The phrase "reserve tank" refers to an extra storage space for something, typically fuel or energy, that is kept in case the main supply runs out or is not enough. Full definition
I discovered the damaged reserve tank cap immediately but didn't make the connection until just now. (mechanics.stackexchange.com)
The rear - wheel - drive brings 1000 Nm to the road, and the model has a range of up to 650 kilometres thanks to the additional petrol reserve tank. (skoda-auto.com)
My Dad (who used to be a mechanic in his older days) said to give it a couple of days for the water to dry up that came out of the coolant reserve tank where they forgot to put back on. (mechanics.stackexchange.com)