FWIW central locking has a dedicated electric pump under the right rear passenger seat. Vacuum to lock, pressure to unlock. If the locks are sluggish, there's a leaky pod or line in the system. The pump will run for less than 5 seconds with a tight system. It will time out after about 30 seconds if there's a major leak. You might might hear a hiss near the leak while the pump is running. You should find the leak before the trunk and fuel filler get stuck in the locked position.

Sixto
MB-less