If it has been a couple of years or more since renewing the antifreeze, this would be a good time. So flush the system with water, since there WILL be some spillage inside the car when you undo the hoses at the valve and water is nicer than Glycol in your carpets.

Drain the system via the engine block drain below the rear exhaust port and also via lower rad hose. I used a garbage bag laid on the carpet under the valve to catch the spillage. Removal of the old valve was not complicated, but fiddly and you are in an awkward position. Have a thick carpet or pad available as you will be kneeling outside the driver's side while leaning in there.

After the job and with the coolant refilled, warm the car up thoroughly with the reservoir cap screwed firmly on so there is pressure in the system. Then check the valve area very carefully for leaks. You do not want one there.

You will want a cold one and a back massage when it's all done.
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Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.