Bongo Fury

I've now replaced the coolant sensor (in addition to most of the vacuum hosing and two of the four solenoids) but don't think my issue is solved.

With a spare coolant sensor and some ice, I can see that the vacuum system is working in that the actuator does move to increase rpm when the sensor is cold. However the actuator doesn't move enough to make much of a difference to rpm.

I can move the actuator and throttle by hand with little effort so it seems like there is insufficient vacuum being generated or there is a leak somewhere. Does anyone know if there are values of the vacuum strength that I could measure somehow or how best to go diagnose a leak in the system?

This ties in with the van cutting out when slowing down, my thinking is that braking is also using some of the vacuum force, causing less of a vaccum to remain on the cold start actuator, causing the idle rpm not to be increased anymore and the van stalling when cold. Make any sense???

The vac pipes can collapse when they get old or have been swapped with thin walled pipe especially when warmed up.
One I did work on years ago had similar issues. Start from the pump and the feed pipe to the solenoids, this comes up the back of the injector pump in metal and can get flattened or damaged. See if you have good vac at the end of the steel pipe from the pump then look for corrosion around the pump. The pumps won't last forever either.