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Aftermarket Cooper S disc brake caliper piston problem
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Author:  miniron [ Mon Oct 26, 2009 5:43 pm ]
Post subject:  Aftermarket Cooper S disc brake caliper piston problem
:?:
Been having some brake fade problems lately so I replaced the pads with EBC Green Stuff pads. Brakes are now good.
Bad news is the old pads, only about a quarter worn, had started to wear in a tapered manner. This is bad as it causes the pistons to tilt in the caliper bore when the brakes are applied and I wonder if this is the cause of the brake fade.
Several years ago, but not many miles ago, I overhauled the calipers and fitted new stainless steel pistons that I had had on the shelf for many years.
The new pistons didn't have the relieved area on the face that contacts the pad and I wondered if this was necessary and was told at the time it didn't matter but it appears it does. A bit of research in the recesses of my brain dredged up some old info I had forgotten about. I'm claiming a senior moment here.
I checked some old original genuine caliper pistons (Never throw anything out) and they have a 0.015" relief around about a third of the piston circumference that contacts the pad. The pistons have to be installed with the relief at the top when the caliper is bolted into position.
The relieved area is there to prevent the pads wearing in a tapered manner due to the self energising effect pulling the leading edge of the pad into contact with disc. Its the same principle used in drum brakes to pull the leading brake shoe into contact with the drum. Good for drum brakes but bad for disc brakes.
So my questions are:
Has anybody else experienced a similar problem or is it only me?
Do replacement pistons you have used have the relieved section?

Regards
RonR

Edit: spelling mistake
Author:  drmini in aust [ Mon Oct 26, 2009 6:29 pm ]
Post subject: 
Ron,
Replacement pistons easily available here (none of which seem to be Lockheed) have not had this relief for many years.
I agree that without it the pads can wear tapered. It also cants the pistons over in the bore which is not a good look.
I have got around this problem by periodically introducing my Greenstuffs to Mr Linisher, but next time I rebuild my calipers I might put a relief on the pistons. :wink:
Author:  GT mowog [ Mon Oct 26, 2009 7:12 pm ]
Post subject: 
Thanks Ron, I did have similar concerns when rebuilding some calipers only very recently. The Larger Discs / Calipers do not appear affected, only the Cooper S (and Cooper?) types.
Author:  miniron [ Tue Oct 27, 2009 8:20 am ]
Post subject: 
:D
Thanks Kevin and GT,
Seems like I'm not alone which is good. I'm not going batty in my dotage after all.
Seems like the problem only occurs with 2 piston solid mounted calipers. More modern designs with single piston flexibly mounted calipers dont seem to suffer this problem. I seem to remember one of the other BMC cars, Morris 1500?, had a caliper which used tapered pads which became flat as the pads became worn. Very strange approach!
To solve the problem I'm going to remove the caliper pistons and grind a small relief around the edge as per the originals. Since the are stainless steel there should be no problem in grinding them, unlike the hard chrome plated ones. Only time will tell if this is the way to go. I don't like the idea of grinding my new Green stuff pads every so often to keep them flat.
These green stuff pads seem to be the ducks guts. After only a few Kms I had to do an emergency stop (d*** h**d in a 4WD) and the Mini just about did a somersault when I hit the brakes. I'm sure the back wheels were off the ground.

Regards
RonR
Author:  GT mowog [ Tue Oct 27, 2009 4:46 pm ]
Post subject: 
miniron wrote:
:D
Thanks Kevin and GT,
Seems like I'm not alone which is good. I'm not going batty in my dotage after all.
Seems like the problem only occurs with 2 piston solid mounted calipers. More modern designs with single piston flexibly mounted calipers dont seem to suffer this problem. I seem to remember one of the other BMC cars, Morris 1500?, had a caliper which used tapered pads which became flat as the pads became worn. Very strange approach!
To solve the problem I'm going to remove the caliper pistons and grind a small relief around the edge as per the originals. Since the are stainless steel there should be no problem in grinding them, unlike the hard chrome plated ones. Only time will tell if this is the way to go. I don't like the idea of grinding my new Green stuff pads every so often to keep them flat.
These green stuff pads seem to be the ducks guts. After only a few Kms I had to do an emergency stop (d*** h**d in a 4WD) and the Mini just about did a somersault when I hit the brakes. I'm sure the back wheels were off the ground.

Regards
RonR


That's quite OK RonR, I have learnt a quite a bit from your post. My experience with Cooper S and 8.4 Calipers, with original type pistons is the pads do wear remakably flat, even when the calipers are in less than ideal condition. Thanks too for the info on the 'green stuff'. I've not long ago bought some (for 4 spots) and have yet to fit them, so it's good to hear something that's not a sales pitch. :D

I dare say that relieving the pistons will cure the problem.

Do you or anyone else know if the replacement chromed pistons have the relief?
Author:  drmini in aust [ Tue Oct 27, 2009 6:15 pm ]
Post subject: 
I bought chromed pistons last year for a rebuild when I couldn't get S/S ones. No relief on them... :x

I don't see a problem grinding a relief, whether S/S or chromed. Sure you will go through the chrome on the edge, but it's miles away from anything that matters, ie the fluid seal.

I think only genuine Lockheed ones had the relief.
Author:  GT mowog [ Tue Oct 27, 2009 8:45 pm ]
Post subject: 
Ta Doc.
Author:  miniron [ Wed Oct 28, 2009 9:28 am ]
Post subject: 
Hi Kevin,
I was concerned about grinding the hard chromed pistons as it would let rust start under the chrome and eventually work its way into the seal areas. It would take a while though.

Regards
RonR
Author:  drmini in aust [ Wed Oct 28, 2009 6:22 pm ]
Post subject: 
Ron, I'd just copper plate it after grinding. Easy peasy to do- some copper sulphate, a battery and a bit of copper. Mask off the chromed outside with tape first, or just polish the copper off afterwards..
Author:  miniron [ Thu Oct 29, 2009 9:24 am ]
Post subject: 
8)
Kevin,
I haven't done that since high school chem class. I'll keep it in mind for the future and dust off my old chem books. As I said earlier, never throw anything out. Doesn't it always seem you need it the day after you throw it out.

RonR
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