I got the loaner pressure tester from advance auto the other night, pulled the spark plugs, made sure the coolant was topped off (or at least I thought it was), , poured some UV dye in, cleaned the area where the radiator cap is to be sure to get a good seal, and pumped it up to 15 psi. It started loosing pressure fairly quickly. I'd say within about 15 minutes it lost about 5 psi. It kept going lower and lower, so I released the pressure, pulled the pump off, and it looked like the coolant level in the radiator had gone way down. I checked for leaks in the cylinder, around the radiator, tried to visually inspect the water pump, checked the engine and transmission oil, and all of the hoses. No leak/sign of dye. I figured, the cooling system wasn't burped properly. since I had the spark plugs out and intake/air filter housing out of the engine, I couldn't start it to burp it properly. I just topped it off at the radiator and gently squeezed the hoses and, sure enough, there were air pockets. Topped it off again, burped until I didn't see/hear bubbles, and re-applied the pump and pressurized again. PSI dropped again, but about half as slow as the first time. I took one of those old people audio amplifier things with headphones on, and listened to see if maybe the pump itself was leaking, or maybe there wasn't a good seal at the radiator. Didn't hear any air leaking, but when I had the amplifier near the radiator cap, I could hear quiet bubbles. That made me think that there was still some air trapped. I waited about an hour to see if the psi kept dipping, and it did. I let it get to around 8 psi, and took the pump off again. Low and behold; the coolant level in the radiator had dropped again. I checked for leaks in every possible area again, and there was nothing. Burped it, and more bubbles came out. Topped it off, burped it again, cleaned the cap area, and reapplied pressure. This time, the pressure dropped from about 15 psi, to about 12.5 psi. It was late, so I just left the pump all night (like I had originally planned) and checked it in the afternoon the next day (12 hours later). I was completely expecting the pump to be at 0 psi, but it wasn't! It stayed at 12.5 psi. I checked again for leaks in the cylinders and everywhere else; nothing. I released the pressure, took the pump off, and the coolant in the radiator was a little low again, but not as low as before. Pretty sure it STILL wasn't burped properly prior to pressurizing for the last time. Which might explain why the pressure dropped slightly, and then held.

Didnt intend to write a short novel, but there you go. Don't know if my logic about the air pockets is spot on, but thats what made sense to me.

Assuming my logic is correct, I think whenever the previous owner last had the coolant replaced, they didn't burp it properly, therefore the coolant was fairly low. Who knows how long it has been that way. The last record I saw of it being replaced was the khameleon stuff back in 2015.

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Read this thread. It talks about the dealer compression test and how a common radiator pressure test misses HG failure.

Coolant Reservoir Bubbling and Filling Up. | Tacoma World/

Dealer does a near 100 psi 24 hour test. Testing at combustion pressure. Which is why I said dealer.

That said maybe you found your issue. But 03-06 1GRFE at highest risk for HG failure.

So there was air in the cooling system, and you've mostly removed it. Set the coolant level w/ the radiator all the way full, mark the level on the reservoir tank cold, and then keep an eye on it after a few cycles of driving. It should fall to the same level when the engine is cold.