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How To Rebuild Calipers
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Peter  



Joined: 04 Nov 2002
Posts: 379
Location: New York, NY

PostPosted: Tue Jan 09, 2007 2:14 pm    Post subject: How To Rebuild Calipers Reply with quote

This “How To” details rebuilding 931 (5-bolt) front calipers. Although mounting hardware is different between the 924/931 5 and 4 bolt calipers, the rebuilding technique outlined below applies equally to all 931 and 924 calipers (not to mention 944 and early 928 calipers).

Parts & Tools:
1. Caliper Seals: Porsche Part # 928-351-941-00. Fits 944, 944S, 928 (’78-’81).
2. Caliper Mounting Hardware: Porsche Part # 944-351-95201. Fits 924 Sport/Turbo/GT/S, 944/S/Turbo.
3. Sandpaper: Grits 600, and 1000 (or equivalent).
4. Hammer, metal punches(wide and narrow), and awl.
5. MAPP/O2 gas torch (if unavailable, then a good penetrating liquid such as PB Blaster).
6. Sand blaster (if unavailable, then wire wheel for electric drill and metal brushes).
7. Paint (any hi-temp paint, or from my own positive experience on my 924 track car, Rustoleum enamel works excellently).
8. Air Compressor (if unavailable, then wide-jaw pliers and plenty of elbow grease).
9. Needle nose pliers.
10. Metal pick (or equivalent bent needle/safety pin)
11. Simple Green Degreaser (or equivalent dish/ clothes washing detergent)
12. Metric taps to clean bleeder screw and brake hose connections (or clean bolts of equivalent sizes).

Step 1
Remove calipers from spindles as per Porsche factory or Haynes manuals.

Step 2
It is always a good idea to take a picture and notes of how the caliper and associated hardware is installed before you take it apart. See illustrative PIC below.




Step 3
Removal of the brake pad mounting pins. The safety wire (denoted with the blue lines) holds the pins and is easily removed with needle nose pliers. Use a torch to mildly heat the pins where they come in contact with the brake mounting assembly (denoted with a green line) to break the rust/corrosion. The red lines denote the pins and the points that should be heated. Then use a hammer and wide punch to knock out the pins. If a torch is unavailable, then use plenty of penetrating fluid, and tap the parts to set up vibrations that help loosen the rust/corrosion. You will still need a hammer & punch, in addition to plenty of hammering.




Step 4
Removal of the caliper hold-down wire.
Once again, use a torch to heat the wire at the point it enters the brake mounting assembly (denoted with blue lines). Then knock the wire out with the narrow punch. If a torch is not available, use plenty of penetrating liquid in addition to hammering.




Step 5.
Separating the brake mounting assembly.
Basically same as above, in that you use a hammer & punch to separate the parts, in addition to a torch or penetrating liquid.


Step 6.
Marking piston to caliper bore alignment
There are two notches marked on the piston. To maintain proper piston alignment, use an awl to score a mark on the caliper perpendicular to each notch. I have used red paint to illustrate the notches on the piston. The scoring mark is not visible in the picture




Step 7
Removal of the brake cylinder from the caliper bore.
This is best done with an air compressor, either by using a threaded hollow rod that threads into the caliper bore at the brake hose inlet and the air gun (as per PIC below) or by using a rubber nipple on your air gun and pressing it firmly into the brake hose inlet. For safety, I place the caliper, piston side down on a rag on the floor, and gradually pressurize the caliper. For me, this has worked a 100% of the time. If compressed air is unavailable, then using a set of wide-jaw pliers, firmly grip the piston top only (the top is the part that comes in contact with the brake pad) and begin twisting and pulling the piston cylinder out. Good luck. I tried the pliers technique only once, and found it exhausting and time consuming.




Step 8
Cleaning the piston and caliper bore. Remove the caliper seal by using a pick to pullout the rubber seal on the inside of the caliper bore. Discard this item.
Next clean the piston and caliper parts by washing them with Simple Green degreaser. Towel dry. Using 600 grit sandpaper, wrap paper around the piston, and while submerging it under running water rotate the sandpaper 360 degrees around the piston several times until clean and shiny. Then repeat using 1000 grit sandpaper. If the piston is not pitted and resembles the piston pictured below, then it can be reused. Do the same for the caliper bore, by rubbing sandpaper along the inside walls and the ridge where the rubber caliper seal rests. It is important for this area to be free of pitting because leaking will occur if a tight seal is not made. Examining the caliper bore may reveal some minor pitting on the bottom. This is acceptable. However, pitting is not acceptable on the bore walls, ridge seal, or piston. If your piston and caliper bore cannot be reused, discard them and buy a good used set and try again.






Step 9
Cleaning and painting calipers.
I like to sandblast my parts with an inexpensive Harbor Freight sanding gun (looks like a paint gun). Make sure to protect the piston and caliper bore from sand by wrapping them in duct tape. Although not as effective, one can also use a wire wheel attached to an electric drill and wire brushes.
With metric taps, or equivalent metric bolts, clean the threads where the bleeder screw and brake line screw into the caliper.
After cleaning, paint the parts to protect them from the future rusting. Although for these calipers (going onto my 931 track car) I used red hi-temp spray paint, I have used Rustoleum enamel for my 924 track car and the calipers and drums still look good.

Step 10
Putting it all back together.
With fresh brake fluid, spread some fluid over the caliper bore, new rubber seal, and piston. Press the seal into the ridge of the bore. Place the piston over the bore, and align the piston notches to the marks on the caliper made in Step 6 above. The fuzzy pic below shows me using a metal ruler to align the notch on the piston with the mark I earlier scribed onto the caliper housing.




Place the caliper face down on a rag and firmly press the caliper down onto the piston.


Install the dust shield by pulling it over the piston first, then the caliper. Then place the circlip over the dust shield on the caliper side.




Next, push the caliper onto the brake mounting housing. Refer to your notes/pics from Step 2 if you forgot which side is right side up.


Step 11
Piston to Bore Alignment Check.
The notches in the piston must be oriented at 20 degrees to the brake mounting assembly. Why? I don’t know, but both the Porsche Factory Manual and the instructions with the caliper- rebuild kit say so! (Seriously, I have researched the subject but have not found a satisfactory answer.) To verify alignment, I made a simple 20 degree triangle out of cardboard to recreate the Porsche tool used to align the piston. The tool rests against the brake mounting assembly and the 20 degree angled cardboard must align with the notches on the piston. As per the pic below, the initial markings from Step 6 are spot on and no further adjustment in alignment is necessary. If alignment is necessary, simply turn the piston.
Below are pictures of the gauge being used, along with the gauge, and finally a close-up of the instructions showing alignment procedure.








Step 12
Screw in the bleeder screw, bolt it back up to the spindle, and attach the brake hoses.
Bleed as usual. You are done!


Last edited by Peter on Wed Jan 10, 2007 2:54 am; edited 1 time in total
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Ozzie  



Joined: 12 Mar 2005
Posts: 4413
Location: Townsville, Qld. Australia

PostPosted: Tue Jan 09, 2007 8:52 pm    Post subject: Reply with quote

Very informative and great pics.
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Peter  



Joined: 04 Nov 2002
Posts: 379
Location: New York, NY

PostPosted: Wed Jan 10, 2007 7:01 am    Post subject: Reply with quote

Ozzie wrote:
Very informative and great pics.

Thanks for the compliment Ozzie!
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macBdog  



Joined: 16 Aug 2004
Posts: 1100
Location: Brisbane, Australia

PostPosted: Wed Jan 10, 2007 10:48 am    Post subject: Reply with quote

Great job. You have high attention to detail. I will be trying to match your quality of work for the set of 911 brakes I am doing at the moment.
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Peter_in_AU  



Joined: 29 Jul 2001
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PostPosted: Wed Jan 10, 2007 9:28 pm    Post subject: Reply with quote

WOW that is soooooo good!!
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Joes924Racer  



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PostPosted: Thu Jan 11, 2007 12:29 am    Post subject: Reply with quote

Outstanding Peter
just what I was looking for.
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Peter  



Joined: 04 Nov 2002
Posts: 379
Location: New York, NY

PostPosted: Sun Jan 14, 2007 6:18 am    Post subject: Reply with quote

Thanks all!
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Rasta Monsta  



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PostPosted: Sun Jan 14, 2007 6:54 am    Post subject: Reply with quote

Hey Peter, are these still off the car? I don't suppose you can share the stampings on the different parts. . .

I swear, they looked identical until after I threw them all in the cleaning pile. . .


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Peter  



Joined: 04 Nov 2002
Posts: 379
Location: New York, NY

PostPosted: Mon Jan 15, 2007 5:21 am    Post subject: Reply with quote

Rasta Monsta wrote:
Hey Peter, are these still off the car? I don't suppose you can share the stampings on the different parts. . .

I swear, they looked identical until after I threw them all in the cleaning pile. . .



Guess you missed Step 2 above.
This is what I have for the passenger front, 5 bolt, 931 brake caliper unit.
ATE 17: Mount with bolt holes, which attaches the caliper unit to the spindle.
ATE 28: Caliper bore.
ATE 07: Mount to which the brake pads attach onto.
I hope it helps.
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Paul  



Joined: 02 Nov 2002
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PostPosted: Mon Jan 15, 2007 5:50 am    Post subject: Reply with quote

Great work!!

If you don't have an air compressor, you can pop the pistons most of the way out with the brake pedal (before removing the brake hoses).
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Rasta Monsta  



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PostPosted: Mon Jan 15, 2007 8:22 am    Post subject: Reply with quote

Actually I was referring to left vs right.
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Lizard  



Joined: 03 Nov 2002
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PostPosted: Fri Jan 26, 2007 5:03 am    Post subject: Reply with quote

a couple of notes I would like to add to this,

left and right the only parts that make a difference are the piston housing.
You can tell left and right piston housing apart by the location of the bleeder valve, it ALWAYS is pointed up to allow air to escape. As long as you know this you never need to mark the caliper as to which side it went on.

I would strongly recommend while you are doing this to replace the front rubber brake lines, preferably with stainless braided lines, but factory replacement are fine as well. The reason is these lines rot and send rubber particels into your fresh new caliper.

You should not need to sand the inside of the bore with sandpaper once it is cleaned.

Make sure that if you sandblast the parts, do everything EXCEPT the shiny part of the piston, in other words you can do the outside lip and the inside of the piston no more. You may also consider just threading an old bleeder into the hole when doing the caliper housing.

When doing this I doubt the kits come with new bleeder valves but I would STRONGLY recommend that, nothing worse than putting it all together then getting to your next brake fluid change and breaking the old bleeder off in the caliper.

When removing the blake piston, DO NOT disassemble the caliper first! Remove the pins holding the old pads in and either put the old pads in or a block of wood in for the piston to exit onto when you remove it with compressed air. If you do not there is a very high likely hood of that piston ejecting across the room and possibly killing someone. Whatever you use for the cushion you want it to be thick enough so that the piston cannot exit the bore of the caliper.
There is one other way to do this. You can disassemble the entire unit as shown above with the piston left in the bore and then put the unit into a vise, close the vise so that it is touching the piston and the other side of the caliper and then apply air, MAKE SURE to watch your fingers if you wish to keep them all. as you hold some air into the caliper undo the vise this will allow for a controlled removal of the piston. Once the piston exits it will just pass the seal and it will tend to cock up or down, if you use too much PSI of air the cocking could mark the piston making it non reusable.

please excersize caution when performing this task though
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Rasta Monsta  



Joined: 12 Jul 2006
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Location: PacNW

PostPosted: Fri Jan 26, 2007 5:13 am    Post subject: Reply with quote

Lizard wrote:
a couple of notes I would like to add to this,

left and right the only parts that make a difference are the piston housing


Glad to hear this. I swear, they look absolutely identical, but when I saw they had different stampings I became a little concerned. . .
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kcoyle  



Joined: 16 Jan 2011
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PostPosted: Thu Dec 15, 2011 11:14 am    Post subject: Reply with quote

Excellent writeup. I'm in the middle of it now and would like to add that one should not get too crazy with the cheeze whiz (torch) heating the caliper hold down wire too much or you will wind up with this.



$40.23 for two new wires.
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sbarc  



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PostPosted: Thu Dec 15, 2011 12:43 pm    Post subject: Reply with quote

This is an excellent thread.

Paul wrote:
Great work!!

If you don't have an air compressor, you can pop the pistons most of the way out with the brake pedal (before removing the brake hoses).


That's the way I do it and I have an air compressor.
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