ATE caliper knockback mechanism

[AndrewSlater]

I recently bought a pair of newly plated ATE front calipers from a DDK'er that I intended rebuilding and fitting to my 912.

I notice that one of the calipers has the knock back mechanism pins fitted and one has them missing.
My preference is to retain the knockback mechanism if possible.

Has anyone had any success of refitting the pins into the caliper halves and if so would they share the technique?

Also does anyone know of a source for the pins or how I would remove them from a donor caliper?

As always any help would be much appreciated.
Regards
Andrew

[Nine One One]

Andrew,
Don’t know if this is of any help its the ATE article on knock back mechanism

https://www.alfabb.com/bb/forums/attach ... iption.pdf

Then there is this (it’s near the bottom of the page next to the 4 pistons shown)

http://www.pmbperformance.com/brakehowto.html

For early model calipers (pre-1975) you will probably have pins and knock-back mechanisms in your pistons. All but the very early cars will have these stamped in the piston so they will be impossible to remove without damaging the pistons. The rears should all be removable with a small c-clip inside the back of the piston.

http://www.914world.com/bbs2/index.php? ... &id=605693

http://www.914world.com/bbs2/index.php? ... &id=605694

Ian

[AndrewSlater]

Hi Ian,

yes I have read all of these articles before - but thanks for the heads up.

My dilemma is that when I 'restored' the calipers myself a few years back I didn't get them plated and painted them instead.
Although the paint finish is great on tearing down the front calipers they have already started to rust on the internals.

I then bought the front pair of newly plated ATE pistons - but one has the knockback mechanism pins removed.

My dilemma is whether to buy new pistons without the function, and 'snap' off the pins in one of the calipers, or to try and reinstate the pins to keep the function.

My rear calipers currently have the knock back mechanism and the pmb article suggests that 'If you do this to the front, do it to the rear as well. Do not have a mismatch of early and late style pistons in your calipers.'

I could get my rears refurbished by Lee at ClassicBrakes ( he's done my later 911 and 914 calipers before ) - but my understanding is that he would snap off the pins on the earlier calipers and fit later pistons.

I am really trying to establish whether new ( or old ) pins can be refit into calipers where they have been removed.

Andrew

[Nine One One]

Andrew,
I suppose Lee might be able to answer it for you, I know he keeps plenty of spares from callipers that are sent in for refurbishment and may be able to help out if he still has some pistons with the knock back pins still in them?

Never knew about not having a mismatch front and rear and wonder how many people are running cars like this without knowing?

Ian

[AndrewSlater]

I spoke with one of the guys at ClassicBrake today and they remove the pin/post as it's not compatible with the pistons they buy.
Their opinion is that the brakes work better without the pin/post.

I'm not trying to start a debate about which is better just trying to work out my options.

as for the 'Never knew about not having a mismatch front and rear and wonder how many people are running cars like this without knowing?' I suspect the mismatch might be scaremongery but I probably shouldn't take the chance.

I think I will get my rear calipers rebuilt by ClassicBrake - but ask for the old Ribe bolts and pistons returned.
On the fronts I will attempt to remove the pin/post on the one caliper where it is fitted and rebuild with new pistons.

That will mean I'm changing from the knockback mechanism all round to refurbished calipers without the function all round.

The main purpose of all this work was that although the 912 passes the MOT braking tests I've always found them to be a little too leisurely for my liking.
I realise the car will never stop as well as my 997 but as I am using the 912 more these days I would like an improvement in brakes.
As such I 've bought new rotors and pads, and thought I would go the extra mile with the calipers whilst I am working on the car.

So my next question is - Does anyone know a good technique for removing the pin/post without damaging the caliper?

[fetuhoe]

'Knock back' is an interesting issue and is something that we tend to ignore with modern brake systems.

The idea that piston retraction is a 'good thing' has been widely promoted for the last few years as a means of improving fuel consumption by eliminating brake pad drag.

In an ideal world the energy stored in the piston seal will retract the piston to a 'zero drag' position and no further.

The reality is, however, that many calipers do suffer from an excessive amount of piston retraction which helps to produce a longer than ideal pedal travel.

This problem can be caused by refurbishing calipers with pistons that are slightly oversize of have a surface finish that increase the piston/seal friction. Austenitic Stainless Steel pistons also tend to increase retraction as they will expand around 40% more than original steel pistons.

In the limit pad knock back can be a real PITA and on race cars can result in having to 'double dip' the brakes with the resulting impact on lap times.

The 'modern' approach to eliminating knock back is to either use a small residual spring which will generate a force of around 4lbs when compressed or to use a very low pressure check valve.

The ATE pin/bush system achieves a similar result in a reasonably effective manner but the down side is that the piston cost is relatively high as the bush that fits to the piston muse be secure in its axial location but free to float radially.

I could easily understand a difference in machining cost of between 3-4 times for the 'damper piston' as they were initially described by ATE.

The pin fitted to the caliper was a steel part with a knurled end and was cast into place. They are difficult to remove and virtually impossible to replace once they have been removed. The only real way is to try to snap them out but this can damage the casting.

Removing the bush from the piston is more straightforward.

The piston is help with a Seeger Orbis retainer on the early piston and they are difficult to remove. The steel bush, however, has a brass sleeve inserted into the bore and these buses can be easily drilled out which will stop them working.

later pistons have the bushes held with a circlip and are easy to remove.
Depending on the knock back developed by the refurbished caliper it is possible that having one caliper with the knock back system and one without will cause a side to side imbalance during initial braking.

I don't agree with the caliper being 'better' without the knock back device. I do agree that it is cheaper to refurbish but you will have the potential from increasing the effect of pad knock back.

[AndrewSlater]

Thanks for the detailed response Chris, always helpful. I had read up on the subject which included some of your previous posts.

If the majority of companies offering caliper refurbishment remove the posts I would guess most cars are driving around with them long since removed.

Today I used my Dremel with a small diameter cutter to remove the posts in the odd one I have.
I've rebuilt the calipers with a Frentech kit that included pistons, and hope to get them fitted next week.

I will send the rears to ClassicBrake for restoration, and then front and rear will be the same.

Thanks again for the help.

Andrew

[AndrewSlater]

I have now finished the work on the brakes.

The fronts have had new rotors, rebuilt calipers and greenstuff pads.
I'd bought the greenstuff pads a matter of days before the DDK post questioning whether they were any good.
A few days later and I would have probably bought something else - but having bought them I fit them.




The rear calipers have been rebuilt by classicbrake and also fitted with greenstuff pads. The rear rotors are in pretty good shape so didn't feel these needed replacing.




I have also changed the flexi brake lines with a set of Goodridge braided hoses all around, as I realised I didn't know the history of my current braided ones and clearly you can't inspect the rubber element.


As I had the car in the workshop I also treat it to a service, valve clearances, new spark plugs, a change of oil and filters, distributor lube etc.

Trying to make the most of the nice weather I took the car out for an hours drive last night to try to bed the brakes in.
Fellow motorists must have been surprised by the old car out accelerating them and leaving them for dead only to brake suddenly in the far distance.

First impressions was that the brakes were slightly better from the go and after an hour of testing they improved further.
The car definitely brakes more confidently that it did previously so all the work has been worth while.
The pedal does seem to feel a little lower without the knockback mechanism but nothing I can't get used to.

The only issue I encountered was the car seems to have an occasional backfire on deceleration which wasn't as noticeable before, which I can only think is down to a valve clearance not being quite right - but that's one for another day.

Andrew

[fetuhoe]

One of the issues you may find with Green Stuff pads is that you may have a relatively long pedal.

In my experience EBC pads tend to compress quite substantially which will add to the pedal travel.

The position of the pedal shouldn't be influenced by the knock back device as all this does is prevent retraction of the piston so it is more likely to be the pad.

[AndrewSlater]

Thanks for the feedback Chris.

I had read your original response to a different post here
viewtopic.php?f=2&t=62779&start=15&hilit=vice

As I said I had just bought the pads a day or so earlier otherwise I would have gone for something else.

I'll see how I get on with them and worst case change back to something more standard.
I original had Pagids on so maybe go back to them to see how different they feel.

All useful advise and much appreciated.
Andrew