(FIXED) Caliper issue after tire swap

[dzeleski]

Hi all, had my local shop swap out my right main tire the other day and now my right brake has significant free play in it before it grabs. Basically I need to push the pedal further before it starts to brake. Once it does braking action feels normal. Looking at the brake caliper I see that the outer pad has a 1/8 inch gap where its just not touching the rotor, that correlates to the larger pedal travel. Once brake pressure is released the gap slowly increases to what you see in the attached photos. No fluid leaks.

Any ideas on what is going on here? Something assembled incorrectly? Slide pins stuck?

Edit: Fixed:

[Shadrach]

The caliper piston should not retract. It should self adjust in such a way that the piston rests against the pad which should in turn rest against the rotor, compensating for the elimination of friction material as the pad wears.  Perhaps the pad is not sliding properly on the caliper pins causing it to flex when pressure is applied but returns to its original position when pressure is released.

[dzeleski]

2 minutes ago, Shadrach said:

The caliper piston should not retract. It should self adjust in such a way that the piston rests against the pad which should in turn rest against the rotor, compensating for the elimination of friction material as the pad wears.  Perhaps the pad is not sliding properly on the caliper pins causing it to flex when pressure is applied but returns to its original position when pressure is released.

Thats what I thought, that parts diagram just shows a normal caliper with slide pins. Ill see if I can get a small pry bar in there and gently try and see if the pad is stuck on the pins.

Edited September 9, 2022 by dzeleski

[Shadrach]

10 minutes ago, dzeleski said:

Thats what I thought, that parts diagram just shows a normal caliper with slide pins. Ill see if I can get a small pry bar in there and gently try and see if the pad is stuck on the pins.

The MM calls for a dry film type teflon lubricant (Tri-Flow) on the pins (called anchor bolts in the MM).  

[dzeleski]

4 minutes ago, Shadrach said:

The MM calls for a dry film type teflon lubricant (Tri-Flow) on the pins (called anchor bolts in the MM).  

Ty, ive got that and will get some on them.

[Jim F]

Any time you remove the pad plate you should clean the pins and sockets.  Scotch-brite the pin and use a piece of scotch-brite with a coat hanger bent over that you can put in a drill to clean the socket.  I use a light coating of DC4 silicon grease(from A&P school) on the pin to keep the pin from rusting.  DC4 should not have a static charge and so it doesn't attract dust.   Also the piston can be stiff in the caliper if the oring is leaking and its caked up with thick 5606.  

Jim

[dzeleski]

Ran down to the hangar. Caliper moves freely, as does the pad. Mechanic called me back and thinks there is air in the line. Hes gonna swing by on Monday and use a pressure bleeder. Ill keep this updated if that fixes it in case anyone comes across this in the future.

[PT20J]

When I've had to remove and replace a caliper to replace an o-ring, I've always just opened the bleeder and let the fluid run out into a suitable container being sure to keep the reservoir full. Since the only air that was introduced is in the caliper, is readily is bled off by the fluid flowing down through it by gravity.

Skip

[EricJ]

On race cars we call that pad knock-back.   It can be a result of vibration or distortion between the caliper and rotor that forces the piston back into the caliper.   It should not be happening on your airplane and may be due to either the rotor or caliper not being properly fastened or the rotor being bent or distorted.   It doesn't take much. 

If you can jack that wheel up a little bit, rotate the tire and make sure the brake rotor tracks true.   If the wheel has play, i.e., has some play in it or the bearings, it can do the same thing, so try to wiggle the wheel/tire assembly to see if it moving around and knocking the pads back.

I don't think air in the line would do that.   

[dzeleski]

13 hours ago, EricJ said:

On race cars we call that pad knock-back.   It can be a result of vibration or distortion between the caliper and rotor that forces the piston back into the caliper.   It should not be happening on your airplane and may be due to either the rotor or caliper not being properly fastened or the rotor being bent or distorted.   It doesn't take much. 

If you can jack that wheel up a little bit, rotate the tire and make sure the brake rotor tracks true.   If the wheel has play, i.e., has some play in it or the bearings, it can do the same thing, so try to wiggle the wheel/tire assembly to see if it moving around and knocking the pads back.

I don't think air in the line would do that.   

I come from a racing background as well and am very familiar with knockback, which is why floating multi piece rotors are lovely for brake feel. This isn’t knockback though, it retracts back to this position without moving the wheel. I also do not have any braking vibration or judder when braking.

He seems to think it’s air, I’m personally not quite so sure. Comparing with the other wheel I can’t see anything obvious that it would be installed incorrectly. I’ll update this with what he says on Monday.

[Guest]

Your pressure plate is either stuck or jammed on the pins.  When you apply the brakes the pedal travel is excessive while the piston moves to take up the gap between the inner pad and the disc.  Cut the safety wire, remove the bolts and clean the pins and make sure the pressure plate is flat and slides freely on the pins, reassemble and safety. 

[dzeleski]

26 minutes ago, M20Doc said:

Your pressure plate is either stuck or jammed on the pins.  When you apply the brakes the pedal travel is excessive while the piston moves to take up the gap between the inner pad and the disc.  Cut the safety wire, remove the bolts and clean the pins and make sure the pressure plate is flat and slides freely on the pins, reassemble and safety. 

That’s what I thought as well but using a small screw driver the pressure plate appears to move completely free on the pins. I’ll yank the wheel fairing tomorrow and take a closer look.

[PT20J]

I would jack up the wheel so I could spin it and have someone else operate the brake while observing the action. 

[Wildhorsetrail]

On my 172 the brake is always a little mushy after removing a tire. The piston gets pushed in a little when maneuvering the wheel off of the spindle. After several good braking applications, the piston finds it's happy place kissing the rotor and it all goes back to normal.

[dzeleski]

Alright for anyone following along... There was no air in the system, and the caliper pins and pressure plate were all perfectly free and clean. The issue ended up being the wheel fairing was installed incorrectly. This put pressure on the caliper and after the brake pressure was released the whole fairing acted as a giant spring and pushed the caliper away from the rotor.

You can see in the photos how the fairing is touching the caliper, with the second photo you can see how both pads are nice and tight to the rotor after pressing the brake with the faring removed. After more carefully comparing it to the other main I could see that at least two washers were missing. Im not sure if they were dropped, forgotten, etc but I had a couple spare ones and popped them on. Mechanic is swinging by later to check my work and make sure hes happy with it but it was at least a simple issue.

Thanks everyone for the tips and ideas!

[EricJ]

Good find!

A gear swing would be a good idea to check how it sits against the wing.

[DonMuncy]

whenever you remove a gear door, you should always be careful to note how many washers are in each position. It is easy to lose count of how many go where.

[PT20J]

1 hour ago, DonMuncy said:

whenever you remove a gear door, you should always be careful to note how many washers are in each position. It is easy to lose count of how many go where.

I find that the washers sometimes fall out during removal. To make sure I get them back in the right places, I’ve used a black Sharpie to note the number of washers next to each bolt hole on the door.

Skip

[N201MKTurbo]

I write the number of washers with a Sharpie too.

Getting your gear doors to close right is the best speed mod you can do.

[carusoam]

Great follow-up Dzel!

thanks for sharing all the pics and details!

Go MS!

Best regards,

-a-

[PT20J]

I wonder how long it takes the factory to set those up initially?

[Jblanton]

If you retracted the gear with the fairing like that check the retraction tubes in the belly, and check rigging/preload.  I have personally seen a bent retraction tube from this same scenario.