Filters are maybe 1.5" diameter, 2.5" long. Not tiny, not what I would call full-size either. Pictures are difficult as the bus is parked in storage a good bit away from me.

What you are hypothesizing about the line pressure dropping below the vapor pressure of the gasoline makes sense, but it is hard to reconcile with some of the observed behavior. Specifically, if it were caused by high fuel demand lowering the input line pressure, this problem would be self-correcting the moment the engine stopped running, yet the only remedy I have found is to park the bus and open the hood for 5--10 minutes and let the heat dissipate. It is clearly heat related, both from the point of view that it only happens on warm days, and only happens when the engine is generating extra heat under heavy load. And yet the entire intake side of the fuel system is well away from heat sources (exhaust, engine) while nonetheless behavior points towards vapor lock on the input side of the pump (pump runs as though it is not receiving fuel).

I guess I'll just have to pull those filters out and the next hot day take the bus out for a nice hilly drive.

What's your best guess as to whether the output (pressurized) line can vapor lock? Might it be dependent on carburetor design, that is, how does the carburetor handle vapor instead of liquid entering the float bowl? It seems likely that the float bowl would be vented to accommodate such an occurrence.

tanstaafl.