Indeed, it can be tricky soldering under baths, especially if they're acrylic. I agree with Belboz, the simple method if the cylinder is vented would be to use plastic pipe and push-fit fittings. However, you can use compression on plastic. When using plastic it's important to pipe inserts at every fitting, also I would advise to use compression iso valves, as the plastic ones are a bit carp.
If you're using push fit, make sure there's no crud on the copper pipes which could interfere with the o-ring seal.
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