Any help would be greatly appreciated on this one. I'm tearing my hair out.

My little C-class is suffering from poor vacuum. Braking is OK. There is enough assistance there. I can park overnight and when I pump the brake pedal next morning there is still vacuum there. But changing from 1st to second gear is a bit abrupt which I'm led to believe can be due to lack of vacuum. Also the rear headrests have barely enough vacuum to operate. There is nowhere near enough vacuum to operate the recirculate flap on the heater. This is the only vacuum operated part of my heating and ventilation system. It has 2 rotary controls for fan speed and the usual screen, floor etc. I have no air conditioner. No climate control. (the flap isn't seized. I can operate it with my finger.)

There is JUST enough vacuum to operate the headlamp height adjustment. But only just.

Here's what I've checked out :

I've traced all the vacuum lines that I can find and they all seem to be connected, and there are no leaks. I've checked the resevoir tank too. No leaks. I have even disconnected all other lines and connected the rear headrest and recirculate lines individually and the results are the same. Not enough vacuum.

I have checked the pump in the trunk/boot and it only has ONE outlet which operates the central locking ONLY. IE - no vacuum assistance to the rest of the car. (The central locking is working fine by the way)

Everything else seems to be fed directly from the manifold. I checked where the vacuum feed joins the manifold and it seems to be very poor. Hardly there at all actually. Nowhere near as much as I'd have expected. And certainly not enough to operate a vacuum element.

I am baffled.

Has anyone else got a C-class like mine which has poor vacuum and no vacuum assist from the central locking pump ?

I entertained the thought that maybe a previous owner had replaced the central locking pump with the wrong one. IE one with no vacuum assist. But this doesn't seem to be the case. It all looks undisturbed and original and there are no signs of any extra vacuum pipes that would have been connected to the "proper" central locking pump.

Please help !!