Brakes

check your vacuum hose to the brake booster also check brake fluid level and for leaks.

The master cylinder probably needs replacing.

The suction noise you hear is from the hydroboost unit. Located on the firewall behind the master cylinder. It is comprised of a large diaphragm with a spring which provides the boost (vacuum) when applying your brakes. The diaphragm internally is ruptured and leaking vacuum which will also make it hard to apply the brakes.

So which part do you think I should purchase?

First disconnect the hose going to the booster on the firewall behind the master cylinder. Hook a vacuum pump up to it and pull a vacuum on it and see if it holds. If it leaks down and it's audible then it's the diaphragm leaking. Then you will know for sure. Easy to change out. Shouldn't be to terribly expensive. Make sure that is what's leaking. But, I'll bet anything that's what it is based on your description of the sound and brake pedal response. I have replaced many of these over the years of working on vehicles. Usually what destroys them is brake fluid leaking from the master cylinder back into the booster. The brake fluid destroys the diaphragm in the booster. Good luck.

Anyone willing to help me isolate the master cylinder so I can figure out which part I have to buy?

Compensation will be given obviously.

You don't need to isolate the master cylinder. Start the vehicle apply light but firm pressure on the brake pedal. If it continues to travel all the way to the floorboard in your vehicle then the cups inside the master cylinder are leaking. To test the vacuum booster connect a vacuum pull down pump to the hose that goes to it. If you hear your sucking/hissing noise then the diaphragm inside the booster is leaking. Then you can buy the correct replacement part you need. Very simple process for you to perform. You can do it.

If the brake pedal goes all the way to the floorboard then the master cylinder is bad. If the hissing is present upon applying a vacuum to the booster unit then it is bad and needs replacing. A master cylinder does not make a hissing noise just FYI. Which leeds me to say it is the vacuum booster that is bad.