Some of the cheaper Weinmann brakes had non adjustable caps holding the "suicide lever" in place. The problem with this is that you cannot tighten them to stop them rattling. The work round is to wrap some fuse wire or similar around under the cap so it takes out the slack and stops the rattling. If this is why you want to tighten them, you should be able to peel the hoods back far enough to tape under them.
The other Weinmann's had a screw to hold the lever in place - which was adjustable but always worked loose.
Most serious cyclists would advocate getting rid of the suicide levers - hence the derogatory term used for them (which was earned by the fact that they were totally inefficient in an emergency situation)
If you like riding on the tops it's worth retro fitting some auxiliary levers (also called top mount or even frog leg) they will cost you approx £20 on top of the cost of new standard brake levers and you will also probably need to budget for replacement brake cables and new bar tape. An easy job and a worthwhile project.