The choke "pull-off" vacuum diaphragm is not supposed to open the choke completely, it is supposed to pull the choke plate open a small amount once the engine starts. The choke thermostat has the job of slowly opening the choke plate to a wide-open position based on the thermostat temperature. I would research and test the choke thermostat. Is the diaphragm being damaged by gasoline or backfiring?

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy