FF81,
Im not familiar with a GLP090 but it sounds like its a fairley large truck. It looks like the truck is not getting enough gas to start it especially when you say it trys to start when primer button is pressed. what type of gas system is on this truck Beam, Asian, etc. I think I would remove the gas outlet hose from the regulator at the carb end and stick it in a bowl of water turn the engine over this will tell you how much gas is coming from regulator, lots of bubbles lots of gas hardly any bubbles no gas and progress from there,

Ihope this gives you some fresh ideas where to start looking.

Titus