Lost Stopping Power

[particlewave]

Thanks guys.
The lines all look good (checked them at flush) and no fluid loss.

I have an IR temp gun and planned to check the rotors, but I’ve never heard of a caliper getting stuck open. The hydraulic pressure seems like it would be too great.

Could it be something to do with the master cylinder or pressure valve?
I should also add that this cropped up over night. It was not gradual.

I really have to press very hard to slow down from 35mph. It feels like someone greased my rotors.

[piper6909]

Originally Posted by particlewave

Thanks guys.
I’ve never heard of a caliper getting stuck open. The hydraulic pressure seems like it would be too great.

Yes, they can get stuck open. Depends how long you had it sitting. Keep in mind that the fluid pressure get distributed to all the other pistons, so you really won't build in so much pressure to get the one piston unstuck. Unless you're really standing on it and locking up the other wheels.

Since you've already checked the booster and your pedal isn't hard, your next suspect is one of the calipers.

[blue62]

Originally Posted by particlewave

Thanks guys.
The lines all look good (checked them at flush) and no fluid loss.

I have an IR temp gun and planned to check the rotors, but I’ve never heard of a caliper getting stuck open. The hydraulic pressure seems like it would be too great.

Could it be something to do with the master cylinder or pressure valve?
I should also add that this cropped up over night. It was not gradual.

I really have to press very hard to slow down from 35mph. It feels like someone greased my rotors.

the rubber seals that are on the plunger in the master cylinder could have an issue and no longer making a proper seal with the master cylinder barrel thus allowing fluid to bypass or the barrel could be pitted causing the same result.

If this was the case your pedal would eventually go to the floor under pressure if you held it long enough.
you could try to test that theory by starting the car apply the brakes hard and hold them for a good period to see if your pedal goes towards the floor.

The only other things I can think of without looking at the car are.
Some sort of obstruction in the master cylinder. (not very likely) but possible.
If they are floating calipers a caliper could be moving farther the its design feature.
But you have done a visual right? so any problem there should be evident.

Or back to - one or more of the soft lines swelling under brake pressure.
This is not something you can usually see on visual inspection.
Usually take a hands on feel test as someone applies the brakes while you feel each hose in turn for swelling. Also it is not very common. But does happen.