The mist sprayer is mounted higher than the pump, so unless the laws of gravity are suspended, the water runs back down the tube, not out the sprayer when the pump stops.

 

Where did you get the boost switch and nozzle from?

 

Nozzles at mcmaster-carr

Boost pressure sensor a la carte from a WI vendor like Cooling Mist or mcmaster-carr may have them

 

WI Parts List
Tank
250 psi shurflo water pump
6.2 GPH nozzle (this is what I have)
12.2 GPH nozzle for engines with more HP (they are 1/8"
NPT)
Tee (if you are using 2 nozzles)
1/8" NPT X 1/4" hose barbs
Hose barbs that will fit the pump you buy
Metal check valve 1/4" NPT F ports
20 feet of 1/4" OD (.180" ID) poly hose
a little 1/4" ID poly hose to connect .180" and the 1/4
hose barb
Boost switch (0-25 psi is what I have)
Wiring
Fuse
Switches and LEDs

 

I don't think gravity is the main concern. Thermal expansion or accidentally energizing the pump with engine off would be more of a problem. Not that I would undo what you've done - but I might consider a little different design starting from scratch...

I'm really just regurgitating some of what I've read on the 'net. The truth is YOU are my go-to guy on this subject!!

Another tidbit I took away from my 'research' was it may not be a bad idea to use a water-based cutting oil in the water to mitigate corrosion in the intake chambers.