leaking grey water outlet

we have just noticed that the grey water outlet at the back of our van is leaking whilst the 'd' lever is full engaged.  could anyone give us some advice as to how to stop this happening please

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Have you this 3" grey water drain?

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hello  yes this is exactly what we have

I've had that for 10.5 years.  First time it started to drip with the cap off I replaced the drain unit - it unscrews.  It does it again now but not when the cap is on.  Replacement caps are available from CAK tanks.  Get two and modify the second to allow a hose attachment:

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many thanks peter I will pass this on to my hubby

regards