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Seized Caliper Piston/Partial Rebuild - 4 Pots

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I fitted new brake rotors and pads yesterday and discovered a seized inner lower piston on both front calipers. Managed to press them back in with a crowbar to get the new pads fitted and was hoping they might free up, but no luck. The front rotors are too hot to touch after a casual drive around the block and they're binding with the car jacked up.


Today I've chocked the rear wheels, jacked the front up and removed the wheels again. Removed the brake pads, then unbolted the front calipers and removed the new rotors:



I've fitted some old rotors, then bolted both front calipers back on:



Then started the car and pumped the brake a few times to push the caliper pistons out:



Then with a drip tray in place I unbolted the brake lines, making sure to keep the banjo bolt washers:



Both front calipers then removed and drained:



Dust seals removed with a flathead screwdriver, then the pistons pull out pretty easy:





Apart from the seized one on each caliper. Which needed a soak of brakeaway, rotating and some brute force:



Three pistons from each caliper cleaned up really well, with no visible corrosion or pitting on the inside surface. The one seized piston from each is well pitted and will be getting replaced:



Just been quoted $52.55+gst per piston (40.4mm), through BNT so not keen to replace the lot if not needed. 4 pot seal/rebuild kits are $76.34. I've ordered 2 new pistons and a full seal kit, hopefully be here in the morning to be back on the road by tomorrow night.


Here's a look at the oil seal from the seized pot. Just need to grab a few cans of brake cleaner and wait for some new parts:



OEM 4 pot seal kit p/n 26297FA051, does both front 4 pot calipers. Then seal kit p/n 26697FA000 does both rear 2 pot calipers. Numbers taken from kamineko's garage thread, with more caliper pics here:


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I've cleaned the 6 good pistons up, mainly around the top edges and dust seal seat. Using a bench grinder, dremel and a small file:



Also given the cylinders a quick clean up with some 800g wet and dry, but they're pretty good anyway:


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Yeah they can go back in any pot, or any caliper of the same model. The piston sizes only change single pot/twin pot etc. Couldn't find any tears or damage to either dust seal, which is the normal culprit. Maybe the seals weren't fitted properly or that's just the dirtiest, wettest part of the front hub.

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Unfortunately the new pistons and seals didn't turn up Saturday morning.


So, I grabbed some paint instead:



Thoroughly cleaned with brake cleaner, then sanded with 600g paper, then thoroughly cleaned again. Then masked up:



Then 2 thin and a final thick coat of VHT SP731 Real Red to match the rear calipers:



Then oven baked at 93c for one hour:



Let cool down completely, then the masking tape removed:



I was hoping to use a sponge to paint the lettering but it didn't really work, the sponge I had was too soft. Ended up getting the modelling paintbrush I use for stonechips:




First coat almost complete, two more to go:



Then baked again to cure the white paint (SP118/129) and here's the finished product:



Patiently awaiting a few new bits and a couple of clear coats (SP730) before they go on:



Rotor paint job:




2 Pot caliper paint job:




If you have the newer style 4 pot calipers with a Subaru sticker rather than raised lettering, you can get the high temp vinyl decals from Ebay:



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The 4 pots got two clear coats yesterday, then baked again:



Then today the new brake caliper parts turned up:

Pistons - Frenkit 40mm P402902 x2

Seal Kit - Autostar SP-S109P (one per caliper)

Rubber Grease - Bosch RG4

Brake Fluid - TRW 1L (full system takes 500ml)



I've lubricated the caliper cylinders with brake fluid. Then smeared the oil seal/piston seal with rubber grease:



Then worked the piston seal into the bore groove, making sure it's not twisted:



Then coated the dust seal completely with rubber grease on both sides. Then smeared the inside of the piston that will eventually go rusty. Then also put a smear around the pistons dust seal lip. The rubber grease stops the seals drying out and perishing, helps them seal and also stops corosion:



I fitted the dust seals onto the piston, then coated the bottom of the pistons with brake fluid and slid them into the cylinders. Needing a little wiggle and shove to slide passed the piston seals:



Then with the pistons pushed right in the outer dust seal lips easily slide in place:



I've stretched the spring clip on the left open, then clipped it on itself so it stays stretched:



Then slid the back over first, held it in place and flicked the front edges over with my fingernails:



All 8 pistons lubricated and installed. New ceraminc pads, pins and clips back in:



Now I have two rebuilt freshly painted 4 pots ready to go back on the car:


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I ran into a problem when refitting the F/L caliper. While originally cleaning them I found what I thought was an old stripped thread inside the calipers lower bolt hole. I ripped the lose thread out and biffed it, the bolt looked fine.


On refitting the caliper, the lower M12 bolt hole is so stripped and slogged out that the bolt slides straight in. The lose thread I removed was a stainless steel thread packing coil, so that the OEM bolt could still be used. IIRC this is the third time I've had these calipers off and I've never noticed it.


I've re-tapped the stripped/slogged caliper bolt hole, going from M12x1.25mm up to M14x1.5mm:



Then drilled out the matching hole in the F/L hub, going from 12.5 up to 14mm so the new bolt slides through:



Then went to a local wrecker and found a useable M14x1.5mm bolt of the same grade, also with a matching 17mm head. I've fitted the new bolt into the caliper, getting the length set just right:



Then used a 4" grinder/cut off blade to cut the bolt excess off:



I then removed the new cut bolt and tidied the end up. Then dabbed it with some paint:



Then fitted the last caliper:



Then gave the system a good bleed:


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  • 1 month later...

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