Repairing Port Water Tank Leak
In early April, 2012 it became apparent that there was a leak in the Port side water tank.  The symptoms  were water under the  cabin sole, the bilge pump coming on hours after the tank  was filled, and most compelling - the water level in the tank dropping when it was not the active tank.
There are three water tanks on Sarah, one under each of the settees and one in the bow under the V-berth.  I only use the bow tank when on a passage and it is the first tank used in  that situation.  Sarah sits bow-down without the weight of 50 gallons of water in the bow. 

Sliding Bunk Board Moved  Fully Inboard

Access Port in Aft Section  of the Tank
There is a sliding bunk board under each Settee mattress.  These boards are moved inward about 6" to widen the settee when used as a berth underway.
When the bunk board is slided fully inboard, as far as the table will allow, the single access port to the tank is exposed.  Since there are no water level gauges on any of these tanks, the access port is prinicipally used to monitor the water level.  If I intend to fill the tanks to nearly the top I expose the access port as shown on the left so I can shut off the water before the tank overflows. 
When I am dockside I normally just use a timer when filling a tank.  With normal dock water pressure it takes over 10 minutes to fill a tank.  I use 8 minutes whenever I fill a tank without monitoring the level throught the access port.  This has normally provided a large margin of error to avoid over-filling the tanks.  The Pearson Owner's Manual warns of not overfilling the tanks because the excessive hydrostatic pressure can burst the fiberglass tanks.
I'm guessing that is what caused the leak in the Port tank.  I believe I got a false indication of that tank being empty (water pump not able to bring the fresh water system up to pressure), and added 8 minutes of water into that tank when it may have been nearly full.
Regardless of the cause, one of the principal water tanks on Sarah will not hold water.  So the first step is to find where the leak is and seal it.  Then maybe add gauges to the tanks so  I  can avoid  overfilling in the future.
Below are pictures I took from the access port.  They do not show a visible crack in the tank walls in the aft section of the tank.  As shown below there are two baffles forward of the access port.  The water under the cabin sole indicates that the leak is likely in the center section of the tank.  I will have to cut an access port to that section of the tank to determine if the leak is there.
From the level of the water it appears the leak is very low in the tank wall, but probably not in the bottom.
Searching For the Leak

Outboard end of 1st Baffle

Baffle, Looking Forward to Next Two Sections

Inboard End of 1st Baffle

Inboard Side of Tank

Inboard Side of Tank, Looking Aft

Aft End of Tank

Aft Outboard Corner of Tank

Outboard Side of Tank
When I pulled up the hatch in the cabin sole, just inboard of the settee, I found water was in the plywood core.
This job just got a lot bigger.  Not only do I have to find and seal a leak in the tank, I may also have to replace a big chunk of the cabin sole.
To make things worse, I discovered water in the hatch inboard of the Starboard settee.  Looks like I have a leak (although much slighter) in that tank as well 
Water in the Core of the Cabin Sole
Plywood Sub-Floor Under the Water Tank
I also took pictures under the cabin sole to see if any water was coming through the cabin sole or down the hull on the outside of the tank.
This area is very dry. 
Since I don't see any tabbing under the sub-floor and no moisture below the sub-floor, it appears the water entry into the cabin sole is on the inboard side of the tank.
The picture on the right from the galley cabin sole.  The water hose from the tank to the manifold for the pressure pump can be clearly seen.  Also it can be seen that the hose is connected to a short pigtail hose from the hose barb on the tank.  The two hose clamps are very corroded.  Looks I'll be replacing the water hose as part of this project.
Water Hose From Tank to the Manifold

Retaining Guide for the Bunk Board Removed
So the first step in finding the leak is to gain access to the other sections of the tank.  To do this I first had to remove the bunk board.  The board will almost slide out, but the mast stops it a few inches short of being free of the retaining guides at each end of the settee.
On the left I have removed the top of the retaining guide at the aft end of the settee and could then pick up the bunk board and remove it.  The only difficulty with removing the guide was to remove the screw plugs with an ice pick.
Things would be a lot easier if I could remove the plywood cover over the tank, but it is tabbed into the hull.  Removing this piece of plywood would require a lot more destruction that I am willing to perform at this point.
So I'll have to cut some large holes in this cover to gain access to the top of the tank, then cut holes into the tank to find and fix the leak.

Tank Cover Tabbed Into the Hull
Inboard Bulkhead on the Settee
I did attempt to remove the bulkhead inboard of the tank by removing the screws from the tank cover into cleat stock on the inside of the bulkhead.  Then I removed the small moulding at the forward end of the settee.  This removed all visible fasteners, but the bulkhead would not move.  It's either tabbed into the forward and aft bulkhead or secured with screws inside the tank compartment.
If I need to get access to the inboard outside of the tank I will have to cut an access hole in this bulkhead. 
In the process of attempting remove this bulkhead I peeled back the plastic coating.  I don't know what this coating is made of, but it looks like it was put over the standard mustard-colored formica covered plywood used throughout the early P424s. 

Hole Cut in Bulkhead Aft of the Tank
Before I started cutting more holes in the tank to find the leak, I decided to gain more access to the outside of the tank.  The first access was gained by cutting a hole in the bulkhead aft of the tank.  There is a large open area between the tank and the bulkhead.  Many other P424 owners have cut a similar access hole to gain additional storage space.
I will cover this hole with a piece of HDPE.
In the picture on the right the hose barb from the tank is visible.  Pearson should have provided this access for no other reason than to get at this hose connection. 
This picture also confirms that the tank rests on the subfloor, which explains how the water has gotten into the cabin sole.  Water on the subfloor can seep directly into the core of the Teak veneer plywood.

Space Between the Tank and the Aft Bulkhead

Space Between the Tank and the Inboard Settee Bulkhead
On the left is picture of the space between the tank and the inboard settee bulkhead.  No leak is apparent here.
With the additional access to the outside of the tank I now have a chance to locate the leak without cutting several holes in the top of the tank.
One Leak is Found
The next day I partially filled the tank and observed water forming under the tank near the hose fitting almost immediately.  Within a few moments water started to ooze out of the cabin sole in the cutouts for the galley and port sette hatches.
Looking under the cabin sole I could see water dripping from the delivery hose.
I  decided to remove the delivery hose before going further, but when I put a nut driver on the hose clamp the hose barb came loose from the tank.

Water Under the Aft Inboard Corner of the Tank

Water in the Galley Hatch Cutout

Water in the Port Settee Hatch Cutout

Water Dripping From the Delivery Hose

Hose Barb Fitting Broken Off the Tank
I have found at least one leak, and it is probably the easiest possible source to fix.  The only problem would be if Pearson decided to bed the fitting to the tank with Expoxy or 3M 5200.  Thankfully not and I could reach the inside piece and remove it as shown on the right.
There  maybe another leak, but I won't be able to determine that until after this leak is fixed.

Inside Piece of Hose Barb Fitting
Fixing Leak and Testing

New Host Barb Fitting Installed
Fortunately West Marine carries the exact same fitting.  In the picture on the left the fitting has been installed on the tank and sealed with clear Silicone. 
I cleaned up the inside and outside of the tank surface around the hole with acetone after letting the area completely dry.
The fitting is only hand tight.  I will let the Silicone set for 12 hours or more before I tighten it and then hook up new hose.
The next day I checked the fitting and it did not require any tightening so I ran the new hose from the manifold under the galley sink to the tank and secured each end with new hose clamps.
Then I  filled the tank to within 2" of the top and watched for any leaks.
As you can see in the pictures below, after ten minutes there was no sign of a leak where there was a lot of water before.

Tank Filled to Within 2" of the Top

No Moisture Around the New Fitting

No Moisture in the Cabin Edge
When I attached the water hose to the tank fitting I noticed that the bend in the hose does exert some downward pressure on the hose barb.  This possibly caused the failure.  Of course it took 35 years to fail, so I'm not too worried about it.  In any case I now have easy access to the fitting.
I left things opened up for the next 24 hours just to be sure, then I put the cabin back together again.
On the right I have secured the new water hose.
Water Hose Secured to Hatch Supports