Water spurts out top of cistern when flushing

28 Jul 2018
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United Kingdom
I've just installed a Grohe concealed cistern. All seems good, however, when I flush the toilet a bit of water spurts out the top of what I believe is the flush valve mechanism. I've attach a photo of the part in question:


This only occurs when I press the flush button, if I reach in and manually lift up the valve with my hand this doesn't happen.

Any ideas why this is happening? Is there something I need to adjust?
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Also, bonus point if anyone can tell me what this part is for:


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1. Some valves do just spurt when flushed. Provided it doesn't come out of the cistern, not a problem.
2. The parts illustrated secure the flush pipe to the cistern.
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Thanks for the response.

1. The water shoots about 1-2 feet in the air. I can put the plastic blanking plate back over the hole on the top, but a few drops still manage to get out through as its not a water tight cover.

2. The flush pipe is already secured to the cistern, and there is no thread where the flush pipe meets the cistern to screw this part onto.

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