16v rear door locks

[revbond]

Both rear door locks on my 1987 16v are malfunctioning: they do not lock consistently when I lock the front drivers-side door. Most of the time, I have to push the locking "pins" down manually. Both doors automatically unlock OK when I unlock the drivers-side front door.

Anyone have any suggestions as to the fault? I would guess a part common to both rear doors ( a vacuum something or other?) since they exhibit the same problem at the same time.

Thanks

John