Mazda Bongo Header Tank – Includes Radiator Cap – Genuine Mazda Item.
A header tank is a container of liquid positioned at a higher level than a main tank, so that the level of pressure can be maintained. The coolant in the radiator header tank should have a mix of water and antifreeze.
If your reservoir is yellow and difficult to inspect, then why not replace it?
2.5 Turbo Diesel – (WL-T) – Chassis (SGL3, SGL3F, SGL5, SGL5F, SGLW)
2.5 Litre V6 Petrol – (J5) – Chassis (SG5W)