Expansion and top-up tanks – can I change to later system?

[AlanR25]

Hi. My first post on any forum, ever!
T25 (1983) DF Engine - Leisuredrive conversion – a charmingly retro nightmare.
My coolant tank arrangement is as section 1-4 of the Haynes manual (the expansion tank has a metal cap). Examining the cap, it seems to be a simple “pressure-relief valve” (i.e. as the water system heats up, it lets water out into the top-up tank). As the system cools, I can’t see how water from the top-up tank is ever going to be drawn back into the expansion tank. Can anyone please explain?
Later systems with the “dalek” cap seem more logical (one valve to let water out and a separate valve to let water back in). I would feel more confident about the water system if I could change to the later version, but the metal cap is a bayonet fit while the dalek cap is threaded and the expansion tank that suits the dalek cap has different sized hose connections from mine. Can anyone tell me how much of the system I will have to change to fit a dalek cap?
Thanks.

[ajsimmo]

Just change the pressure tank for the (later) early type.

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[Mocki]

from memory i think you need the expansion tank from a late diesel to fit your pipes, but if there is nothing wrong with your actual tank, you can buy new baynet caps
it works the same way as the darlek cap, it sucks back because the outlet pipe is below the seal when the cap has contracted by the pressure, so before the cap closes the coolant is sucked back as it is cooling.

[bigherb]

As the system cools, I can’t see how water from the top-up tank is ever going to be drawn back into the expansion tank. Can anyone please explain?

There is a vacuum valve in the cap, you should be able to lift the disc up under spring pressure.

[AlanR25]

Thanks bigherb, from memory my cap doesn't look the same in the centre - I will have another (more careful) look.

[marlinowner]

I have the same on my kitcar with a Triumph Vitesse radiator and, remarkably, it works as intended.

[AlanR25]

Thanks for pointing out the vacuum valve. As I have the expansion tank out at the moment, I've been able to test the whole setup. The vacuum valve seems to be working, but the seal between the cap and the top of the tank is suspect - maybe the condition of the sealing surface on the tank or lack of compression from the bayonet. Think I'll follow Mocki's advice and invest in a new cap.
Thanks for your help.