At the moment it's necessary to 'pump' the buttons a couple of times, which I'm sure shouldn't be necessary.
When mine did that it was because the bellows were cracked. Replaced it and all good for the last few years.
I suspected that but they both seem intact. Hard to tell though given that they're only sealed against the red plastic part when actually installed.
Did you find replacements without having to buy that whole kit I linked to?
In my case the bit you push had failed. I replaced only that end.
@Brun started
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When mine did that it was because the bellows were cracked. Replaced it and all good for the last few years.