Brake noise and hissing from radiator cap?

I drove a bit longer than I usually do yesterday afternoon and I had the ac on the entire time. I've never had this problem before but the brake pedal started making a noise, somewhat like a grinding noise.
Checked the brake reservoir and it looked over filled. I had the brakes changed about a month ago so I'm not sure if that may be the reason for it being overfilled.
Not only that but I heard the radiator cap hissing so I let it cool down for a while and drove it again during night and haven't heard any hissing or brake pedal noises. Just wondering if anyone has had this problem before, I'm afraid the brake master cylinder is beginning to fail or the radiator cap is breaking down. Thanks in advance.

 

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What do you mean by 'overfilled' brake reservoir? It's not uncommon to have brake fluid very close to the top at the lid..... Being overfilled won't harm anything.

Have you checked your radiator overflow tank? Is it 'at the full line'??

Look around the radiator cap and/or around the area - look for white residue.
If you see white residue...you might have a leak. I would wash the white residue off and keep an eye on things. Make sure your reservoir tank is up to the FULL mark. And....Go out and buy yourself a new radiator cap. Radiator caps are CHEAP and easy to replace.

I had two OEM radiator cap fail/leak on my 07 while I was under warranty. The 3rd OEM cap failed/leaked after my warranty and I ended up buying an aftermarket cap.

 

Yeah I'm also thinking the cap may be failing, sounds like air is leaking underneath it. And also the brake fluid is above the full line. I think it may be because of the brake pad change, piston sits closer in the caliper that might've pushed fluid back up the reservoir. Thanks for the advice, I'll give it a good look around.