560SEL passenger door lock problem
I have a strange passenger front door lock problem on my 1990 560SEL.
Central locking works fine when I'm the only one in the car, meaning only the driver's door is opened/closed during the trip.
However, when I have a front seat passenger accompanying me, after leaving the car the passenger front door will not lock on the first central locking attempt. It does lock after a second attempt. The behaviour is consistent.
Is this as simple as a failing vacuum element, or is there something deeper going on here?