Melitta CI

My 10 year old Melitta CI (>13000 cups) unfortunately started to spin today.
Instead of in the cup, the coffee runs into the drip tray, just like the water from the cleaning cycle. I.e. at the actual outlet nothing comes out.
With the tips and advice from this forum, I was able to disassemble the machine enough to see that the water/coffee is running away at the bottom of the brewing unit.
The transition to the outlet hose in the upper area of the brewing unit remains dry.
Now my question, is the brewing unit broken here or just the drain valve after all?
How can I narrow this down more so I can order the correct replacement part? (So brewing unit vs. drainage valve. After all, it's about 15€ vs. about 70€.)
Is there anything else that should be replaced prophylactically at the same time, if an older machine is already disassembled?
Thanks for your tips!

Does the water run centrally down from the brewing unit or when you look from the front rather left down the brewing unit.
If it comes centrally, look at how the lip seal on the upper brewing piston looks.
If it comes rather left it can be there drainage valve. Can also be the nozzle that runs into the drainage valve. Possible that the then has a crack.

Hello Schlawi,
thank you for the information.
That helps in any case already times further.
The fluid runs out the bottom left.
I will look at the nozzle and the valve on the weekend again more closely.