There are several blog articles already dealing with seals and the know leaks in the section joins (which I will link below), but the focus here is how to check what might be leaking. So the staring assumption is that your Lewmar Old Standard opening port(s) is/are leaking.
Closing seal leaks
Generally speaking, though the seals do compress with age (they are a hybrid of solid and skinned sponge EPDM) they should still operate if smooth. To help maintain this it's important to periodically clean then with a wet (plus washing up liquid) cloth to remove the inevitable airborne debris. If you find the seal is starting to crumble to the touch then times up and you need these replaced.
So what if they are still smooth, but a little squashed? Time for the paper test...
Paper test
This works well on any opening units, including hatches.
Take a piece of standard printing/copier paper (A4 in Europe, Letter across the pond) and cut into strips approx 25mm (1") wide. With a dry seal, place these as best you can around the seal and close the port/acrylic/hatch. Then try to pull each out, one at a time. There should be some resistance. If you have to tug, then the seal is probably fine, but if they just slide out with virtually no effort then 'Houston, we have a seal problem!'.
What to do with 'gappy' but smooth seals?
If the gap is on the lower part of the seal you could remove the inner handles (pop the central plug to get to the single fastening screw) and then replace with a washer against the acrylic to bring the handle away from the acrylic. This will push the acrylic towards the lower seal. Check by repeating the paper test.
If the gap is on the higher part of the seal you could do similar, but rather than a washer I would suggest cutting a spacer to match the footprint of the hinge mount (as just a washer could stress the acrylic in the area of the relatively narrow fastener). Then repeat the test.
Build up any spacer(s)/washer(s) in small increments as too much and you'll find it hard to close.
If after this you still have leaks, wait until dry and then try and put some paper towel just under the acrylic (but above the catch blocks (we'll come to that below). Then do the hose test again. If the towel says the leaks are at the level of the seal then new seals are probably on the cards. If not, then let's move swiftly to my favorite cause below....
Section leaks
Probably the most likely source. The rubber insert between the outer frame sections (3 and 9 o'clock) hardens and shrinks with age and water enters here. It then travels around an inner channel and ends up existing from a hole behind the nearest catch block. You can actually see this hole as it's between the two machine screws holding the catch block in position. It's clearly located below the main closing seal, so if you've checked the closing seal and are still getting leaks this is probably the culprit.
If you do suspect this as a source, you can always put some duct tape over the joins and do a hose test. If this solves the flow then you need to address the section joins. Without going into too much detail here, this involves cutting out the rubber seal, cleaning the section ends (thoroughly, plus a final wipe with Acetone) and then applying silicone which hopefully (if clean) will adhere and provide a long term solution.
Why do seals fail?
This isn't rocket science and will probably explain why seals on one side or area can be worse than in other areas. Sun (UV/heat) and airborne debris are the main causes
Sun man!
Not a problem in parts of the world where the sun seldom comes out to play (as an Englishman I feel duty bound to include the UK in this list particularly during the depths of a cold, dark, wet and thoroughly unpleasant UK winter! - there I feel better for getting that off my chest - almost certain to get a hot summer now). Where was I? Oh, yes UV causes virtually any rubber, EPDM, neoprene, butyl etc to shrink and harden when exposed to UV (in the same way as acrylic crazes). For boats not on a swing mooring this can explain why the windows on one side suffer more. Other than installing outer covers there's not much you can do in this regard, but this is something to consider if leaving the boat for some time unused.
Airborne Debris
There's load blowing around but clearly some areas will be worse, particularly where sand particles are involved. These may collect on top of the seal and when opened fall between the seal and acrylic. Rather like sand papering your seal.... Left uncleaned this will abrade the seal to the point where it crumbles (helped by the sun). Again, covers will help reduce the build up, but regular (you need to judge how often based on your location) cleaning with a wet cloth wish some washing up liquid will inevitably help, particularly if they have been left close for an extended period (so the build up will be more).
Fitting new seals
I'll link to some other pages below but this is not a straight forward procedure and can only be done well with the portlight out and on a bench, not in situ. See below for more information.
Blog links
I've penned a few posts and inevitably there will be some overlap (as I'm old and can't remember what I've written before), but read these and you should have a full(ish) picture...
Lewmar Old Standard Seal Fitting etc (guidance on fitting closing seals)
Maintenance parts for Lewmar Old Std Ports (handle and catch block spares)
Lewmar Old Standard Port Options (general replacement options and discussion of New Standard replacements)
Lewmar Old Standard Openers - known leaks (section leak info)
Lewmar Old Standard Port Closing Seal (sizing and pricing)
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