funny you say this...I started filling the what I thought was the washer fluid reservoir with a bout 3/4 gal of it going into the coolant side before I realized oh ****. luckily the washer fluid didn't mix and stayed at the top and I was able to syphon the majority of it out.
the coolant fill reservoir is a siphon typer overflow. if you fill and empty it while cold and not running there's pretty much no mix with the active coolant loop.
when the truck is cooling down the coolant pressure in the loop goes down and it vacuums in coolant from the reservoir to keep the system full with no air/cavitation.
When the truck warms up and the active loop coolant expands, it pushes open the valve to dump into the overflow tank (where we usually top up.) if there's too much expansion, the excess dumps out the overflow drain on the reservoir onto the ground under the truck. this is not a normal behavior for a working system and is more likely during an overheat or overfilled system.
When the active loop is low on coolant the hot air expands much faster than the coolant boils off and burps out into the reservoir, and the burps take the overflow tube to outside.
The vacuum left by the lower volume of air in the system combined with the vacuum created by the water pump will pull fluid back in from the reservoir to the radiator eventually burping away all the airspace in the active loop.
In day to day we should never have to use the pressure cap to top up our trucks coolant systems anymore. Leaks happen, service interval flushes happen, and an evacuated coolant loop won't push/pull without fluid so the top pressure cap is needed for high volume coolant exchanges.
As for the washer reservoir, the pumps for the sprayers are typically attached mechanically to the bottom of the fill tank, one per sprayer circuit (i.e. one for front sprayers, one for rear).
our trucks don't have headlight washer/wipers, rear window washer/wipers. what I would usually do is check the rear wipers vs the front to check for a reservoir leak compared to a ruptured line on the front or rear sprayer tube.
Fun but from Jeep days. The rear reservoir had a flex tube running from the tailgate to the tank. the flex tube for the wipers dry rotted often and turned the rear glass wiper/washer into a pisser that would directly spray the tailgater behind me.
