Air trapped in calipers?

[R18Civic]

I witnessed something very odd today...

When replacing my front pads and rotors today I noticed a few things. First despite the driver front sounding like it was dragging or hitting high spots on the rotor, the passenger side pads had a little over the legal limit of material left while the driver side had double that.

Despite that the pistons went all the way back in. However the boots looked puffy or swollen on both sides. Half the boot was folded in between the caliper and piston, the other half was puffed out and looked worn. I tried to gently push it back into place with a flat head, but it just popped back put like there was a cushion of air behind it.

Any idea what would cause this? I have two repair kits on the way with new boots and retaining rings for the inner groves.

 

[Ducked]

Swollen dust boots sounds like attack by mineral oil or organic solvent.

Air in the system would be expected to make the pedal spongy, but wouldn't of itself cause uneven wear in an intact system, since its all supposed to be at the same pressure.

If the spongy side is blocked/clogged, perhaps by a degraded brake hose, so its partly or wholly isolated from the pedal pressure, and it has some air and/or water in it, I suppose that might produce these symptoms.

Though compressed air is the commonest recommendation on't internyet, the easiest way to take the calipers apart is to blow the piston out with pedal pressure. If it won't blow, you'll know.

Or at least that'd tend to support this (speculative) diagnosis.

 

[Ducked]

I suppose the return could also be partly ndue to some residual elasticity in the trapped bit of dust boot acting like a weak spring.

 

[Lubener]

I hope you had the cover off of the master cylinder when you pushed the piston back in to allow the excess fluid to escape. If the pads were that thin, brake fluid was added to make up for the worn pads over time. If that's not the case, I agree with a brake hose issue. I never had a caliper spring back out after being compressed.

 

[R18Civic]

The piston didn't spring back out, the swollen portion of the dust boot did. And to be honest the brake fluid cap slipped my mind but in the process of compressing the piston the excess fluid came out of the rear of the cap.

As far as spongy brake pedal, it's slightly spongy. I power bled the brakes in 2014, Got to the last caliper and forgot to top off the reservoir. Ended up bleeding the MC in the car with the cheap parts store kit, Got most of it out but not all.

I took it out last night and bedded the pads in. Everything feels solid but I'm still going to replace the boots.

 

[Ducked]

Originally Posted By: Ducked
Swollen dust boots sounds like attack by mineral oil or organic solvent.

Air in the system would be expected to make the pedal spongy, but wouldn't of itself cause uneven wear in an intact system, since its all supposed to be at the same pressure.

If the spongy side is blocked/clogged, perhaps by a degraded brake hose, so its partly or wholly isolated from the pedal pressure, and it has some air and/or water in it, I suppose that might produce these symptoms.

Though compressed air is the commonest recommendation on't internyet, the easiest way to take the calipers apart is to blow the piston out with pedal pressure. If it won't blow, you'll know.

Or at least that'd tend to support this (speculative) diagnosis.


Could also be that an obstruction (corrosion ring/lip and/or a bit of jammed boot) is allowing the piston to go in but isn't allowing it to come out fully. That'd explain the uneven wear, and is probably a more likely explanation than the caliper being isolated, as I suggest above.

Since you're replacing the boots anyway, you could cut them off and clean up behind them with a pick/scraper, to make it easier to blow the pistons out. I was re-using my boots so I just dribbled brake fluid in there as a lubricant. If I was doing it again, I might use vegetable oil in a hypodermic syringe.

 

[R18Civic]

Welp I got new square cut seals and dust boots from both sides. However I'm a bit confused. The passenger side had the least pad material, but after a drive this afternoon which included a few hard stops the driver front smelled like it overheated. Passenger had no burnt odor or any odor at all.

I got the piston out of the driver front caliper, however after installing a new square cut seal, I can't get the piston to go back in. It went back in the first time, I used a c-clamp and 34mm axle nut socket. I had to remove it again to properly seat the dust boot, however the second time and every attempt after the piston goes in cock-eyed every time and needs 35-40 psi of air to remove.

Both the piston and piston bore and clean and no nicks/scrapes.

Any suggestions before I go drop the caliper off with my mechanic?

 

[R18Civic]

He just saved me a bit of money and recommended syl-glide. This is why I love my mechanic. I was just gonna drop it off and be done with it.

 

[R18Civic]

She's back on the road. Any more brake issues and I'm dropping it off for a check up.