1998 W210 E200 Central Locking
 

Need help to troubleshoot problem with the central locking. When i unlock the car, the boot/trunk could not open immediately as it suppose to be. However after about 1 minute, it can open as normal. What is the possible cause, is it te vacuum pump? Please help.

I would say you are right, either the PSE pump or the vacuum solenoid/element at the trunk latch? The PSE will "give up" after a time period, I woud not think it would keep trying for a minute though. I would lock the car,then unlock, and listen in the trunk area (even from the outside) to listen for the element to unlatch. Alternatively you could remove the rear seat cushion, then unlock the car, and listen to the pump to find if it is running longer than it should.
Gilly

Thank you Gilly for the advice.

Did you ever resolve this issue? Mine did the same and yesterday the vacuum pump stopped working all together. The pump PCB has some water damage but has worked with this water damage for more than a year. Pressing the RC button does not start the vacuum pump at all. Is there a way of testing the unit before replacing seeing as it is incredibly expensive? There is 4 pin connector on the vacuum pump unit that is not connected to anything, does anybody know what this is?