I am having the same problem with my new Procon pump. The outgoing pressure builds up and it does not allow the presoak to work. This is getting annoying!
Well, I decided today to replace the Procon pump (ppcb2507x) which is associated with the pre-soak problem. Maybe it could be the Procon’s fault? Only time will tell.
The presoak did not work again and there was pressure built up on the outgoing side of the check valve so the check valve could not open. I just took the hose off to remove the pressure. I am not really sure what the problem is so I will need to look at it again tomorrow.
It was not me this time! I pulled up to the carwash and there is an abandoned car, which is always nice. They actually came back with a Windex container full of gas and off they went!