Whistling sound when pressing brakes

Hello,

I drove my Fiero today for the first after months being parked, and everything checked out to be fine while driving -- knock on wood -- but naturally I discovered a whole new peculiar issue -- and this time it is regarding the breaks.

Firstly, the brake pedal isn't firm -- I have to press hard on it to stop -- but I don't understand why this might be so on the account of the fact that my brake fluids still remains at max level.

Secondly, whenever I press on my breaks, even as lightly as possible, an audible whistling sound can be heard at my front passenger side -- presumably by the wheel. I googled what could be causing this sound, and apparently the issue may range anywhere from the accumulation of brake dust to the warning tabs on the brakes being reached to even rotor/caliper problems. Last year, I had replaced my two front calipers and breaks, but the breaks were the Nascar brand (the very cheap ones but it hasn't even been 10'000 kms since replacement... ). Do warning tabs on breaks make a whistling sound like a boiling kettle?

Thanks!

I was going to reply that could be the problem as I store my car and try to get over to the storage and get the car moving even if just around the block:)