i had the same problem. inside the throttle cable from the pedal to the pump there is a spring (its inside the cable cover cant see it ) i think its to give a smooth throttle responce, but they get all gunked up and it dosnt matter how fast you push the pedal its the spring that has to pull the pump arm, if its all gunked up it pulls slowley, to fix the problem i used the magic fix all liquid WD40, take the cable off of pedal end and push through bulk head.
then you can stand in front of the van and pull on the cable to see it move the pump arm, i held it up in the air and squirted wd down it and pulled and relessed till it was free, also spray arm and spring on pump