Replacing the brake master cylinder isn't too hard of a job. When you get the replacement part, make sure it has a lifetime warranty. That way, you'll never have to pay for it again.
First, here is a link that shows how in general one is replaced. You can use this as a guide.
The new brake master cylinder will need bench bled before install. Here is a video that shows how that is done:
Here are directions specific to your vehicle. The attached pics below correlate with the directions.
2009 Hyundai Sonata L4-2.4L
Vehicle Brakes and Traction Control Hydraulic System Brake Master Cylinder Service and Repair Removal and Replacement Repair Procedures
Do not spill brake fluid on the vehicle; it may damage the paint; if brake fluid does contact the paint, wash it off immediately with water.
1. Remove air cleaner mounting bolts (B) from the air cleaner mounting bracket and air cleaner body (A).
2. Disconnect the brake fluid level switch connector (A), and remove the reservoir cap (B).
3. Remove the brake fluid from the master cylinder reservoir (C) with a syringe.
4. Disconnect the brake lines (A) from the master cylinder. To prevent spills, cover the hose joints with rags or shop towels.
5. Remove the master cylinder mounting nuts (B) and washers.
6. Remove the master cylinder(C) from the brake booster (D). Be careful not to bend or damage the brake lines when removing the master cylinder.
1. Install the master cylinder on the brake booster with 2 nuts.
2. Connect 2 brake tubes and the brake fluid level sensor connector.
3. Fill the brake reservoir with the brake fluid and bleed the brake system.
I hope this helps. Let me know if you have other questions.
Take care and God Bless,
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Friday, March 12th, 2021 AT 10:12 PM