Coolant and Heating Header tank top and up tank

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Dalek cap, header tank cap, coolant pressure relief cap

Top up tank to fill mark, when engine cold....... Engine gets hot, coolant expands, builds up pressure with temp, and opens header tank pressure cap (VW Blue, after market black, Dalek shape, left hand rear of engine bay (right hand on diesel Ed) ) forcing coolant into the top up tank........ Engine cools down when parked, coolant contracts, pressure turns to vacuum, and coolant is sucked back from top up tank to header tank, returning top up tank to fill level from its over full state when hot..... So, if the pipe between is split or leaking, it would be pushed out of the header tank and out of the split pipe onto the zorst, not into the top up tank, and when the engine cooled down, the header tank would be not full......

However, if you overfill the top up tank ( fill it when cold with Coolant at least 50/50 mix with water) it will push excess out of the breather holes in top of the top up tank, onto the zorst, and find its correct level.....