Rear Caliper Assembly - Help!

I just rebuilt both rear brake calipers and reuniting the piston housing with the floating frame has proven to be just about impossible.  The tension spring that holds them together is a real bear to compress in two different directions simultaneously.  Also, the Teflon slides get knocked out easily in the process and complicate things even more.

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< Piston Housing with Tension Spring >                  < Teflon Slide on Floating Frame>

Does anyone have experience with the assembly process for this type of caliper?  There has got to be some magic trick to do this!!

Thanks in advance!!

Here's a video that might help. While this guy didn't technically assemble his properly, it might give you an idea of what to do.

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Hotrodz of Dallas wrote:Here's a video that might help. While this guy didn't technically assemble his properly, it might give you an idea of what to do.

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Thanks Bob...but this video is for a front caliper rebuild, not the rear.  I managed to figure out the proper assembly sequence for the front and it wasn't so bad.

The rear calipers on the other hand...  that deformed spring is just nuts.  It seems the legs are just too short to engage both sides on the floating frame simultaneously and that leads to canting of the cylinder housing...and subsequent failure.  in my first attempt, I did manage to assembly one caliper but then realized I left out the Teflon slides.  They take up just enough of the wiggle room to impact the required alignment and they pop out if you look just at them the wrong way!!

I'll keep looking around for guidance and will likely give it another try later today.  It just shouldn't be this difficult!!

Having three hands helps with this [You must be registered and logged in to see this image.]

When I rebuilt my rear calipers, I recall needing a vice to hold one side steady while I tried to simultaneously expand the spring and hold the little nylon slides in place. Took a few tries and I may have invented a few new swear words in the process.

Thanks for your input... I discovered the same thing!  Read on...

n_l wrote:Having three hands helps with this  [You must be registered and logged in to see this image.]

When I rebuilt my rear calipers, I recall needing a vice to hold one side steady while I tried to simultaneously expand the spring and hold the little nylon slides in place. Took a few tries and I may have invented a few new swear words in the process.

One of the last things on my "To-Do" list was rebuild and install the rear brake calipers and parking brakes.

I covered the front caliper rebuild process in a previous post:
Front Caliper Rebuild
Post  Raymond-P Wed Sep 01, 2021 12:29 am

The rear caliper rebuild process is nearly identical with two (2) significant differences:
         1)  A particular orientation is required for the caliper piston which has a step in the brake
               pad contact surface.
         2)  The Guide Spring that holds the floating frame to the brake pad housing is an
               asymmetrical design and difficult to install.

NOTE: All the online “how-to” videos and write-ups I was able to find blow right past these 2 issues with barely a mention!  Fortunately, the ATE instructions that came with the rebuild kit shed some light on the subject.

Getting Started
Remember to take all the pictures you can of your caliper BEFORE you disassemble it.  It’s plenty easy to get disoriented, and looking back at the pics will be invaluable!  Do one caliper at a time.

Note: Pictures in this post are from both the LEFT and RIGHT caliper rebuild.

Below are some of my “before” pics.  I have over a dozen more just for reference!

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< Top Side View, RIGHT and LEFT REAR Calipers >

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< Underside View, RIGHT and LEFT REAR Calipers >

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< Fully Disassembled RIGHT REAR Caliper>

Disassembly - The rebuild process started with complete disassembly and salvage of everything except the replaceable items: piston seal, piston dust boot and lock ring, Teflon slides, and old brake pads.  This is all straight forward and similar to the front caliper.  I did learn that using compressed air is the easiest way to remove the piston from the cylinder.  Be sure to close the bleeder valve, place a piece of wood or folded shop rag between the piston and the frame, and don’t use to much air pressure or your piston will go flying!!  ~25 psi did the job for me.

Media Blasting - Blast cleaning revealed some corrosion on the brake pad housing frame, but it was not severe enough to affect function.  One benefit of blast cleaning is the process provides the perfect surface prep for good paint adhesion.  I elected not to use primer for this reason.  Blasted components were all cleaned with Brakleen and thoroughly dried prior to painting with VHT 900 deg caliper paint.  Remember not to touch surfaces to be painted with bare hands to avoid skin oil contamination.

Lubrication - This time around, instead of using NAPA Sil Glyde, I opted for Prestone Brake Parts Lubricant (~$6) which is 100% synthetic and specifically formulated for lubing seals and all sliding metal parts on the caliper.  Do not use any petroleum-based lubricant on rubber components. The new ceramic brake pads have a factory applied dry anti-squeal coating on the back, but I applied lubricant on them just the same. Avoid getting lubricant on the brake pad material that contacts the rotor.


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< Frame Corrosion >                                                 < Reconditioned and/or New Parts >
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< ATE Piston Seal Kit and Teflon Slides >                               < UAP Ceramic Brake Pads >

The ATE caliper rebuild kit (PN: 11.0441-3601.2 ~$32)  included a new piston seal, a new piston dust cover and lock ring, and new Teflon floating frame slides.  
Reassembly

Piston Seals - After clean-up and refurbishing, installing the new piston seal was the first step in reassembly.  The piston, cylinder wall, and especially the seal grove and seal, all get a generous application of lubricant.  

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<  Lube on Piston Seal and Groove >                                      < Pushing in the Piston >

Piston - The piston is then simply pushed into the cylinder/piston housing by hand… although not the whole way.  Just get it partially inserted and then install the piston dust boot.  With the dust boot installed, push the piston down until the bottom of the dust boot is just above the lip of the piston housing. Then, the outer ring of the dust boot can be folded down over the lip on the cylinder housing and the lock ring carefully installed. However, don’t do this just yet…

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< Piston Dust Boot Placement >                               <  Fully Installed Dust Boot and Lock Ring >

Guide Spring - The next step is installing the guide spring on the piston housing.  It only goes on one way with the more bent leg on the trailing side of the brake pad housing frame.  Again, knowing the forward rotation is important.

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< Guide Spring Installed >                               < Piston Housing Installed in the Floating Frame >

Piston / Cylinder Housing -  With the guide spring installed, the piston housing can be installed into the floating frame.  Check that you have it turned the right way and carefully align the slots in the housing with the rails of the floating frame. I used my bench vise with aluminum soft jaws and cranked it right it.  You can check your work by making sure the housing has engaged the tab at the back of the floating frame.  

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< Pressing in the Piston Housing >                          < Full Insertion of the Piston Housing >

Piston Alignment - This is where difference No. 1 shows up.  Note in the previous pics that the piston surface has a step with about 40% of the brake pad contact surface machined down.  From the ATE instructions, I learned that this 40% stepped area is to be oriented toward the forward rotation of the rotor, specifically, at a 20-degree angle with a radial line from the hub center.  This is difficult to wrap your head around without illustration, so I added the ATE schematic and instructions below.  The schematic part is the same one I saw mentioned for about 2 seconds in a YouTube video.  Reportedly, it’s out of the Porsche Manual.  

The schematic makes a distinction between brake calipers IN FRONT of the axle, and brake calipers BEHIND the axle.   Curiously, they are just inverted mirror images.  

The key take-aways are:
1) The piston step must face the forward rotation of the rotor, and
2) The edge of the piston step is rotated 20 degrees from radial with the outside corner back toward the rotation of the rotor. The inside corner remains on the radial line.

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< ATE/Porsche Piston Alignment Schematic >           < 20-Degree Alignment Template >

Unlike the ATE template, I made a template which is a simple 20-degree wedge that has the same outside radius as my rotor (~5 5/8”).  The point of the wedge defines the hub center (or axle) while the rounded end represents the outside edge of the rotor.  To use the template properly it is essential to identify the forward rotation of the rotor.

With the piston housing in the floating frame, I could now use my template to check the orientation of the piston step relative to the straight rails of the floating frame.  The blue arrow on the template in the photos below indicates the direction of rotor rotation for forward movement.
 
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< 20-Degree Piston Alignment LEFT REAR >             < 20-Degree Piston Alignment RIGHT REAR >

When I first assembled the pistons in the piston housing, the angle was off.  Consequently, I had to make alignment adjustments and the easiest way to accomplish this was some backtracking.     I noted the precise amount and direction of piston rotation I needed, and then removed the dust cover lock ring, peeled back the dust cover off the cylinder lip, and pushed the piston back out with compressed air.  I could then rotate the piston by hand as needed, pushed it back in the cylinder, and reinstalled the dust boot lock ring as before.

WHY???
At first, I thought Porsche was shooting for the stepped area of the piston surface to be tangent with a radial line from the hub, but the 20-degree alignment requirement negates that theory.
My Internet research revealed no explanation for the piston surface design or the 20-degree alignment specification.  I do however believe the design is intended to stop brake squeal.  The 40% stepped area effectively reduces applied pressure on the leading edge of the brake pad which is where high frequency vibration (squealing) normally originates.  The 20-degree angle creates a pressure distribution across the leading edge of the brake pad which may also help negate squealing.  Supporting this theory, new premium brake pad designs have a chamfered leading edge to stop squealing.  
Anyway…that’s my 2 cents worth!!

Reuniting the Floating Frame and Brake Pad Housing - This is where the 2nd difference shows up.
The Guide Spring in the rear caliper has compound bends in it that lock the frames together.  To make the union, you must engage one of the spring legs in the brake pad housing and physically deform the other spring leg down and together, simultaneously so that it engages…and…the slide bars of the floating frame slip into the guides on the brake pad frame…and be sure you don’t knock off a Teflon slide.  If you’re thinking the operation requires more than two hands, you would be 100% correct.
This is no simple task and I struggled with it for hours until I found a sequence that worked.
Below is how I did it.

First, I installed the new Teflon slides on the floating frame and lubed all mating surfaces.  Then, I laid out both frames on a table to preview how they fit together.  Next, I positioned the lower (less bent) guide spring leg close to where it is supposed to go and clamped it into place on the floating frame using a small pair of long-nosed VICE-GRIPS. (This is Extra Hand #1)

Next, with the brake pad housing frame inserted through the floating frame, I locked the bottom section of the brake pad housing in my 4-inch bench vise. (This is Extra Hand #2). Aluminum soft jaws helped protect the new paint job.

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< Teflon Slides Installed >                                   < Preliminary Alignment RT Caliper >
 
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< Pushing the Lower Spring Leg, LT Caliper >                    < Completed Union, RT Caliper >

The bottom leg of the guide spring is bent inward much more than the top leg.  Using one of your real hands, (Hand No. 3) start the top spring (clamped) and slide bar of the floating frame into the brake pad housing and hold it firmly in place.   While maintaining pressure on the floating frame so the top spring leg stays in the guide spring groove, use your other hand (Hand No. 4) and a wide tipped flat screw driver (or equivalent) to push over the bottom guide spring until it is engaged in the bottom guide spring groove.  With both guide spring legs partially engaged, check that the Teflon slides are where they belong and if so, push the floating frame completely into the brake pad housing.  Done!!

Caution:  With the frames all clean and lubricated, they separate easily compared to disassembly and if this happens, “rinse and repeat!!”  Ask me how I know…

Brake Pads – I elected to install new brake pads before putting the calipers back on the car.  This is a straightforward process and it begins with making sure the caliper piston is fully compressed. I lubricated all the caliper contact surfaces including the brake pad plates, the retaining pins, and the expanding springs.

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< Lubricating the Brake Pad Plate >                   <  Checking the Rotor Gap and Pad Anchor >

To avoid problems installing the caliper on the rotor, be sure the outboard brake pad anchor is fully engaged in the brake pad backing plate.  Lastly, check the space between the brake pads.  My rotors are ~3/4 inches thick and the gap measured ~7/8 inches.  Good to go!!

Wrapping things up, I installed the retaining pins, expanding springs, wear sensor cable mounting wire, hitch pin clips, and new bleeder valve caps.  My new paint job got a little beat up during assembly so after some touch up, the calipers are ready to install on the car.

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< Completed Rear Caliper Rebuild >

Next Step… Emergency Brake Shoe Replacement