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Help with brakes please


Well-known member
16 Sep 2010
Can anyone help me with my brakes....
Inherited these calipers on the car. The part number 20.4691.01 on one half 20.4692.00 on the other. The piston sizes are 36/32mm FRONT and 34/26mm REAR. The rears are fitted with a couple of penny washers to space away from the hub (not too clever!) the pad fitment are standard 964.
I need to get them re-furbished and need Some advise.

Are they O.K to be fitted to this C2 with the standard master cylinder as I have heard many differing problems with none standard brakes?
Oh and they appear to be aluminium....

I need your help guys.


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Chief, those piston sizes don't sound right. You'll find all the usual piston sizes for a 964 and 993 and big brake upgrades on this chart:


I don't proclaim to be a brake expert but comparing the piston sizes you quote to the standard 964 piston sizes (40/36 front and 30/28 rear), I would say you would have too much rear brake bias with them. :?:
Thanks for the reply here and on Rennlist.
No one knows what these calipers are off, which is flustrating :x
I have measured the piston diameter as best as I can without pulling them out to far so could be a little off with the size but not too far.
The pads are 964 size and fit. There are no adapter plates just a straight fit all be it with the penny washers on the backs.

I guess my real problem is that the travel on the brake peddle feels odd...
You can push (by hand) so far then there is resistance and it goes again.
I think to be honest that this could be the linkage to the master cylinder or indeed the cylinder it's self. Anyway I guess I need to poke about a little more.....

Thanks again for your help Steve
I thought they were identified at 993 C2 front and rear?
PeterS said:
I thought they were identified at 993 C2 front and rear?

I saw that reply on Rennlist Peter but am sure it is wrong. For a start, 993 front calipers are bigger and take the next pad size up. Chief already mentions that both front and back take the standard size 964 brake pad. Therefore the fronts he has can't be off a 993.

Although our calipers are made by Brembo and Chief's have the correct caliper body, the Brembo decals look original so I doubt they started out on a 964 (or any Porsche for that matter). My guess is that they could have come off a scooby? And maybe that's why the piston sizes are different to what they should be?
Front calipers has 40/36mm for sure and rear might have 30/34 or 28/30. It's hard to say from the picture.
I will say they might be original 964 c2 calipers and someone has smooth the face off.
Only Maserati and Ferrari use the same plates on the calipers as Porsche and even them they are not smooth.
I'm only wondering why they have those washers for :?:

Master cylinder gonna be ok with them as the pistons are the same as standard calipers.
Right, I have found a listing that says they are off of a 968.
So should I stick with them and get our man above to refurbish them.
Of shift them on?
Thinking what Steve says above regarding too much rear brake bias coould this be causing the 'unusual' handling?

I think you can adjust brake bias but you need to be happy these are up to the job for the car. Begs the question why they were fitted in the first place :dont know:
Chief said:
Right, I have found a listing that says they are off of a 968.
So should I stick with them and get our man above to refurbish them.
Of shift them on?

Theyre the wrong calipers for thE car, the fronts especially providing too little piston contact area

I would move them on and either upgrade or get proper 964 Calipers
Chief said:
Right, I have found a listing that says they are off of a 968.
So should I stick with them and get our man above to refurbish them.
Of shift them on?

That makes more sense. Looking at your first pic, I presume the FR on the tape means Front right (as it was on the 968)? At the moment the caliper is still set up for a 968. By this I mean that the linking pipe is on the wrong end of the caliper for a 964. This is because on a 968 the caliper bolts on to the trailing edge of the disc, whereas on a 964 it bolts on the leading edge. To make it fit a 964 you need to remove the pipe and put it where the bleed valves are and put the valves where the pipe currently is. As they're all 968 calipers, you'll have to do this with each one.

Once done the calipers should sit on the front of the discs with the pipe at the lower point of the caliper and the valves at the top. If you leave them as they are, you'll never bleed the calipers properly and get a spongy pedal. This pic (front right) shows how a caliper is mounted with the pipe and the valves in their correct positions:


While all this will make the calipers theoretically a correct fit for a 964, I'm basing my experience on what can be done with 928S4 and 944 turbo rear calipers which have the same size pistons as a 964. However, I don't know if 968 pistons are the same size? Your measurements suggest not, which makes me tend to agree with 917K that you'd be better off finding the proper calipers.
Thanks for all you replies and help with this.
Thomas, thank you for the e-mail....
It has caused me a sleepless night!
I guess i am on the look out for a set of calipers then, sounds like a bit of a mine field :eek:

FOR SALE SET OF UNUSUAL CALIPERS.....etc etc :frustrated:
Chief, I've asked on the 968 forum on PCGB, what piston sizes are in the calipers of a 968. I've been informed that they should be exactly the same size as a 964 i.e. 40/36 front and 30/28 rear. Therefore if 968 calipers have the link pipes and bleed valves swapped round (as I mention above) they should bolt directly on to a 964. However, none of this explains the fitting of penny washers and the odd piston sizes you've measured.
Again, thanks for your time and effort here :thumb:
I shall pull the dust seals off and extract more of the piston to be sure.
The 'Fr' you have noticed just was me writing FRONT before the end of the pencil broke :oops:. The link pipes are the correct way up, however as all have mentioned the penny washers are a no no for me.....
Still not sure where to go with this at the moment :dont know:
Right.....here we go!
I have removed the pistons and measured them exactly (on the bench, not the floor) and we are as follows.....
FRONT 40/3
REAR 34/30
This now suggests the front are standard C2 pistons sizes. However the rears have the RS piston sizes. After close inspection and removing some paint it appears the faces have been machined smooth then re-painted.
1 - Just live with the larger rear piston size and refurb all calipers.

2 - Replace the rears for standard C2 four piston calipers, however these won't have the machined faces like the front ones.

3 - As the rears are RS size, blow the budget and go 'Big Reds' on the front. The only down side will be trying to find the correct master cylinder without selling the family silver.

4 - Just throw them all in the bin and buy standard calipers from Thomas at Porsche Calipers.

Any advise/ Wise Crack's appreciated as i'm loosing the will to go on!
How about a part ex with Thomas for standard set up? Secondhand sets do come up on the bay but you may need to have refurbished for peace of mind. Equally those that have updated to big reds may off load their old ones. If you offer up the German helmet and leather jacket you might get some interest :dont know:

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