fuel pump key on prime

The fuel pump key on prime does not work when hot on my 2008 6L Gen4 VE fitted with a 500kW modified supercharged LSA engine retaining the standard E38 ECU. It is fitted with an in tank Walbro 535 lph pump with a modified tank cover retaining all the existing internal tank components to suck fuel from the opposite side of the tank into the surge tank and keep the fuel level sensors working. The in tank fuel pressure regulator and non-return check valve had to be removed to make the pump fit. A new 10mm fuel line runs via a large fuel filter up front through the fuel rails to an AEM fuel pressure regulater returning the dumped fuel via the original fuel supply line back to the tank. The fuel pump key on prime works fine when cold and it starts perfectly when I wait for the fuel pump to complete its current reset 8 second prime time.

I do not want to fit an in line non-return check valve because the 535 lph pump is already running at its limit to maintain 400 kpa fuel pressure at full power and probably won't deliver enough fuel/pressure with any additional restrictions in the fuel line.

Can anyone advise me how to make the fuel pump key on prime work at all engine temperatures assuming this is the trigger/culprit?