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First off, the car: Toyota Matrix 2010, Base model, 1.8L engine, FWD, 5SP manual (Canada).
I have a sinking break pedal. When I pump the breaks with the car off, the pedal is hard and does not sink. If I hold it that way and turn the car on, the pedal sinks (rather fast, and quite far). I have some breaking power but nowhere near what I would expect.
I changed the front pads and rotors yesterday, and also checked the rear (disk) brakes (one year old, decent condition). None of the callipers are seized, and all the slide pins are free. There are no fluid leaks.
I’m about to try bleeding the breaks to eliminate air in the system from my list of possible causes, but I how could I isolate issues with the master cylinder? What would the symptoms be, and what would cause them? What else could be at fault?
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Take a step back before you work PAST the problem.
Did you have an issue before the pad slap? Did you open the brake system anywhere that air could have gotten in?
Did you pump the pedal 7-8 times to make sure the caliper pistons were out as far as they needed to be?
Bleed the brakes of air. Keep an eye on the fluid level at the master cylinder during the bleeding process. Do not let it run dry. Keep us posted.
To answer some of the questions, yes, I had the same problem before replacing the pads/rotors (except that there was some grinding from the excessively worn pads).
I did pump the pedal after changing the breaks, and was able to get good pressure on the pedal with the car turned off. The fluid is at the correct level, has never run dry, and I’ve never opened up the system.
I will go ahead with the bleed and get back to you guys…
If you didn’t open the system/ bleed the brakes when doing the pads/ rotors, I would indeed take a hard look at master cylinder and brake booster. There are classic master cylinder symptoms if you do not have air in the system/ fluid loss, etc., Here is a helpful article from Eric, http://www.ericthecarguy.com/faq/solving-brake-problems?start=7 Good luck with it…
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