Evap trouble codes P0441 and 0455.

All the Toyota's I've had with this type system like my 2001 sienna the problems never been up under the hood it's always back in the back near that evap menagerie of crap in that rectangular formation that it's in there's one or two evap switches back there that get corroded and disgusting and stop working there's even YouTube videos of guys and third world countries breaking those switches apart cleaning them showing you the damage putting them back together and they work I believe in my sienna van that's what would put the light on that switch back there at the purge bank or container whatever that's called right above the tank in front of behind it whatever you want to call it I believe in the Prius it'll probably work out to be very similar If you go back there and look you'll see one or two of those electric valves but 12 volts to them see which one doesn't click break it open if you're so inclined and see the rust and nastiness clean it up and it'll work or the switches are like $13 online several manufacturers make them And my sienna when the check engine light came on I went back and replaced that valve or cleaned it and fixed it and the light went off.

 

Evaporative Emission Control (EVAP) System - P0441 Diagnostic Code - Trouble Code Diagnosis Guide | Toyota Parts Center

Causes and Fixes for Toyota P0441 Code - Toyota Car Mechanic

Toyota Prius P0455: EVAP System → Leak Detected (Large) | Drivetrain Resource

 

I can't tell you if there should be pressure or not. Seems you have vacuum only one way.
So vacuum from the engine pulls the vapors into the engine. And not allow it to flow back
into the tank. The cap is vented to allow air in, but not allow the vapors out....

Doing diagnoises can be a royal pain sometime, having to trace lines out. I would think
something would be ovious and you could easily see it. Check the engine compartment
for any hoses that might have popped off, or are cracked...

I always like to check the easy stuff first.... And older cars seem to have cracked plastic
where they connect to the fittings...

 

Those switching valves or evaporative switching valves in the back near the canister are always suspect because they're in the rain and the weather The one under the hood is protected generally speaking unless somebody hits it with a hammer pulls it and dislodges it or whatever I know in my other Toyotas that have this similar setup rarely is the underhood part the problem more or less It's the solenoid valve in that rectangular charcoal canister evap system business right up above the tank where water and salt and stuff can smack in to the solenoid valve which is you can see from looking under the car it's usually reddish brown brick color with gold or silver metal trim It's just a little thing with two or more ports on it and hoses hooking to it and like ASR said you shouldn't be able to blow stuff back to the tank that should be being sucked towards the engine and not allowed to go back generally speaking a couple of those lines may allow you to blow through them but the important one shouldn't allow those fuel vapors to go back towards the canister they should only be able to pull forward towards the engine.