Toilet: Slow flush unless i hold onto the handle

[Xad]

I have two toilets in my house, the upstairs ones flush has been weak for a long time. I know it's not the toilet pan itself because that was replaced a few months back and the problem still remained.

If i flush it, then roughly half the tank only empties only so the flush is weak because the water stops gushing out fairly quickly.

If however i push the flush handle but hold it down, then all the water empties from the tank and the flush is really good.

Why is the flush getting cut off so quickly and abruptly before the water gets a chance to empty? What's most likely the cause?

Thanks.

 

[TicklyT]

That's the way one of the older 'half flush' water saving syphons worked - press and release the handle for a half flush, press and hold for a full flush.

For full flush only, a plastic blanking plug was fitted in a hole in the side of the syphon - perhaps that has come loose?

If the syphon doesn't have that hole in the side, it may be excessive wear in the bushing around the rod that lifts the diaphragm. That would allow air into the syphon, breaking the vacuum once the water level drops to the top of the bush. Holding the handle down holds the diaphragm against the underside of the bush, stopping the air leak.
If that is the case, it's time for a new syphon.

 

[Xad]

It is very old, over 20 years old. I can't remember how it was when it was new as i was only a kid but the strange thing is, the downstairs toilet never had this issue and always gave a powerful flush. If it was a hold down flush for a full flush, i would have thought both of the toilets would have the same system?

I had a look around, this wouldn't be the piece would it - or maybe it's from another part as it's was repaired recently due to a overflow issue:

Also took some other pics, but i can't see any kind of ports in the sides of the syphon that could be plugged, so i'm guessing your other assessment that it needs to be replaced is probably correct. Although i can't remember it ever giving a good flush unless it was held down

EDIT: I've been informed that apparently some of the water is supposed to remain like that. Having checked both toilets it seems a similar amount remains at the end. Can't quite figure out why one flush is more powerful than the other