nvu

water pump shaft seal is bad

unplug the switch, see if it goes away. if not, look for the tail/illumination relay, pull that out. if that doesn't turn it off, something aftermarket or is wired different from factory. This is from a 98 impreza, similar setup as legacy

it just sits in there spring loaded. the tension is enough to keep it from backing out. these aren't high torque bolts so that should be good enough

It's water getting past the rubber boots and collecting in the hole. Just pull the plug wires and look at the clamp inside them. Might as well pull the plugs and inspect while you're there. Might be a good idea to blast the holes with compressor air before taking out the spark plugs. I found a picture, see the wavy areas where the plug wires are. Water likes to collect up there and eventually makes it inside the hole.

My ej22 was acting up weeks ago when the rain was constant in socal. It had similar issues of running fine with gas and idling poor. Turned out there was water in the spark tube and it corroded the connector so it didn't clamp on the plug tightly. Worth a check if you haven't done so.

Sender port is the simplest for a temporary install if you want to diagnose the problem. You could remove the alternator belt and swing it out of the way. The car could run for an hour before batteries get low. It's likely just the sender, the engine would already make bad noises if it were actually low pressure while driving. With the engine warmed and real pressure gauge attached, rev it a bit and see if the pressure stays good.

Not sure on 99's, I had a similar issue on an 03 and it did turn out to be the relay. For the 03 it was next to the fuse box on the drivers side, had to remove lower dash to access it. It was a cylindrical relay.

Maybe check the timing tensioner? A failing one would make valve knocking sounds.

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remove the lower dash, should be only Philips screws, unhook some switches attached to the plastic panel. then if needed you can also unhook the fuse box from the frame, it's only some 10mm bolts/nuts. no need to unhook anything, but should give plenty of room

you have the version where the 2 bolts are external to the tank. i don't think it needs any washers. middle one. the one i have the bolts are internal to the tank, directly touching the fluid. the washers go inside between the bolt and the internal tank bottom. right one. rtv wouldn't hurt, just give it a day to set. it really shouldn't need any to seal. thinking that bottom plate is tweaked, check that it's flat and square. If the crush washers helped, maybe the bolts are too long and not original. Here's the link to the pic https://parts.subaru.com/a/Subaru_1996_Impreza-Outback-Wagon-22L-MT-4WD/_54100_6028409/POWER-STEERING-SYSTEM/G10-346-02.html

If it's just at the reservoir I'd rtv the mating surfaces. The copper washers are for the newer style reservoir.

push the piston 80% of the way into the block, tighten the compressor a bit more, tap around the edges so the compressor gets into the chamfer, tap the piston in

are you using the ratcheting sleeve compressor? place the piston face up on a flat surface, place the compressor over it and lightly tension it down making sure everything remains square. pick both up and put it them on the block, tap it so the skirt protrudes into the block and the rings are around the last tension band. lightly tap around the top of the compressor and tension a bit more. make sure it's squared and centered. the piston should slot in with moderate tapping.

Center diff takes the brunt of the mismatch. In a safe place, drive in figure 8's and see if there are any odd noises under the car. Center diff failures are usually a binding/release feeling, then quick knocking sounds, then catastrophic failures that could lock all 4 wheels. It's probably safe if there are no noises or binding.