Everyone is right about swapping parts with a different vac. Being that it happens sometimes it's going to be hard to troubleshoot.
If it happened everytime, I would say possibly the Multitron. But since it's sparatic I would point more towards the GS-9.
Don't weep the brush
You DO NOT want to weep if you are using Methanol Soap. It will defeat the purpose of having it.
I don't see the advantage of weep the foam brush anyways. When the water freezes it will usually start on the brushes. Then customers are scrubbing their cars with iced up...