Both. The reservoir is the overflow. But in the event your radiator is low, your reservoir will back it up.
coolant overflow/reservoir tank
The radiator cap could be the cause of coolant reservoir overflow or backup. Have the cap checked to see if it is bad.
you don't fill the radiator directly, there is no radiator fill cap. you fill the overflow reservoir and it keeps the radiator full. overflow reservoir is spherical plastic near firewall. power steering reservoir is up front near battery.
If you mean the plastic overflow tank, replace it. If you mean the plastic tanks on either side of the radiator core, replace the radiator.
the overflow reservoir, it should have marks for full cold and hot
Remove the radiator cap and add it directly to the radiator. Then fill the overflow reservoir to the full mark.
If the engine coolant reservoir has a thread on cap , that is your pressure cap / " radiator cap "
for most latest car and SUV model it is located beside the radiator. You can find it easily by following the overflow hose attached in the neck of the radiator. the end of it is the location of the coolant reservoir
To put in more antifreeze in a 1995 Plymouth Voyager, located the reservoir near the radiator. Antifreeze is no longer added directly to the radiator. It is put into the plastic reservoir that is labeled radiator or coolant overflow.
if its the same as the lhs 97 model its on the overflow reservoir not the radiator its self the reservoir is next to the firewall on passenger side mine has a red relieve valve on a silver cap
Once your vehicle is warmed up, if you have antifreeze coming out of the overflow you have too much in your radiator.