Hey everyone, I've been having this issue with my brakes for the last few weeks. When the cars off, the brake pedal feels nice and firm, then once I start driving the pedal with go soft randomly throughout the drive, it won't go all the way to the floor, just soft. I'll have to push the brake pedal down almost twice as far as I would to get some firmness in the pedal and for the car to actually feel like it's braking.
I've tried bleeding the brakes multiple times with no luck, I'm not seeing any signs that the master cylinder is leaking, I've checked all my brake pads and they have plenty of meat left in them, and I've walked around the car after driving to see if any of the wheels are hotter than the rest because I've heard that could be a sign of a failing caliper and they're all about the same temp.
I'm really tempted on taking the car to a mechanic to get a diagnosis but I wanted to check with all of you to see if anyone has any ideas of some things I can try to help narrow down my issue.

 

Likely the master cyl. They can leak internally which will result in the soft pedal.

 

It could also be a sized caliper piston and/or sliders. Piston can be tested by removing the brake caliper from the bracket and having a helper step slowly on the brake pedal. I've seen this more commonly on toyota vehicles.

 

Alright, so when the helper steps on the pedal it should extend out, and if it gets stuck then that most likely the one that has been seized? I'll have to check them all later this week once I get back in town. But if it was a seized caliper, wouldn't I feel excess heat coming from that wheel after driving?

 

Yes if you have a stuck caliper it would likely create excess heat if it is stuck on. However if it is seized in the not applied position then doubtful.
With the car off pump the brakes a few times and get them firm and hold pressure. If the pedal slowly goes to the floor then your master is probably worn out.
If you are unsure then it would be wise to have a professional look at it.

 

I've personally seen it both ways, seized on and off. Even when seized in the off position, it would give a similar symptom as a bad master cylinder or air in the system. The first pump of the brake pedal is trying to compensate for the bad piston that doesn't want to move.

 

This might sound silly but you cant compare the brake feel whilst the car is off vs when the car is on. What you are actually describing is normal.

Car Off = Rock solid pedal with very little movement - no more than an inch (will take a couple of pumps of the pedal to get this)
Car On (with a well bled system) = About an inch of travel free play then an additional inch of travel of actual brake application, maybe more.

A poorly bled system will increase the travel of the pedal when the car is on but not off. There is no way in the world you would have the leg muscles to apply same amount of force compared to the system when running.

 

Does it fade more, when brake are hot (have been ridden hard, for a long time)? Could mean areas within master cylinder is bad & can't hold pressure.