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1G How to rebuild front single piston brake calipers

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I can't stand working on anything brake-related...never have, never will. And I'm not really sure why: Maybe I got brake fluid in my diaper a few decades ago, or I am still haunted by the rear drum brakes on my '77 Camaro...I dunno. But whether it's replacing a master cylinder, pulling rotors, or just changing pads, I'd much rather be having a beer or doing...well just about anything else instead.

But I also hate paying a lot of money for something when I don't have to, and I hate it even worse when a car won't stop when I want it to. So with all that said, I decided to rebuild my brake calipers and write this tech article.

DISCLAIMER: Screwing around with your brake system is not something to be taken lightly. If you don't feel that you are 100% capable of taking on a project like a caliper rebuild and doing it correctly, leave it to a professional. Duh.

Note: This article assumes the calipers are on the bench and ready to be rebuilt. Refer to the factory service manual for instructions on removing them from the car. Also, this is based on the front calipers from a '92 Talon TSI, but other models will be similar.

Now that we have all of that out of the way, let's get started.


What You'll Need

  • front caliper seal kit (for '92 TSI AWD - Mitsu #MB699707, or NAPA #1776 )
  • a dull flat blade screwdriver
  • a source for compressed air
  • rags
  • brake cleaner or Simple Green, etc.
  • 1500 or 2000 grit sandpaper or emery cloth
  • DOT3 brake fluid
  • a drill press and 2" sanding drum
  • duct tape
  • a dental pick or something similar
  • latex gloves (unless you like that warm fuzzy feeling of brake fluid under your nails)
  • factory service manual (highly recommended as always)

Note: The Mitsu seal kit is around $39 a side, and contains the piston seal and boot, as well as new boots for the mounting pins. The NAPA kit doesn't contain the pin boots, but has the piston seal, boot, and retaining ring and is only $8 a side. NAPA sells all 4 pin boots for both sides in a separate kit (#83062) for about $14.50. If you've never done this before, I highly recommend that you go ahead and spend $8 more for an extra kit, because odds are that you will rip a boot the first time you try to install one. :)


Disassembly and Cleaning

The first step is to get the piston out of the bore. The easiest way to do this is to use some compressed air behind the piston to push it out. The problem is that if you aren't careful, it's really easy to turn the piston into a highly efficient finger crusher, especially if the piston doesn't want to come out easily and the pressure behind it builds up. So it is extremely important to stuff a lose rag or some type of foam between the piston and the outside of the caliper, and KEEP YOUR FINGERS CLEAR. You also want to make sure that the piston won't fall or go flying once it pops free.

Note: With only about 30psi being pumped into the cylinder and no resistance, the piston will be generating a force of around 150lbs. If it's stuck and then suddenly comes loose, it will be even more due to the inertia of that heavy piston. Needless to say, if your fingers are between it and the caliper when it pops free, you're gonna have a bad day.

Here's how I do it... With the rag/foam in place, remove the bleeder screw if you haven't already, and slowly inject no more than 10-20 psi or so into the brake line port on the caliper. Start with the air nozzle away from the port, and move it closer into it while watching the piston. You can also place your finger over the bleeder hole to modulate the air pressure pushing on the piston, and gradually work it out. If it doesn't want to move, try spraying some WD40 or something similar into the brake line and bleeder ports and let it sit for a while, and then try it again. Turn up the air pressure to force it out only as a last resort. If it's really stuck, there is probably so much rust built up in the cylinder that you'll need to just replace the caliper.

Note: You'll want to place a paper towel or rag around the air nozzle as well, because when you inject the air, brake fluid is going to spray back out of the port. Also be prepared for fluid to drain out of the cylinder once the piston is out.

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When the piston pops out, the rubber boot may or may not come with it. The boot is held in place with a steel ring, so you may have to carefully pry the ring away from the top of the cylinder to get the boot free. I can usually just pull the boot towards the inside of the bore, and it will pull the ring out with it... assuming the boot doesn't tear and leave the ring in place.

Once the piston and boot are out, it's time to start surveying the damage and remove the inner piston seal. The seal sits about 3/8" down from the top, in a groove cut into the side of the cylinder. If you can't get the seal out with just your fingers, try using a dental pick; but be very careful to not scratch the cylinder. Work slowly to get the pick under the seal, and it should come right out. In the image below, you can see that the piston is pretty nasty (and popped right through the boot)...but's it's really not as bad as it looks and cleaned up nicely, as you'll notice in the following pictures. The second image shows another (freshly painted) caliper with some light corrosion visible on the inside of the bore.

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After everything is out, clean the cylinder and piston as much as possible with some brake cleaner, Simple Green, etc. Rinse everything off really well and use some compressed air to help dry it out as quickly as possible to keep rust from forming before you get it back together. (Since my home's A/C is always running, I set parts on the top of my heat pump fan outside. The warm air coming off it dries them in no time.).

The best way that I've found to refurbish the piston is to mount it on a sanding drum in a drill press. Wrap a few turns of duct tape around the drum for a tighter fit, then slip it into the piston and tighten it down (make sure the piston can't come flying off!). With the drill running at a medium to high speed, use some 1500 or 2000 grit sandpaper to lightly remove the corrosion. If the piston is pitted, you're probably better off just replacing the whole caliper, since the cylinder is likely to be pitted as well.

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For the cylinder bore, I just use some 2000 grit sandpaper and lightly remove any corrosion by hand. It's not the easiest thing in the world to get at, but it can be done. You may not be able to get it shiny clean, but as long as you can't feel any rough areas it should be clean enough. Be sure to keep your hand moving around the inside so that you don't sand too much in one area. Also be sure to remove any deposits from the seal groove and the top of the bore where the boot sits.

Note: You can buy caliper cylinder hones, but I don't like taking the chance of getting them out of round or over-honing them. My philosophy is that if I can't get the cylinder in shape with cleaner and some light hand sanding, I'll just replace the caliper. But YMMV.

After sanding, blow the cylinder out with compressed air, and spray everything down again with some brake cleaner to remove any dust. Set the cylinder and piston aside for a bit to allow them to dry thoroughly. If something comes up and the caliper will be apart for a day or two, lightly coat the piston and cylinder with some brake fluid and put them in a plastic bag.

Believe it or not, the piston shown in the image below is the same one as in the first two pictures above... after spending some quality time with Simple Green and the drill press. ;)

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If you want to paint the calipers, you can do it now or wait until the piston goes back in. If you do it before reassembly, make sure to plug and block off the cylinder bore really well, and plug the brake line and bleeder ports. I've done it both ways, but I usually prefer to wait until it is reassembled for two reasons. For one, you are going to get brake fluid on the caliper during assembly which will eat off the nice new paint, and two... it's easier to cover and protect the boot and open end of the piston from paint than it is to block off the whole cylinder bore. Be sure to use a good quality paint designed specifically for calipers.


Assembly

With the cylinder nice and clean, coat the inside of the bore and the new piston seal with some brake fluid. Push the seal into the groove on one side, and then work your way around until it is in place. Make sure it isn't twisted and is fully seated. Once the seal is in, coat the piston with brake fluid and slide it into the bore. Push it straight in...don't turn it. It may be a tight fit with the new seal, but it will eventually slide in.

Once the piston is in place, lubricate the piston with a bit of fluid where the boot sits, and place the boot over the piston and into place.

Note:

The FSM actually states:

"Fill piston edge with specified grease. Install piston boots.
Specified grease: Repair kit grease (orange)"

I'm not sure what "Fill piston edge" means, but I assume they are referring to the groove that the boot sits in. I've never actually seen any grease here, and have always just lubricated them with some fluid to help the boot slip into place and seat properly. The FSM also states "Caution - Do not wipe special grease on piston seal", but then diagram 14K070 shows grease on the seal and not the boot, which is definitely not right.

There are many ways to go about getting the piston, boot, and retaining ring into place. I've heard of people putting the boot in first, and then pushing the piston through it with the help of some compressed air, but I've never gotten that to work. I've also heard of people putting the steel ring inside the boot and then installing them both together, but all that got me was torn boots. I prefer to slip the boot onto the piston, slide the piston into the bore, then seat the boot, and then insert the retaining ring. As usual, YMMV.


In the first pic below, I've pulled the boot back so you can see how it should be positioned on the piston.

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This is where the fun begins, assuming you are a masochist. Ok...it's not that bad, but getting the silly ring to sit properly in the boot can be a total pita the first time you try it. To begin with, make sure the piston is all the way down in the cylinder, and then work the outside edge of the boot into the groove at the top of the cylinder. The boot has a "lip" on the outside edge that sits perfectly flush in the cylinder groove if it isn't stretched and is seated properly. I use a dull flat-head screwdriver to CAREFULLY work my way around the boot, pressing the lip outwards and into the groove. There shouldn't be any wrinkles in the boot, and the outside edge of it should pretty much dissappear into the cylinder.

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Once the boot is seated properly, install the retaining ring by starting at one end and working your way around the cylinder. The ring pushes against the outer edge of the boot, about 1/8" down from the top. The boot has a very shallow groove in it for the ring, which you can just barely see and feel as your working the ring into place. The frustration comes from the fact that if you aren't careful, it's easy to get the ring under the boot edge, or to roll the boot edge over the ring. When it's correct, the ring should be about half way into the cylinder wall against the boot, and barely visible while looking straight down into the cylinder at the edge of the boot.

The image below shows me installing the ring. You can see that part of the boot has worked up and under the ring in one area, so this attempt was destined to be a do-over. (You'll also notice that I didn't worry too much about the inside of the piston...I'll coat it with some caliper grease when I re-install the caliper).

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As a final check of everything, I like to slowly inject about 5-10psi into the brake port to partially extend the piston, to make sure the boot is seated properly and isn't torn or leaking. If you do this be VERY CAREFUL, because it's extremely easy to over-extend the boot or pop the piston completely back out... in which case you get more practice at all of this.

BTW - The caliper shown below was painted with Duplicolor caliper paint after the piston and boot were re-installed, by covering the boot with the cap from a jar of Wyler's bullion cubes (which is almost a perfect fit) and plugging the ports. Funny how I keep finding uses for those caps. (bottom of page) :)

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Now that wasn't so bad was it?
 
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