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MG Midget and Sprite Technical - Refurbishing Twin Master Cylinder

I've got three 7/8" twin master cylinders, and I've been cleaning them up and test-honing them to find which is the least pitted. One of them is very good, but it's also the rustiest throughout.

How would you go about cleaning it up? The bores are the easy bit.

BTW, my new Sealey multi-purpose hone set goes down to 3/4" dia. but the cone which seats the back ends of the hone legs is approx. 1" dia! So that wouldn't go the length of a master cylinder, would it? (it does now)
Nick and Cherry Scoop

Evaporust is what I use. Doesn't harm rubber parts and doesn't remove any good metal, only rust. You may be able to find it over there.
J Bubela

Unless you enjoy the challenge of messing about just buy a new one. Probably works out cheaper anyway - under 50 quid on EBbay.
f pollock

Nick
If you consider buying new then do first look at MGA Guru website regarding problems with TRW brand master cylinders.

Some three years back I had a complaint from a Frogeye owner who tried three TRW master cylinders before settling on one that worked O.K.

Your old master cylinder bodies, even if the cylinders are pitted, are probably suitable for sleeving by a Co such as Past Parts although the service is not cheap. They have sleeved items for me.

Alan
Alan Anstead

Yesterday, I fitted a TRW master cylinder on my MGA and had a problem. I bought it in 2010 and it has been sitting in a draw, so cannot say whether current production run is better.
For those that are interested, the problem is with the relief valve. When I press the brake pedal and then release it, the front discs remain locked for over five minutes. The rear drums release as they should, apparently because the springs are able to return the shoes, but obviously there are no springs on the discs.

Apparently there is a fix that involves drilling the tiniest of holes on the relief valve but I don't know the full details as thats not for me. I have bought a new Caparo master cylinder which I hope will work straight from he box.

Graham
Graham V

Graham, thats exactly the problem I had with the second TRW one I had. The first one leaked straight out of the box so was returned and the second one arrived and I had that problem. Solved by adjusting the rod until they just released. I certainly would not buy another TRW one.

Trev
T Mason

Graham,

With disc brakes the main piston seal acts to retract the piston back into the cylinder when the pressure is released. During operation the piston seal is lozenged and when the pressure is released to seal relaxes and due to its grip it pulls the piston back a small distance to release the grip. It may be in the case of the TRW master cylinder that the residual pressure is maintained at too high a level so preventing that action.
David Billington

David
I don't understand the detail but I'm sure youre right. Apparently with drums all round, it works ok, though I wouldn't recommend that master cylinder to anyone having read so much bad press, and based on my own experience.
Graham V

David,
Having just replaced the pistons and seals on my Golf, and having done the same on my Sprite I don't believe the idea that the seals act to pull the pistons back. The seals themselves are very soft and flexible and I very much doubt that there is sufficient energy stored when they "lozenge" to have any effect at all on the pistons.

Agreed, there is no spring to release the pistons (or pads). The beauty of disc brakes is that there is virtually no movement needed to bring the pads into contact with the disc, so pedal travel is minimised. What does release the piston is very slight run out in the discs. This should be very slight but that is all that is needed to ease the friction between pads and disc.

This is easily seen if, with the wheel jacked off the ground, you firmly apply and release the brakes. Then turn the wheel by hand. The initial resistance is quite high but as soon as it begins to turn the pads are nudged back, but still drag. But by the time a full revolution has been done the brakes should be fully released.
GuyW

Well, I definitely won't buy a new one!

I knew the answer really. Just start, and go on thinking of ways to clean it until it's clean.

Alan, I remember getting an old unit from you a dozen years ago, for the yellow Sprite, and sending it off to be sleeved. Can't remember how much it cost, but it was worth it at the time, because 3/4" models couldn't be found at reasonable cost: the MGA unit was nearly £300.

Nick and Cherry Scoop

Can someone explain the issues concerned with the two m/c cylinder sizes. I have the 3 options of building my frog with either the original twin leading shoe brakes, a similar twin shoe set up but using 8" drums or fitting discs.

I have read that if using discs, on would ideally use the later m/c, but also that this isn't actually essential. So how do the m/cs differ?
GuyW

Guy

The original 7 inch drum set up on a Frogeye uses a 7/8 bore master cylinder.

The 8" modified system uses a 7/8 bore master cylinder. Back plates are taken from an Austin A40 Farina as are the slave cylinders, shoes, springs, and drums. This is what I have had on my Frogeye for the 48 years that I have owned it.

The disc brake set up uses a 3/4" bore master cylinder. The master cylinder pushrods have to be shortened by 1/2".
The rear brake cylinders are changed to Morris Minor 3/4" bore to give a balanced system. There are those that use a 7/8' master cylinder with discs but in the past there were complaints of a "pedal like porridge".
This is the system that I fitted to Gary Lazarus' Frogeye at the NEC. To fit discs also means a change of hubs / stub axles etc etc perhaps even wishbones if they cannot be separated from the stub axle. What was once a cheap conversion can get expensive these days especially if fitting new items. Whilst doing the dry build on the bench, at home, prior to the NEC it was surprising how much fettling had to be done.

Depending upon which clutch slave you use it is possible to have a master cylinder re-sleeved with 3/4" bore for the Discs and 7/8" to suit the original 948 slave cylinder.

MGA Guru website outlines the problems of residual pressure with TRW brand master cylinders and how to make modification to correct.

Alan
Alan Anstead

Thanks Alan,
As I say, I have the components for all three alternatives, but wasn't sure about the possible interchangeability of the master cylinders.

You state that the original Frogeye and the 8" modified system both use 7/8" bore master cylinders. Are they in fact the same component or is the 8" one of the same bore size but a different item taken from the A40?

If the disc braked cars use a smaller diameter cylinder, then it will displace less fluid but at higher pressure, unless the stroke is also altered to compensate.

I have 2, or maybe 3, master cylinders in the boxes of parts that came with the Frog. I will need to give them a good check over to see what I actually have!
GuyW

"Are they in fact the same component or is the 8" one of the same bore size but a different item taken from the A40?"

They will be the same item. The A40 used separate 'tin can' master cylinders, of a similar design to the Mini.
Dave O'Neill 2

Guy
As Dave says the A40 Farina had tin cans. The 8" conversion uses the standard Frogeye 7/8 bore master.
The two types of Master have different strokes requiring 1/2" to be lopped off the Master to pedal pushrods on both clutch and brake cylinders when converting to discs.
Alan
Alan Anstead

A picture of a disc brake conversion being done on the midget and Sprite Club Stand at this year's Practical Classics Show NEC with assistance from Ian Cooke and John Clark..
We replaced the wishbones and fitted polly bushes as well.
And for next year's show........
Alan


Alan Anstead

Alan, what is wrong with using rear drum brakes from a later Sprite/ Midget if converting to front discs. That's exactly what I intend to use on the Frogeye together with a 3/4" tandem master cylinder. As I understand it, the 3/4" cylinder (from the 1098 Spridget) is designed to work with disc calipers (to release the pistons) whereas the 7/8" one isn't.
Bill
W Bretherton

This brings back happy memories of just how confused I was last time it came up. In the end, it was clutch performance that drove me to a 3/4" master on my last Sprite. The gearbox and slave were from a 1275, and my 7/8" master threw the slave pushrod too far. Don't remember the "pedal like porridge", but then it was my first experience of discs.

Why do the pushrods need shortening, Alan? I remember doing it, but can't remember why.
Nick and Cherry Scoop

Just emerged,blinking into the sunlight, from the depths of my garage. I have two of these dual master cylinders, though there could be more of them in there!

One consists, so far of a cast steel body and a couple of piston things with a domed head at one end and a disintegrating rubber seal at the other end. Its grubby, with some surface rusting but the bores look like they would probably clean up and hone ok. The bores are 7/8 " and on the side in raised cast letters it has a casting number (?) 38183C and says LOCKHEED 7/8. But so far there is no top cover, filler cap or end plate.

The other was wrapped in newspaper and well oiled, complete with the oblong pedal box, all powder coated in black with a red top cover and plastic filler cap. It also says LOCKHEED with the same casting number but no size on it. It measures at 3/4" bore. Its not new, but clearly refurbished. The problem is this one doesn't have the double ended piston thingies with it.

Tempting to locate some pistons and use the 3/4" one. Excepting I was leaning towards using the A40 8" drums as I thought it was that bit nearer original for a Frogeye. My guess is that the 3/4 bores wouldn't displace enough fluid for all those drum brake cylinders.

As Nick points out, the decision may be determined by which clutch I use. I am probably heading towards using my spare 1098 engine.
GuyW

When I got my frogeye on the road I kept the 7/8" master cylinder and just fitted the discs to the front, it may be a happy coincidence that I think the local parts supplier provided MM rear cylinders. I never had any issues with the set-up the brakes seemed to work fine. Eventually the master cylinder needed replacing so I fitted the 3/4" intended for the disc set-up and it made the brakes lighter but it was never an issue before. I only later heard that the 7/8" wasn't ideal with discs but didn't encounter any issues myself.

The worst car I've ever driven for heavy brakes was an early VW Golf, apparently they had a reputation for it, the owner said she planned ahead regarding braking.
David Billington

Nick
The drilling in the 3/4 piston is 1/2' shorter and the original length pushrods will not fit if everything else is as is.
Bill
Using later backplates and double acting pistons is another route defending on what parts are to hand.
Guy
The only problem I have ever encountered with the 8" drum set up was brake fade, on stock linings, after a day traversing the (Austrian?) Alps some years ago. Probably why I went up the Italian Stelvio pass, in the car, and not down it: the down side, to Bormio, not being so steep.
Alan
Alan Anstead

Just to add to the confusion, I have: 7/8 m/cyl, calipers on the front, 3/4 on the rear, frogeye clutch slave, frogeye flywheel, rib-case, 1275 clutch fork, spitfire 1200 9-spring clutch cover and all I had to do was lengthen the clutch slave push rod by about 1 inch. Everything works fine. Never had any brake issues.
Simon Wood

I just pity the next owner when they need to buy any spares for it !!
GuyW

Thanks Alan. I like a simple answer.
Nick and Cherry Scoop

I use the 7/8 MC with the later sprite disk set up, Mintex 1144 pads and double cylinder rear brakes. The only modification was to remove the valve in the MC and replacing the rear slaves by smaller ones. Better tires (after 60 years of rubber, profile and construction development) need an other brake balance than 60 years before. In the original set up the rear will lock first.

Flip
Flip Brühl

Twin master owners - do you paint it?
Nick and Cherry Scoop

I did use Lacquer when I used dot 5. Now I use dot 4 I smear some Waxoil or Owatrol on it every now and then.
Flip
Flip Brühl

Thanks, Flip. That sounds good for Dot 4 users like me.
Nick and Cherry Scoop

I used halfords petrol resistant lacquer on mine and it still looks fine after 3 years of using DOT 4.
Bob Beaumont

At the moment I haven't finished work on the bores, but I have cleaned it up to a nice burnished cast finish, and it's had the oily rag all over while it waits for a decision.

Do you think the lacquer has actually resisted Dot 4, Bob, or are you simply very fastidious about never spilling a drop when filling and bleeding?
Nick and Cherry Scoop

I cannot check just now but l think black hammerite resists dot4. Certai nly the "extreme chassis black" that Frost / Eastwood's supply does.
GuyW

Does anyone know -
are the two double ended dumbell-shaped pistons the same item fitted in the 7/8 and the 3/4 master cylinders, just with different sized seals? Or are the pistons themselves different too?
GuyW

Guy
The pistons are different by virtue of diameter and the 3/4 is drilled 1/2" shorter. I don't have the different items to make further comparisons.
Alan
Alan Anstead

Thanks Alan
GuyW

I am careful when I top up or fill the cylinder but have spilled a little occasionally. I am careful to wipe it off but the lacquer has remained intact.
Bob Beaumont

Hi

Pictures of 7/8 and 3/4 twin master cylinder pistons here:
http://www.gerardsgarage.com/Garage/Tech/DbrakeUg.htm

I have a 3/4 brake and 7/8 clutch twin bore single master cylinder setup using the correct pushrods on my 948cc MkII with disc brake upgrade, also using 1098cc Morris Minor rear brake wheel cylinders.

Useful info: http://www.gerardsgarage.com/Garage/Tech/DBrake/TandemMC_Part1e.htm
M Wood

Thanks Mike, for those excellent​ links. They enable me to properly identify what parts l have, which is the first step for me.

Reading that, it sounds like those who experienced non releasing discs with the TRW masters had the wrong type of valve fitted.
GuyW

Can someone explain the difference between a 1098 MM rear cylinder and a Midget 1098 rear cylinder? Are the drums and shoes different? Thanks
J Bubela

Guy

I think you may be correct about the wrong valve and non-releasing brakes.

J Bubela - Alan Anstead will be able to explain better, as it was he who helpfully suggested to me using 3/4 inch Morris Minor rear wheel cylinders when upgrading to front discs on my formerly drum braked 948cc MkII Sprite (early 1962 year). Drums the same.

As Alan explains in a post above this is so can use a 3/4 inch rear wheel cylinder to match the 3/4 inch master cylinder for brakes. The Frogeye/Bugeye and early MkII Sprite have drum brakes all round with rear brakes with single acting single rear wheel cylinder. See pictures here: http://www.gerardsgarage.com/Garage/Tech/DBrake/rearbrakeUg.htm

To get the correct bore rear wheel cylinder either fit a later backplate for a later single rear wheel cylinder of the correct bore - a Midget/Sprite item - which is a dual acting cylinder or do as Alan suggests - fit the correct bore item that fits into the earlier backplate (this wheel cylinder only found on a 1098cc Morris Minor, never on a Mk1, MkII Sprite or MkI Midget)

Cheers
Mike

M Wood

3/4 inch Morris Minor rear wheel cylinder Lockheed part number is: LW11287

http://www.morrisminorspares.com/brakes-c42/rear-brakes-c50/rear-brake-cylinder-1962-1971-3-4-bore-genuine-a-p-lockheed-can-be-used-as-replacement-for-7-8-bore-click-for-details-p829512
M Wood

For comparison 1964 onwards rear wheel cylinder:
http://www.leacyclassics.com/parts/austin-healey/healey-sprite/brake-system/gwc1102.html

(NB front drum brakes mkIand MkII sprites have single acting cylinders, but not 3/4 inch bore: http://www.leacyclassics.com/parts/austin-healey/healey-sprite/brake-system/gwc1114.html

The 3/4 inch bore found on a Morris 1000 model of car rear wheel brake cylinder - confusingly these cars had a 1098cc engine!
M Wood

If the master cylinder body is already in 'rust' applying Kurust will turn the body a nice purple/black which seems unaffected with spills.
f pollock

Brilliant idea, Fergus (I seem to remember it's fergus from years back). I reckon I might do that - these master cylinder bodies are always rusty, even if you can't see it.
Nick and Cherry Scoop

I am converting a '61 to disc brakes. I now understand what needs to be done to the brakes. The clutch is now the question. The 948 engine and smooth case gearbox will be retained. I am going to use a 3/4" MC. The slave for the 948 and the 1098 is the same but the MC bores are different. What effect will changing from 7/8" to 3/4" have on the slave cylinder? Is this just a change in pushrod length? I have the book but haven't found the answer. Thanks to everyone for the information, this is the first time I have added disc brakes to a MKI.
J Bubela

John (?) Changing from a 7/8" master cylinder to a 3/4" one will make the pedal lighter to press but will reduce the fluid displaced so the clutch slave cylinder wouldn't move as far and the clutch might not fully disengage. The solution might be to use a slave cylinder from a later 1275 car as although they appear the same the later one is 7/8" compared to the earlier 1" diameter ones.

The length of the pushrod makes no difference to the amount of fluid displaced but as mentioned earlier it may need altering to match the design of the later M/C pistons used
GuyW

IIRC the flexible hose for the slave cylinder has a different thread, so that would also need changing.
Dave O'Neill 2

Yes, there is an adaptor available.
GuyW

GuyW - changing from 7/8" to 3/4" bore will NOT reduce the fluid displaced (this will be the same in both cases). With the 3/4" bore the pedal will travel a greater distance. This gives a greater mechanical advantage and thus a lighter pedal.
J.E. Davies

It won't be the same for the same pedal travel. On mine the pedal travel is limited by the floor.
GuyW

Guy: Where is this is adapter available? JLH and Magic Midget do not have them. Waiting for a reply from Peter May.
J Bubela

To pick up on earlier points about quality of new twin master cylinders, you can buy the correct AP Caparo twin 7/8 master cylinder (for Mk I Sprite and early MkII Sprite and MkI Midget all on drum brakes) and the twin 3/4 version for late MkII Sprites and MkI Midgets on front discs from Powertrack Brakes: http://www.powertrackbrakes.co.uk/

In terms of unions (J Bubela question above) to switch to later master cylinder (NB chopping footwell) try Powertrack or racing car parts suppliers in the UK such as Merlin Motorsport, Raceparts or Trident Racing Suppoes - look on their Girling brakes sections of their websites.

Best wishes
Mike
M Wood

I can't believe that the cup in the 3/4" cylinder travels 20% further than the cup in the 7/8" cylinder. Is the piston shorter?

As Guy says, all the rest of the gear is identical, with no additional leverage anywhere.
Nick and Cherry Scoop

Nick, I don't know if the piston is shorter, but I don't see how that could make any difference. The amount of fluid moved is determined by the diameter of the piston, and the start and finish positions of the piston seal. The piston behind the seal could by 1/4 mile long, its the distance of movement that matters.
GuyW

The 3/4 piston is drilled 1/2" less than the 7/8" and uses a 1/2" shorter pedal pushrod.
Alan
Alan Anstead

So no difference there, in effective travel.
Nick and Cherry Scoop

It could make a difference, Guy, because the cup would be able to travel further along the same length cylinder. IF more pedal could be applied, which it can't.
Nick and Cherry Scoop

Ah, so the distance that the piston (cup) moves is limited because it bumps up against the end of the cylinder? That sounds bound to cause self-destruction!
I thought the travel was controlled by how far the pedal moves. And on mine that is limited by the floor.

Certainly, at the other end of the system, the slave cylinder piston only moves along a fraction of the available 2" or so of the bore. It is designed this way so that as clutch wear occurs the slave progressively uses a different 1/2" length of the bore, making the clutch self adjusting.
GuyW

You're in a mischievous mood. A shorter piston could move to the same finishing point in the cylinder, but the cup would have started its journey further back.
Nick and Cherry Scoop

Very true, Nick. starting further back would mean it had more available travel without destroying itself with a head on collision with the end of the cylinder. But I still think that the actual distance it moves depends on the pedal travel, limited by the floor.

Unless there are differences between the two versions, in the length of the short lever lug welded to the pedal arm.

The other exciting thing to think about is what makes the M/C piston return to its "at rest" state, and how far back up its travels is that!
GuyW

The 7/8" pistons (top in photo) are the same length as the 3/4" pistons (bottom). A 7/8" pushrod is longer because its hole (in the piston) is 1/2" deeper than the hole in a 3/4" piston - probably because a long hole in the smaller piston would overly weaken it. In other words, if the correct pushrod is used, the COMBINED length of the piston/pushrod combination is the same. For more details, see MASCOT Jan 2008. Part of the trouble here is that MO$$ etc list the longer 7/8" pushrod for both master cylinders. If this long pushrod is used with the 3/4" master cylinder, the brakes will work 1st time but then never release. The inevitable result will be red hot brake drums and a car that will fail to proceed. Also note: when chopping a 7/8" pushrod to 3/4", the end needs a very slight taper and must be rounded, just like the original.



J.E. Davies

This thread was discussed between 21/06/2017 and 14/07/2017

MG Midget and Sprite Technical index

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