It could be a non return valve to stop the water draining away into the main tank.
If you can remove it and try it without you may find that its restricted and may possibly clear by flyshing on a mains pressure tap.
Hot water helps to soften the pipe for removal/fitting
Usually there is too much pressure causing water to shoot over the bowl edge.
You may have poor connections to the pump/fuse (corrosion etc)
------------- John
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