If it did not seal it would be getting forced out the rad when hot into the overflow (positive pressure)...once it cools the rad has a vacuum which then enables it to lift the centre of the cap for water recovery back into the rad. The caps are pressure rated so make sure you have the correct one for your system.i have an over flow bottle. black bottle on the right hand side. series 3 1976
still doesnt seen to seal at the top though.
pretty sure its the correct on . cap just isnt water /air tight. you can blow back down the pipe and hear ait leak out.
That would be right as the return valve in the middle is only lightly sprung ? have you tried measuring top to bas on them both ?pretty sure its the correct on . cap just isnt water /air tight. you can blow back down the pipe and hear ait leak out.
not that type of system it's just a overflow bottle not pressurized no rad cap on bottle on series landyIf its got an expansion bottle shouldnt the rad cap just be a plain version with no innnards and the expansion bottle cap be the pressure type? if you put the traditional pressure type on the radiator it cant draw the water back in from the expansion bottle