Putting a valve on the discharge line is an acceptable temporary repair, and will work indefinitely, so long as the valve leak isn't so bad that a large amount of water is bypassing the filter, when the valve is set on "filter".
You can probably test this by adding DE via the skimmer: if some returns to the pool, you've got water bypassing the filter. If not, not.
But the bypass would likely be worst when the filter is dirty and the resistance to flow through the filter is maxxed.
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