Bourton toilet flush priming

How do other people prime their toilet flush?  The AS manual says not to travel with more than two litres.  When we arrive on site, the flush will not prime/work with just those two litres.  Even if we put another litre in, it will not flush.  We have to top it right up to 7 litres, then it will work.  This has always been like it.  When we leave the site, we have to pour away all the surplus water.  We don't bother putting toilet rinse in it any more as we have to pour it away.  What a waste and what a bonkers system.  Does anyone else have this problem or is it us?  Are we doing something wrong?  We have owned three motorhomes in 40 years so I thought we knew most stuff about them but this flush system gets to us.  Mr and Mrs Jimmee  P.S.  Just seen a similar question elsewhere on this forum.  It seems the answer is to completely fill the tank as we have been doing in order to prime the system?  What a waste.

We usually travel with ours full and am not aware of any problems. I wasn't even aware that there was a recommendation not to do this.

Thank you for your interesting reply Peter.  We are only going by what the AS manual says, only travel with two litres.  We assumed that by filling it fully the system would overflow or cause stability problems or something.   Maybe we are being too cautious.  As you have not encountered any problems, perhaps we should try it and post the outcome on the forum.

Now that I know that, with the Thetford flush tank model which I have it's capable of overflowing internally if you travel with more than the recommended maximum of 2 litres, I won't risk further damp damage. It took 5 years before the damp became noticeable, and a further 2 years before I found the cause. 

I'd be carefull, Jimmee!

Cymro

Just had a look at the manual and found the reference to 2 litres. Live and learn!

Thank you Cymro and Peter.  Yes we will stick with the advice from AS.  It’s there for a reason.  As you say Peter, learn something every day and if it stops the dreaded damp then that’s got to be good.