8D House Battery Dropping Load Quickly

Experiencing a bit of weirdness regarding my house battery's.

I have (4) NAPA Commercial lead/acid 8d battery's to service the house systems. They are about 2.5 years old.

Batt 1&2, which share a common "Off,1,All,2" switch seem to drop the load very quickly, especially when handling high demand such as windless and davit operation.
Batt 3&4, which share a separate switch perform well.

I ran some tests after bringing them to a full charge and allowing the surface charge dissipate. (With batt switch off) Load tester indicated the batterys are within acceptable range and Multi meter indicated about 12.6 volts before and after load test.

Puzzled, I switched on Battery 1, and turned on some DC lights, the water pump and head vacuum pump. Within less than a minute the DC Engine room lights began to dim with each stroke of the vacuum pump.

Thinking the problem may be a bad ground or parasitic load somewhere upstream of the battery, I used a jumper cable from batt 4 to batt 1. Lights and vac pump immediately perked up. (So it's not due to an upstream parasitic load)

So, Battery's 1 & 2 have little grunt despite testing OK and only being 2.5 years old.

What am I missing?

ps: I have no problem replacing the battery's if need be, just trying to figure out what's going on.

Edit: I know someone will ask this. Yes, the electrolite is at proper level, no there is no visible damage to battery case and yes, I shined up the contacts and tightened them good before the tests.

Either test for specific gravity or get an electronic pulse tester. Insure one cell somewhere is screwed up.

Lead acid batteries can get "sulphated up" as there is hard sulphate and soft sulphate (good type) that builds up on the plates. When that happens, the battery will return a "full" voltage quickly without taking a charge. It may read 13 volts but when a current draw is used, it runs down very quickly. It can be fixed by equalizing (heavily charging) that one battery at a higher voltage. For a large 8V battery, you can disconnect it from the bank and charge it with a 12V manual charger (not an automatic charger). Some inverters have a special "equalize" setting so you can equalize the whole bank.