heavy rain + lights

Welcome to PriusChat!!

Are there any OBD2 codes (DTCs) that you can share with us?

 

The timing is likely coincidental with respect to the getting of gas. The problem existed before, what the mechanic did wasn't a fix, and it may involve something intermittent getting jostled when you went through the big puddles.

Ordinarily when you read the trouble codes from the brake system (which is what you should do here), there are a couple hundred possibilities for what codes you'll get. Here we can narrow it down because you also had the high-pitched alarm. That doesn't come on for all trouble codes, but chiefly the ones involving loss of assist pressure. You also directly noticed the loss of assist pressure ("my brakes are also very stiff"). So the computer isn't crazy, it's telling you real stuff.

If you do not have a code reader, you can get basic two-digit trouble codes out of the brake system by using a short jumper wire and counting blinks on the dash lights. There are threads here that cover that.

If you do have access to a code reader, use that; it's faster, easier, and will give you more information.

Either way, if you post the codes here, you can get more advice.

Keep closely in mind that you can have a code showing your brake actuator isn't maintaining pressure and still not mean that the actuator itself is the problem or that you should buy a new one to fix it. There can be other things causing the problem, such as wiring, connectors, or relays. In fact, the intermittent nature of this problem suggests those kinds of other things. Those can be a lot simpler and cheaper to fix, so you should make sure proper troubleshooting is done to find where the problem really is, before anybody talks you into expensive parts replacement.

An old post here (though it is specific to Gen 1, not your Gen 2) will give you an idea of how closely the car's computers monitor the brake components, and how much useful information they can give you if you ask.

 

Massive puddles or small lakes?
Are all of your flaps and panels intact?

As mentioned above, you need a code reader.
These days there is really NO excuse not to have one since they are VERY inexpensive and you can use the interwebs for access to knowledge and advice.