Water header tank...should it have a washer in the cap?

I drove to & from Manchester last weekend & having checked all the fluids before setting off I rechecked it all once I returned home.
The Td4 hasn't used any oil but had used a small amount of water from the header tank.
I've looked for any cracks but can't find any, but I did notice that the plastic screw-on cap doesn't have a rubber washer inside it.
Does the cap seal onto the plastic of the thread on the tank or should it have a rubber washer & mine is just missing?!

There should be a seal to stop air pushing out when the coolant gets hot. Theory is the system becomes pressured when the coolant is hot. Hence why the rubber coolant tubes become harder when hot due to increased pressure as the water expands a bit. The cap has a pressure release value in it which open to vent the excess air pressure if the pressure rises above a certain amount.

Mine has a sort of fixed to the cap rubber ring which pushes down on top of the threaded part of the coolant reservoir. It was bought around 2009 when I changed both the coolant reservoir and the cap using main dealer supplied parts.

This is the exact cap i have had problems with, Have a 1.8 petrol and each time i complained to the company who sold me the header tank it was my fault.

apparently my system was over pressurizing and causing coolant to leak out. Also it hit the bonnet slightly causing the bottle to slowly crack on the top.

fixed the coolant leak by pushing down very hard so it tightened past the clicking stage.


TOP TIP FOR BEARMACH IF YOUR GOING TO PRODUCE A BETTER UPGRADED COMPONENT MAKE SURE IT DOESNT LEAK THROUGH THE THREAD AND ITS SLIGHTLY SMALLER (HEIGHT) THAT IT DOESN'T TOUCH THE BONNET ONCE ITS CLOSED

If you look carefully the threaded part looks like it's got 2 seal at the top a I ringing the centre round bit and another at the top of the thread

Found the washer but it had definitely seen better days.
Had a rummage in the work van and found a new one. Will keep an eye on the water level and see what happens.

also, quite common for hairline cracks to form in the header tank where the threaded neck joins the bottle.

these are closed and sealed until the coolant heats up and pressurises the system, when the open up and let vapour out

tell tale sign is deposits, a bit like crystals around the neck of the header tank