'O'-Rings

'O'-Rings vary in size and shape, material and hardness. As a consequence it is always best to use the Manufacturers replacement seals wherever possible. But they all work in the same way!
(fig.1)

At a natural state the 'O'-ring sits more or less in an undisturbed position - this is the situation on the surface without pressure and would leak every time (fig; 1). Most equipment uses some closing pressure to slightly displace the 'O'-ring (fig; 2), this will stop leakage at surface pressure.
(fig. 2)
 
Failure is generally caused by hair or sand particles on the 'O'-ring seal face, causing a leak path to destroy the camera and photograph's already taken. Other causes of failure are split, torn or fatigued seals which are generally more difficult to spot during casual inspection.

Inspection of seals and seal faces / seats should be regularly undertaken and we recommend carrying this out at each film change. The highest risk time for damage is between dives on board the boat with everybody climbing over your equipment while you are trying to keep it safe. Find a quiet corner and take time to inspect for any signs of damage.

I generally clean each seal and seat face, inspect the seal and face surface, then I re-grease using only enough grease to 'shine' the seal and close up as soon as possible. Some seals do not require greasing and if greased may cause failure, these seals are usually coloured but always check your instruction booklet first.

When re-greasing 'O'-rings only use a small amount of grease, (small pea size for a Nikonos main door seal), the grease only allows the 'O'-ring to move in order to seal under pressure (fig;3). Grease will not stop a bad seal from leaking!
(fig. 3)

Over greasing will only allow more debris to stick to the 'O'-ring surface, increasing the likelihood of flooding.