I had a 84 GMC with an engine mounted mechanical fuel pump design that would not pressure up when it was allowed to warm up to reach the temperature of a cooling engine block. This would routinely happen when ever we stopped for a quick burger on a hot day. It would take 15-20 miles at low power until it recovered normal operation. Two different GMC fuel pumps did the same thing. Was not until I installed a 3rd party pump that the problem was solved.
Turns out when the "genuine" GMC fuel pump heated up and due to thermal expansion a leak developed around a rubber gasket seal between the Hi P output and the Low P return to tank line.
