'big flush' not working despite tube being connected

[pooksahib]

Hi. I noticed my 'big flush' needed a couple of presses to work. This lasted for a week or so. This morning, nothing - just a pathetic hiss of air. I assumed the tube had come loose, but no. The 'little flush' is fine. Picture attached. Can anyone suggest what the cause/cure is? Thank you so much.

 

[undertrained]

Kink or split in the tube when put back in?

 

[pooksahib]

Not applicable - has been working since day 1. Thanks, though.

 

[Lower]

Diaphragm in the big flush button has failed. Swap the tubes around between the button to check that the big flush works when connected to the other button.

 

[pooksahib]

Thanks, Lower. Can the diaphragm be replaced or is it a matter of getting a whole new plate?

 

[Lower]

As far as i'm aware its a whole new push button assembly.

 

[pooksahib]

Actually, both buttons are fine. I switched the tubes on the cistern end (the red bits in the picture) and now it's the small flush button that doesn't work. Which means it's one of those two things in the cistern (do they have a name?) which is the problem.

I'm guessing it's a 'replace' and not a 'repair' job. I think this is what I need:
https://www.amazon.co.uk/Ideal-Standard-Armitage-Conceala-flush/dp/B00I461ZQ0
I'm not a plumber but I previously replaced the rubber seal on the inlet valve. Is this latest job something a layman could do?

 

[just pumps]

Yes. Be sure to isolate the water supply to the cistern first and allow half a day.

 

[pooksahib]

Thanks, just pumps. Good news, though - I found this on a website and, sure enough, after taking off the red caps, one of the diaphragms is turning to dust. An easy fix:
"If only one of the buttons on the S4399AA dual flush plate has stopped working the most likely problem is that one of the SV70367 pneumatic diaphragm washers fitted into the top of the flush valve has failed. If a pneumatic diaphragm washer has splits or cracks then the valve will not operate due to loss of air pressure.
Replacing the SV70367 diaphragm washer is a simple matter of removing the caps on the top of the valve and inspecting the diaphragms for splits and cracks and then replacing with a new one"

 

[dilalio]

Also worth bearing in mind with this flush mechanism is that more often than not, the air tubes are left at their original length and they should be cut down as short as is practical

 

[pooksahib]

Good idea, thanks. And thanks everyone else. I await an ebay delivery...