Gas gauge

I am new to this forum and any advise would be welcome. My son and I purchased a 1969 Dart Custom 4dr. as a project. Gas gauge was inoperable. After doing research we pulled the dash cluster, no easy chore, cleaned and repaired the loose pins and installed a new gauge voltage regulator. An ohm test show all was good. After reinstall the gas gauge will start to come up and then after a few seconds drops back down. Any suggestions? Thanks.