Replace steel pipe with hose?
 

I can't source a replacement expansion tank to radiator STEEL pipe. (On Volvo's cooling system diagram No. 2677988g attached it's labelled "38".) Mine's rusty and leaking.
So could I replace it with a flexible hose, attached to the existing hoses at the expansion tank end and the radiator end? Where could I source a suitable hose?

S40 1999 Auto Petrol.

I don't see any reason why not. If you got a bit long enough you could replace 37/38/39 in one piece IF they are all the same bore diameter. A few cable ties and less joints to worry about.
Motor Factors do water pipe by diameter on the roll.

Thanks.
 

Thanks.

use silicon motorsport hose as anything you buy in an accessory shop will be poor and not last like the steel pipe did , you will fit it and forget it for some years until it dumps all your coolant in a few seconds on the motorway in the dark ...and probably ruin the engine ,

https://www.merlinmotorsport.co.uk/s...e-hose-lengths

they are very helpful and will advise .