My breaks make a sloshy sound when I step on it. It works alright but the sound is unnerving.
Any time someone has a ( brake ) problem they should see a brake specialist . Vehicle not starting is an inconvenience - not stopping is a catastrophe .
The sloshy sound could be the power brake booster making noises it didn’t make before. Likely won’t get better, only worse and that may mean you lose your power assist. It would make it much harder to stop.
Agree with @VOLVO_V70 , not something to put off.
Are you sure the noise is from your brakes? If you have air in your cooling system, you could be hearing coolant sloshing around in the heater core behind the dashboard when you stop quickly.
Another possible source of that sound is water in the heater/AC housing because of a clogged drain hose. The sound would be coming from inside the cabin, under the dashboard, approximately in the center - similar to if there’s air in the cooling system, as lion9car suggested.
Thank you, I will check it out!