Roughly 2 gallons of coolant...All sort of crannies for it to "Hide" in.

(There's also the unmentioned Block Drain)

With your car's front wheels firmly up on ramps (AND the Emergency brake ON)
'Get a plastic "Kiddie Pool" and place it under the lower Radiator hose
(Where it attaches to the Radiator)...Loosen the clamp and remove the end of
the hose from the radiator.'Messy ,But it will drain MOST of the coolant.
To remove the rest...take the Heater hose off the Monovalve and use a shop vac to pressurize the system...
almost all of the coolant will depart.

FSM instructs us to drain the Radiator VIA the Plug...Then open the Block Drain.
(Helps if the Stern is raised)

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'84 300SD sold
124.128