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MG MGA - Dunlop disc brakes

Again seeking advice. Just been attempting to fit new Mintex pads to my Twin Cam which, being new, are a bit thicker than the old ones, naturally. The piston retraction tool shown in the workshop manual doesn't look particularly special so I assumed a similar sized piece of steel (a large screwdriver!) applied carefully to lever the piston inwards should do the trick. However, a reasonable amount of force made no movement so I stopped for the day to take stock.
Question - am I missing something here? Should it take a strong leverage to move?
Bruce.
Bruce Mayo

Bruce,
You need quite a bit of leverage AND a parallel force, otherwise the piston will not move easily.

Don’t forget to extract some brake fluid from the master cylinder first also. Otherwise it will overspill when the pistons go back.

Regards
Colin
Colin Manley

Three Wheatabix!

You could try spraying behind the pads and the brakes generally with brake cleaner to clean and wash/flush dust/crud.

Then careful use of a good penetrating or releasing fluid (not ordinary WD40), to break and/or make the caliper pistons travel easier. I favour GT85 if it's more to lubricating that's needed or PlusGas if it's more to penetrating needed and add GT85 later.

Personally I'd spray then walk away and make a big mug of tea to let the chemical soak in and do its work before trying to use the metal leverage tool.

I don't know if taking the reservoir cap off would make any odds but you know Tesco's advertising slogan.

If you do a good enough job you might need some clothes pegs whilst you fiddle about with anti-squeal shims and/or copper grease or deal with the effects of drinking a big mug of tea.

Once you get the new pads fitted you want to work the calipers to keep them freed up, easiest way is to drive the car and use the brakes with varying pedal travel, i.e. just drive the car regularly on interesting roads at interesting pace.

What are the inside of your flexi-hoses like. Put the kettle on.


Nigel Atkins

Rather than extracting fluid from the master cylinder, I prefer to crack open the bleed nipple when pushing pistons back - not that I have a Twin Cam with Dunlop disc brakes, but just as a general principle.
Dave O'Neill 2

just checked on this and it looks like MG used this special tool to lever the pistons back into the Dunlop calipers.
http://mgaguru.com/mgtech/tools/18G553.htm

On my Lockheed calipers, if I am not removing the caliper from the hub, I remove JUST ONE worn brake pad and then I then refit it the wrong way round so that the friction material is against the piston. This leaves the pads back-plate facing the disc.

I then slip a piece of scrap thin metal into the caliper against the disc to protect it from damage.

This is so that when you insert something in between the disc and the back of the pad to lever the piston back into place, all force applied to the piston is cushioned by the old pad material and it makes certain that the force is applied evenly onto the piston with no damage to it.

You need not worry about damaging the disc because it has the thin shim of metal protecting its surface and the other brake pad is supporting it firmly in place from the other side of the disc.

Once the piston has been completely pushed back inside the caliper, you then remove the reversed old pad (and the thin piece of metal protecting the disc) and fit the new pad.

Then repeat for the other pad.


On the Lockheed caliper you can use a small G-clamp to press the pistons back into place instead of using a lever.

This is a picture of the G Clamp I use.

Is this method going to any work on your Dunlop calipers? The basic principles seem to be about the same.

Cheers
Colyn



Colyn Firth

Thanks you guys. All helpful stuff. Will try again later.
Bruce.
Bruce Mayo

NO no no no
You should never lever off the rotor to push pads back
Remove the old pads and use a tool like this-
----Disc brake piston spreader----

https://www.ebay.com.au/i/142116242213?chn=ps&norover

ebay uk would have something similar
William Revit

This one looks better from a design perspective.
I wonder if it retracts enough for a non ventilated disc and totally worn pads! (MGA)
Art Pearse

Yep
They'll screw right in till the paddles touch each other if you want and will push both pistons out together
or you can just do one piston by having one paddle in the pad hole and the other on the outside of the caliper--they screw either way
William Revit

William
That piston spreader looks to be perfect for the job, also I can't believe the low price, I doubt if you can get a basic g-clamp for that price over here. I can see why using the spreader would be the preferred method.


As for levering the pistons back, I have changed an awful lot of mga pads over the years using the method I described and I have never caused any warping or damage to the disc (rotor).
Maybe this has been the case because I have never had a totally seized piston and so they have always pushed back in fairly easily. (Or maybe I have just been lucky)

But I will see if I can get one of those piston spreaders to keep handy for my next pad change.
Cheers
Colyn
Colyn Firth

Looks a bit like my wood clamps. Got about 6 of them.

Steve


Steve Gyles

Steve,
Technically that's an engineer's/Toolmaker's Parallel clamp.
But if it works, what the hell......I like the correct names!!
Allan Reeling

Forgot to post my URL
https://www.wish.com/product/5b2c93ca53d6cd738ac0ab90?hide_login_modal=true&from_ad=goog_shopping&_display_country_code=CA&_force_currency_code=CAD&pid=googleadwords_int&c=%7BcampaignId%7D&ad_cid=5b2c93ca53d6cd738ac0ab90&ad_cc=CA&ad_curr=CAD&ad_price=19.00&fallback_cids=5b63da0cda1006073bbecd315ba30bd51a12186a637b09425a17f00dff96b67b5876deaf5c14b07becc69664545825bd5b909a85178ef54e116b73c85b63bbdbccc9430969b9c3f5&campaign_id=1643927883&gclid=EAIaIQobChMIy5zsjs6E4wIVBdvACh0YYgy-EAQYAiABEgK_evD_BwE
Art Pearse

An update and a further question.
Firstly, I think some of the suggestions like a conventional g-clamp are a no no for the twin cam Dunlop caliper. Though I do like the parallel flat clamp suggestion.
The Mintex Classic pads duly arrived but the nicely engineered bonded on backing plates (much better than the original fall-off ones!) had unfortunately a slot width about 1mm too narrow to engage with the “dimple” on the piston plate (twin cam owners will understand). Questmead who are supplying these instantly owned up and have arranged for return, re-manufacture and return.
But, here is the question from an ex-engineer - why all this complication of a backing plate with a slot which engages with the piston “dimple” (sorry, can’t think of a better word!) which just makes it a pain to remove and re-insert the pads? I can see that the pads will be pulled back by the piston after releasing the brakes, but why bother? Could not the pads just sit happily in the caliper, with some sort of simple backing plate to take the load, and be pressed and released during normal use? Perhaps I am missing something here!
Bruce.
Bruce Mayo

I have just found and ordered the same item on UK ebay, you can get them from as little as £6.49p on there.

This is one of the lowest price ones
https://www.ebay.co.uk/itm/DISC-BRAKE-SPREADER-PISTON-PAD-SPREADER-SEPARATOR-CAR-CALIPER-HAND-TOOL/273443004087?hash=item3faa791eb7:g:OcwAAOSwTgJcanMz

Thanks William
Colyn Firth

Colyn

There's an interesting footnote at the bottom of that ebay listing:-

"Due to the current weather conditions,parcels to most areas in the UK will be delayed for some days .The courier is working hard to return service to normal as quickly as possible.We appreciate your understanding and patience!"

I know it's been a bit wet today, but really?
Dave O'Neill 2

Maybe they should switch to using waterproof ink on the address labels :^)

Colyn Firth

Bruce

The calipers on MGA T/C are very early versions of disc calipers and basically over engineered, but at the time of manufacture were the latest of the latest

Basic explanation-
After the brakes have been applied the piston needs to retract the pad to give around .006"-.008" clearance between the rotor and pad
On your car that pin and fittings in the piston that attaches to the back of the pad is how they did that back then, weird sort of a springy setup but it works

Later disc calipers have a square section "O" ring in a tapered groovein the caliper--the O ring deforms when the brake is applied and when released the O ring returns to it's normal shape, dragging the piston back a tiddle to give clearance

Just 2 different methods, you have to think of yours as special-------

Found this for you to have a read of--

http://mgaguru.com/mgtech/twincam/tc207.htm

Cheers
willy
William Revit

Thanks Willy. I do understand the subtle difference of the way that these early Dunlop calipers retract the piston to give a running clearance between the pad material and the disc, compared with the way that is achieved in a modern caliper. I get that. However, the point I was trying to make is that, whichever caliper we consider, the principle is the same that the piston will retract to take pressure off the pad to disc interface, with the modern caliper just leaving the pad sitting free but captive in the caliper body whereas the early Dunlop caliper maintains a physical connection between piston and pad and actually pulls the pad back as the piston retracts. My point was why does it bother to do that? It would surely be simpler to have no physical connection between piston and pad and just leave the pad to “float” loose in the caliper body like a modern version.
You might ask why not just accept the way it works, which of course is what I intend doing, it’s just that the Dunlop type pads’ backing plate slotted on to the piston plate is a source of frustration when struggling to remove them, rather than just pulling a floating-free pad out. Not a big deal, just my observation on the subject!
Bruce.
Bruce Mayo

Ahh
Sorry Bruce, I see what you're asking now
It certainly would have been easier to change pads if they had terminated the retractor mechanism at the outer piston instead of carrying it right through to locate in the pad , it would have still worked and just left the pad there to float as you mention
Just a bit of early over engineering I guess
Cheers
willy
William Revit

Allen

Thanks for educating me on the engineers parallel clamps. I inherited them from my late F-in-L. He was an engineer from the 30s to 70s running his own business. He was skilful with both metal and wood. I had always assumed they were from the woodworking side of his work. Now I know! Many thanks.

By the way, his business initially made all the twisted paper cord used for upholstery piping and door fixings (hammering door cards into it) in the motor trade. Then he made the packing pellet (compressed paper - saving on valuable metal at the time) for every .303 round fired by the Allies in WW2, including the BoB. Then he made static caravans in the post war years.

Steve
Steve Gyles

Willy,
Just to clear up a point that you made.
The retractor pin only passes through the hole in the end plate of the piston. Then through the coiled bush and finishes inside the piston.
That end of the pin does not connect to anything.

Mick
M F Anderson

Cheers Mike-
willy
William Revit

Mick and Willy.
Just to be totally clear on this (for me anyway!) - are you both agreeing with me that physically attaching the backing plate of the pad to the piston plate “dimple” by sliding the backing plate slot onto it is probably engineering overkill and a free floating pad would have sufficed? In fact, thinking about it as I write, I now remember that at least two of my pads have backing plates which became detached on a previous occasion when they resisted being withdrawn and came apart. The brakes have obviously been working fine for some time now with some of the pads free floating in their callipers and loose on the backing plate. Maybe that answers the question, but do comment/ confirm.
Finally, when talking to the supplier, Questmead, and the Mintex development guy, they did say that physically attached backing plates are standard on Dunlop for road application users but the racing guys do not use them.
Best
Bruce.
Bruce Mayo

The retractor pin is unnecessary and is missing in most Twin Cams.
If fitted I remove it. Seal deformation retracts the pads.
The slotted, thin backing plate has often come loose and has been. discarded.
However, if your new pads have a plain steel backing plate you must drill a depression for the nipple to fit into or grind the nipple of the piston.
You cannot have just the nipple pressing on the backing plate.

Mick
M F Anderson

Yes and no
Yes I agree it's an overengineered unnecessary system that's been overthought just to get the pad back off the disc which is something that happens anyway without it
but
There's a part of me that says if the whole thing is complete on your car and can be assembled as designed, it's just sort of nice to know in your own head that your car still has it all others don't
Just a semi purist thing I guess which isn't me anyway
Bit like wearing a watch with actual hands on it instead of the usual digital, still tells the time just different
You're lucky Mick came on board --he gives good advice
Cheers
willy
William Revit

That's perfect guys. My car has the pin and nipple still so, now fully understanding the issues, I will just leave it alone and persevere as original! Thanks again.
Bruce.
Bruce Mayo

Mick is dead right - they thought they needed to retract the pads from the rotors; in fact they need to try and keep the pads NEAR the rotors as pad kick back happens any time you corner hard given the end float on (mostly rear end) bearings.

For pressing pistons back into the caliper halves when fitting new pads, I have always found the really large channel lock or water pump pliers do the trick. You need the large ones so the force is more or less parallel and you aren't trying to cock the piston in the bore.

Be aware that while backing plate design has been improved on modern pad versions, not all of the manufacturers seem to have an accurate dimension for lateral size and we have heard that some need to be slightly reduced in size or 'eased' to slide in properly. Needless to sy, the first think to do if that appears to be an issue is to make sue the caliper is clean and there is no build up of crud impeding pad insertion before you star hacking up the pads.
Bill Spohn

Bill. Thanks for that. Understood.
So, given a simple choice of using my new Mintex pads with the, now, correctly dimensioned slot in the backing plates as per original Dunlop 1959 design, or, grinding the slot a couple of mm wider to prevent physical engagement with the retractor pin nipple, which would you recommend? I was intending to stay “original” but now have the option before putting everything back together.
Thanks
Bruce.
Bruce Mayo

Final update. The Mintex Classic pads are installed, bedded in and feel nice and progressive. Very pleased. The old ones, green stuff, worked ok but not much initially then started to work well with lots of pressure. Mintex seem better and predictably progressive.
However, the slots in the backing plates needed easing a touch wider with a Dremel, equal to the slots in the old ones, to make them fit. The supplier swears blind that they fit perfectly on Jag Dunlop calipers without mod, so I don’t understand that, but happy that mine work ok. After lots of chats with the experts it seems the connection between the retractor pin nipple and the pad backing plate slot is irrelevant anyway so I’m not too bothered anyway.
Bruce.
Bruce Mayo

Sorry I’ve come to this subject a bit late, but from working on my Twin Cam since 1964 be aware of the retraction tools mentioned if your pistons still have the retraction ‘dimples’.
Only use that type shown with a worn pad that protects the dimple from being squashed by the retractor.
Remember also if you haven’t changed the brake fluid for a while (I change mine every 18mths) crud and rust is likely to have damaged the cylinder bore – even sized the piston.
The safest way to avoid that is when replacing the fluid ‘exercise’ the piston by removing the pads, fully extending piston travel then retract it. Do this several times on each wheel cylinder. With Silicone fluid this may not be necessary and I would be grateful on member’s views on the performance of that fluid.
Cheers
John.
J Hughes

Glad you have found the Mintex M1144 pads to be as good as I did, they have been excellent on my Lockheed brakes and they give much better braking than the Greenstuff ones.

Cheers
Colyn
Colyn Firth

FWIW, the factory held the same theory in regard to the Lockheed calipers used on the pushrod MGAs. They provided a pin inside the caliper half that rode in the back of the piston.

The replacement pistons for those calipers cost $45 at Moss. If you break off the pin and use MGB pistons, they cost $8......
Bill Spohn

This thread was discussed between 24/06/2019 and 04/08/2019

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