Toilet flushing problems

Any idea why a toilet pan would completely empty after flushing? I mean the toilet works well enough, in that it flushes and there's the expected rush of water from the cistern to carry off the solids, paper and fluids but increasingly there's nothing left in the pan or U bend other than skid marks. No water in the U bend. It's as if everything has been vacuumed through to the external sewage outlet. Could there be some sort of reverse pressure being applied that's the reason for no water being in the U-bend after a flush?

It's not smelling (yet) and doesn't happen every single time but I'm just a bit concerned that I might get a reverse surprise and instead of everything being sucked out from the toilet I might get some sort of blow back if there's a vacuum at work and I'll end up with my bathroom walls coated in a thick layer of bottom sludge...

Calling a plumber in for an intermittent problem is an expense I want to avoid.


Job done......
Two thoughts from me.

You should have a soil stack on the outside of the house. The toilet pipe should flush into it and gravity will take it away into the sewer. The top of the soil stack should be ventilated to allow air to enter so that there isn't any vacuum.

I wonder if the the stack is blocked with something thus not allowing air to enter when the toilet is flushed thus creating a vacuum where all the water is sucked out? Check for a birds nest or something blocking the stack.

If the toilet pipe doesn't enter into a soil stack, you could have a poorly designed system that may need a durgo valve built into it. That's a one way valve allowing air into the system but containing any smell in the sewer pipes.
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