VW T4 Multivan 1997 - Heater not working

Hello,

I have a 1997 T4 Multivan ACV manual transmission without air conditioning, with dual heating, and a D3ws auxiliary heater that is currently not working and is in the workshop.
The heating wasn't getting warm anymore. According to the customer, it stopped working suddenly. I replaced the thermostat for a diagnosis, and it worked perfectly afterwards. However, 4 weeks later, it's only lukewarm again inside. Thermostat checked, removed, everything is perfect.
"Removed the mechanical heating valve, and it appears to be in good condition. Blew out the heating radiator, and there's no sludge or debris inside. The engine temperature reaches the third mark after 7 km and just below halfway on the gauge after 10 km. Both coolant hoses leading to the radiator are hot." The hoses were all rather lukewarm at the top.
Can the radiator be leak-proof? There was also green antifreeze in it before I started working on it.

Was the heat exchanger on the T4 manually bled?

Is it located at the top of the bleed screw on the radiator inlet hose in the engine compartment, near the splash guard? Yes, I vented!

Both heaters are only lukewarm at level 2, despite the hot hose leading to the radiator?
Does water flow from the thin hose to the expansion tank? If not, it's probably a loose WP (water pump) impeller.

I've also been thinking about getting a WP wheel. It's only one year old, and the engine isn't overheating.

Okay, today I warmed it up, performed a CO2 leak test which showed no detectable defects, and pressure-tested the cooling system. The heating system was working perfectly. Throughout the day, I repeatedly took it for test drives with the heating running at full power. In the evening, the heating stopped working again.

After pressing the accelerator, I experienced a white mist/fog extending up to 100 km/h with a warm engine.
behind me.

"Head poetry?!"

Okay, so, today I found the error. After I removed the valves for the WSH, I had an idea. Located at the top of the cylinder head, next to the oil dipstick, a pinhole-sized opening in the coolant vent hose connector had been blocked.

So, the engine was never able to bleed air.

However, water was coming out of the surge tank, which was quite disconcerting. He has another connection where water flows through.