Brake pedal has been sinking unless you pump it?

2015 HYUNDAI SONATA

109,000 MILES • AUTOMATIC

BHAMDOC1

MEMBER
399 POSTS

Bought it with shimming brakes. Replaced front brakes and rotors, greased all pins, some of them were lightly sticky, nothing crazy. Also replaced rear brakes and rotors. Since then the pedal has been sinking unless you pump it. Brake fluid is full.
I see that the passenger side rotor hardly has any marks compared to the rest. unless am imagining. Just need another set of eyes and what would the first thing that comes to mind in this case? First picture is passenger front where my finger is pointing, rest are for the other wheels

Images (Click to enlarge)

Oct 18, 2026 at 8:08 AM

BHAMDOC1

MEMBER
399 POSTS

I have jacked uo frobts and they both stop fine when brakes applied. Regreased pins again just in case. Jacked up rear and applied brakes and both wont move with brakes applied

Oct 18, 2026 at 10:10 AM

JACOBANDNICKOLAS

✓Experienced Mechanic
110,245 POSTS

If the brake pedal slowly goes to the floor when you apply pressure, chances are it is a faulty master cylinder. Fluid bypasses the plungers allowing the pedal to reach the floor.

If you are certain there are no leaks, that is where I would recommend starting. Here is a link that you may find helpful.

https://2carpros.com/articles/how-to-replace-a-brake-master-cylinder/

Additionally, I attached the directions from the manual. Let me know if it helps or if you have other questions.,

Take care,

Joe

See pics below

Images (Click to enlarge)