then it went to nothing on the gauge again. I have also checked this gauge against another and they read about the same...what am I missing here...any insight would be helpful and a thanks in advance