Will modern syphon be ok to replace one in old cistern?

[griff22]

Hi, An old cistern (at least 10 years) has been moved and fitted without the overflow being connected! Not by me of course!
I am looking for the best way of resolving this. Fitting a 'normal' outlet pipe through the wall is not possible.
As I understand it, the modern syphon (with internal overflow) is not designed to cope with the larger quantity of water (7.5 or 9 litres I am told) in an old cistern. The result being that the equally old toilet bowl may not be successfully flushed. Is this correct?
If there is a risk that this is so, would a "flapper" valve, such as the "Fluidmaster" flapper valve, be a solution?
If neither of these I guess I will have to revert to plan A which was to connect the overflow to the foul pipe via a one-way valve.
All comments, advice etc will be gratefully received!

 

[kevindgas]

you can get a syphon with an adjustable flush level 6,7 & 9 litre & internal overflow from plumb center

 

[seco services]

As I understand it, the modern syphon (with internal overflow) is not designed to cope with the larger quantity of water (7.5 or 9 litres I am told)

what do you mean not designed to cope with 9 litres.
this is a modern syphon that flushes 9 litres and has an internal overflow.

 

[kevindgas]

thats the one!!

 

[griff22]

Thanks to both of the above.
I was advised that because modern syphons are designed to flush less water (6 litres?), they will not flush the old design of toilet bowl successfully.
If there is absolutely no danger of this then I will use a modern syphon.
Any views on the flapper - it looks good - quiet and neat too. Would it also be okay in my old toilet combination?

 

[seco services]

these will flush 6, 7, or 9 litres depending on how you set the bungs.
if a new installed toilet then you are surposed to set on the 6 litre flush
if fitting in an old installed toilet then you can set to 9 litre flush.

just another silly regulation.
i always leave them on 9 litre flush.

flappers are ok but not my first choice.