Drain valve of carver water heater leaking

Hi Everyone, 
It's been a long time since we have been able to make use of our dear old Harmony for anything more than transport, so no surprise to find a few problems now we can use her as our home from home again.
Most have been sorted, by a bit of common sense and patience, but this is beyond my limited brainpower.
We didn't know we'd lost the Carver's drain plug O ring last Autumn til we tried to refill the system last week! I sent for and received a replacement for both plug and ring, checking model and age for the right one.
Only trouble is that it still leaks, even when tightened up. Have tried with old plug and new O ring but it still leaks.
Are there different types of O rings ( supplier says I have the right one ) or is something else wrong?
We would be really glad of your wise advice, please, if possible.
Thank you and good wishes,
Gran

Gran so sorry you are having problems , I cannot help re leak, but someone will soon, hope you get it sorted as soon as, then you go a very best wishes to you.
Regards,
Margaret

There are two different sizes of plug but one is pre-1987 and the other later. You can also get an improved quality of seal - both plugs and seals can be seen here...

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are you sure it's leaking from the drain plug and not running down from the pressure relief valve which is in the top left corner above the drain plug. If that's leaking, then you have to replace the valve. It's common fault and not that easy to get the old one out.

Or you might have done what I have in the past - left it in the open position
It looks like the drips are coming from the drain plug, but as Paul says, it runs down the front of the heater and drips off the bottom where the drain plug is.

Frank

Hello again,
What a kind lot you are! Two helpful answers in no time, and Daisy Mae's encouraging good wishes.Thank you.
I've been in and out and around our van on and off all day.Emptied the boiler twice via the leaky drain plug to make sure I'd got the O ring in properly. Checked the water really was coming out round the drain plug, but then had to do my shopping. Noticed a new plumber's shop had opened, so went in on the off chance, luckily having got the old drain plug with me. In a moment another O ring was produced and for no charge. Got home, Took out leaking plug, got another good soak, replaced it plus newest O ring, and, I still can't believe it, but so far so dry!
Not all O rings are created equal, it seems. Gran

This thread confirms how brilliant this forum is!

I have a similar problem to Gran which I also thought was a problem with the drain plug so thanks for the suggestion that it could be the pressure relief valve. Will now do further research!

petern