Q: Brake pedal slowly becomes firm to press after short period of driving

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I had an issue with one of my rear rotors / pads, so I replaced both rear pads and rotors. I plan to do the front ones tomorrow, but i'm having an issue with the pressure building up in the brake system. The brake pedal gets firm / hard to press down after maybe 5-10 minutes of driving. When this happens it feels like the brakes are constantly being applied, and I will have to press the pedal down pretty hard to be able to shift into reverse. If the car sits parked for awhile the pressure slowly goes back to normal. My initial research tells me that this could be due to a bad / damaged flex hose. Not sure if bleeding the lines / air in the system could possibly cause too much pressure as opposed to too little. I will bleed the system when i'm doing the front brakes tomorrow anyways, but just looking for some input with this problem.

My car has 116000 miles.
My car has an automatic transmission.

Hello and thank you for your question.
The problem lies within the proportioning valve in the brake system. The valve could be plugged up with debris in the system making the valve stick. If the proportioning valve is okay and working correctly, then the brake controller plungers has some debris in the system. I suggest getting two large bottles of brake fluid Dot 3 and flushing out the entire system with all new brake fluid. This will help remove any debris in the system and help keep the system clean. If the system is still being pressurized after flushing out the entire brake system, then you may have to replace the proportioning valve or brake controller to keep the system from building up. If you need assistance with diagnosing your brake system, then seek out a professional, preferably a professional from Your Mechanic to help you with your brake system.

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