Half flushes are like the Goldilocks of toilets—never quite right. I’ve had a few where the half flush was basically just a polite suggestion to the bowl, not an actual flush. Have you checked if the cable or rod between the button and valve is loose or stretched? Sometimes it’s not even the seal, just that the mechanism isn’t lifting enough. I once spent an hour cleaning gunk off a seal only to realize the button was barely moving anything... felt pretty silly after that. If you’re already swapping parts and still getting nowhere, might be time to call it and upgrade. Sometimes those old designs just don’t want to play nice anymore.
You nailed it about the half flush being more of a suggestion than an action sometimes. I’ve run into this with a few older dual-flush setups in my rentals. One thing I’d add—besides checking the cable or rod—is to look at the float adjustment if your model has one. If the float’s set too low, you’re just not getting enough water for the half flush to do its job.
Also, sometimes the internal flapper or valve can get warped over time, especially if folks use those drop-in tank cleaners (not a fan, personally). Even if it looks fine, it might not be sealing or lifting right anymore. I’ve had tenants swear up and down they “barely touch” the buttons, but you open it up and see all sorts of wear.
If you’ve already swapped parts and fiddled with adjustments and it’s still weak, yeah, sometimes it’s just time to bite the bullet and replace the whole mechanism—or even the toilet itself if it’s a really stubborn old design. Some of those early dual-flush models just never worked that well, no matter what you did.