Replaced pads and rotors, brake pedal sinking to floor now?!?

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07navi

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Thanks guys! I did go do the break in procedure again and I think it feels a little better (or maybe I just want that to be true...haha) but it's still super easy to push it straight to the floor. Having to hold it for ~30 seconds is more like what I'd expect! I don't think it's a safety issue at this point since the brakes stop fine unless they're super hot, so I'll drive it this week to see if it keeps improving.

Not sure if I already mentioned this, but I thought about putting the old rotors and pads back on the front to test if that was the issue but I already got rid of the pads, unfortunately.

I doubt if that would do anything anyway.
 
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UniqueTII

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The theory would be that the uneven contact patch is causing the pedal to drop as it twists the pad flat, so the old ones wouldn't have that issue. Just a theory!

thumbnail_20200924_221313.jpg

EDIT: This was after the first break-in session.
 

Plati

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The theory would be that the uneven contact patch is causing the pedal to drop as it twists the pad flat, so the old ones wouldn't have that issue. Just a theory!

View attachment 38333

EDIT: This was after the first break-in session.
Is that normal that the pad only rides on 3/4 of the rotor diameter?
On mine the pad is sized same as rotor. Looks odd.
 
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UniqueTII

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Is that normal that the pad only rides on 3/4 of the rotor diameter?
On mine the pad is sized same as rotor. Looks odd.

The pad covers 100% of the rotor diameter, so this is definitely not a good contact patch! I swapped the inner and outer on that side before doing my second break-in and it cleaned it right up, so I'm leaning toward a pad issue and not a caliper/bracket issue.
 

07navi

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You could have the worst pads ever made and they won't cause the pedal to drop to the floor. Cars.com lists the spongy brake pedal on the expys as one of the top drawbacks.
 
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UniqueTII

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You could have the worst pads ever made and they won't cause the pedal to drop to the floor. Cars.com lists the spongy brake pedal on the expys as one of the top drawbacks.

It's clear that my brake pedal behavior (goes right to the floor) doesn't match the expected performance of the vehicle (yours takes firm pressure over time to go to the floor) so there's obviously some sort of problem. There are a limited number of factors that would allow a hydraulic cylinder to move in a closed system, namely movement of other cylinders, bypassing, leaking, air, or hose expansion. It's not my theory, but others have stated that they were getting caliper movement as the non-parallel pad surface was pressed flat onto the rotor which caused the pedal drop. As the pad wore over time the pedal pressure improved. Again, just some experiences of others that I'm interested in testing. Given the volume of fluid needed to move the large caliper pistons, it doesn't take much movement to consume the amount that the master cylinder can displace.

The other factors I've listed have all been addressed to some degree with changing out parts and so many bleeds, but I'm not confident in any part of the braking system at this point. There may even be other factors I'm not thinking of but this is a pretty simple system overall.
 

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It's not my theory, but others have stated that they were getting caliper movement as the non-parallel pad surface was pressed flat onto the rotor which caused the pedal drop. As the pad wore over time the pedal pressure improved. Again, just some experiences of others that I'm interested in testing. Given the volume of fluid needed to move the large caliper pistons, it doesn't take much movement to consume the amount that the master cylinder can displace.

It truly pains me to say this but (cough, cough, choke) @07navi is correct.

You could have the worst pads ever made and they won't cause the pedal to drop to the floor.

If the pad is not close enough to the rotor that is a caliper problem not a pad problem.
 
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UniqueTII

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It truly pains me to say this but (cough, cough, choke) @07navi is correct.



If the pad is not close enough to the rotor that is a caliper problem not a pad problem.

Lol, I welcome any correctness on this ridiculous issue!

Just to clarify, it's not that the pad isn't close enough to the rotor, it's that they're not parallel so the contact patch is uneven (see picture a few posts back). Again, the theory is that the caliper pushes it as flat as possible but that you run out of pedal travel due to the volume differences of the master cylinder piston and caliper pistons. With multiple people saying that their pedal feel improved over time or by repeating the break-in procedure I feel that it's worth entertaining.
 

07navi

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It's clear that my brake pedal behavior (goes right to the floor) doesn't match the expected performance of the vehicle (yours takes firm pressure over time to go to the floor) so there's obviously some sort of problem. There are a limited number of factors that would allow a hydraulic cylinder to move in a closed system, namely movement of other cylinders, bypassing, leaking, air, or hose expansion. It's not my theory, but others have stated that they were getting caliper movement as the non-parallel pad surface was pressed flat onto the rotor which caused the pedal drop. As the pad wore over time the pedal pressure improved. Again, just some experiences of others that I'm interested in testing. Given the volume of fluid needed to move the large caliper pistons, it doesn't take much movement to consume the amount that the master cylinder can displace.

The other factors I've listed have all been addressed to some degree with changing out parts and so many bleeds, but I'm not confident in any part of the braking system at this point. There may even be other factors I'm not thinking of but this is a pretty simple system overall.
Not buying that theory because the caliper pistons move a long way with very little pedal travel.
 
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UniqueTII

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Not buying that theory because the caliper pistons move a long way with very little pedal travel.

That's fair but I don't have a better theory to act on. "These trucks have crappy brakes" doesn't explain a new condition after 130K miles and a hardware change. The condition didn't change after replacing calipers, lines, master cylinder, and HCU.

I just pulled the front wheels again and the pads seem to be sitting okay with no huge gaps on any edge. I'll try to have someone pump the brakes later so I can see if there's any movement as the pedal falls.
 
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