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MG Midget and Sprite Technical - Disc brake conversion

Just a general question, following a discussion I had with someone.
If converting drum brakes to disc is the 7/8” master cylinder retained or replaced with a 3/4” one.
Thanks
Les
Les Robinson

Hi Les

it is normal for the twin master to be replaced with the later Mk111 sprite 3/4" MC. To retain balance the rear cylinders should also be changed to later Morris Minor 3/4"
Bob Beaumont

As Bob says. The 3/4 MC will enable lower pedal effort plus it relieves pressure when pedal released to prevent slow pad release AIUI.
Bill B

I fitted later disc brakes to my frogeye when I rebuilt it and got it on the road in 1985 and used the 7/8" MC as that's what I had and it was all fine and never noticed any issues. At the time the 3/4" disc MC was NLA as were the correct rear cylinders so they were changed for the Morris Minor ones as that's what the local factor had. I was aware that the 7/8" MC would give a heavier pedal than the 3/4" MC but it never seemed to be an issue, it is after all a light car. When the 7/8" MC needed replacing the 3/4" disc MC had become available again so I fitted one of those and while a lighter pedal it wasn't a great difference. Only years later did I hear about the residual pressure device being present in one of the systems.
David Billington

Thanks for the replies.
I converted to disc brakes years ago and fitted a 3/4” master cylinder. I did find that the clutch bite was nearer the floor but not an issue. This is due to the difference in slave (7/8”) and master (3/4”) diameters and their effective areas.
Also fitted Morris Minor 3/4” rear cylinders.
Only asked the question as someone raised the topic with me so just checking what other people have done.
Thanks
Les
Les Robinson

With a 948 clutch the original MC is 7/8 so, with a 3/4, you'd think the bite point will be lower, because the required pushrod travel will occur later. I have a 1275 in the Frog so I fitted a 1" slave as that is the correct spec. I believe the later individual 1275 MC is 0.7" so my 3/4" is close enough - well it works OK.
Bill B

Drum Brakes to Disc Brakes: converting midgets & Sprites.



A few years ago I, assisted by John Clark & Ian Cooke, demonstrated how to convert a drum braked Frogeye to disc brakes at the Practical Classics Show NEC. It appears that nothing has been written, nor archived for reference, on the various options available especially with regard to choice of master cylinder.

Firstly do you want to convert to discs or would a brake shoe material upgrade suffice? Mintex / Questmead (enquiries@mintexclassic.com) can reline brake shoes with a different grade material to suit your requirements. If your ageing legs cannot push the brake pedal with the same force as when in your athletic youth then servos are available to fit.

How to fit disc brakes depends upon the state of the recipient vehicle and its engine size. There are four basic master cylinder options.

1) If the car is at the restoration stage and under bonnet
aesthetics are not a priority it is possible to cut the sloping flitch panel and, with suitable reinforcement, transfer the master cylinder assembly from a later disc braked Sprite or Midget.

2) If the car is fitted with a 948 engine and gearbox, and these are to remain, its original 7/8 bore dual master cylinder can be retained and the brake port can be sleeved and reduced to 3/4” bore to suit the disc brake hydraulics. I have used www.pastparts.co.uk, in the past for fitting stainless sleeves to hydraulic cylinders. For the cylinder at 3/4” the pedal pushrod will requre reducing in length to 3.75”.

3) A 3/4” bore dual master cylinder can be fitted.
( Beware that there have been issues with TRW brand dual master cylinders: see the MGA Guru website ). The pedal pushrods will both require reducing to 3.75”

4) Use a Sebring pedal box with individual master cylinders that allows a choice of cylinder bore. You will require 3/4” bore for the brakes but either 3/4” bore if your clutch slave is 1” bore or 0.875” if the slave is
0.875”. Sebring pedal boxes are available from www.brookfab.co.uk.
T:07548020508 (As at 5/23: £475)

Now to the rear. My personal choice is to fit 3/4” bore Morris Minor rear brake cylinders, part No RBK106 with large washer RBK121 & small washer RBK122, as a direct replacement for the stock items. East Sussex Minor T 01580-200203 or David Manners Group can supply.
It is possible to take the backplates and slave cylinders with shoes from a disc braked car and fit complete onto the rear axle.

Once these choices have been made work can start but do be aware that you are altering safety critical items and ensure that you have an understanding of what you are doing before attempting. Also with a nod to Health and Safety be careful when using tools especially jacks and lifting gear. Several brake line joints will require copper washers to seal so ensure that the washers are imperial and consider annealing them, to soften, before fitting.

At the front of the car the later kingpin / axle assembly will need substituting along with the disc braked hubs. Later kingpin / uprights are available from Moss for outright purchase BTA744 & BTA745 ((handed) £175 ea 5/23)).

A later Workshop Manual will be a guide for fitting but Dave Sewell also provides advice in an article about his fitting disc brakes to his Frogeye in Mascot April 2019 pages 20-22. The archive of past Mascot magazines is available to Masc members on the Club Website although you may require the assistance of Webmaster Mike Bennett to access it. There is an index to technical articles too!


Of course when I demonstrated the conversion, on Gary Lazarus’s Frogeye at NEC, I had, in the weeks running up to the show, fettled various parts and dry fitted, off the car, to ensure that no issues would arise at the show. If the conversion is to use second hand parts then they will have to be checked perhaps restored or replaced so that the conversion can escalate as the old kingpin may not separate from the lower trunnion requiring new wishbones that in turn need more disassembly and perhaps a change of trunnion bushes. Secondhand hubs may require new front wheel bearings that then need an understanding that not many FWBs supplied, even by Sprite & Midget specialists, are ‘face adjusted’ or have the correct radii so if not experienced it is easy to ‘open up a can of worms’.

S/H front hubs are available from Andy Jennings (Spridget Breaker) together with steering arms BTA648 & BTA649.
S/H hubs hopefully will come complete with bearings and wheel studs. A new seal will be required after the bearings have been examined cleaned and re-greased. Buying new hubs will add to costings.

MGB Hive have most in stock:

Brake disc backplate BTA473 (LH) BTA472 (RH)
3/4” Master Cylinder GMC112 (check if TRW see above)
Brake pads can be of any manufacture as preferred; stock, EBC, Mintex?
Steering arm lock tabs (2)
Caliper Banjos (2) and associated copper washers
Calipers (outright) 17H9438/9 LH/RH (£85ea 5/23)
Brake pad pins and clips 17H7963 (£3.50 ea)
Anti squeal shim (4) 17H2460 (£1.00 ea)
Brake Discs (2) BTA383 (£12.95 ea) or Brembo SKU 317 from Peter May @ £58.75 ea (I am currently bedding in Brembos with EBC green stuff pads)
Disc to hub bolts (8 off) at £4.00 ea
Caliper to upright bolts (4) plus lock tabs (2).
Brake hose kit GBH157
.





Alan Anstead

Thanks Alan, very detailed and informative.
At the clutch slave cylinder, should there be some free pay at the push rod? I seem to recall that there should be but don’t know why - if the slave piston moves the push rod then I would have thought that the slave piston is only moved back as far as the lever pushes it. I have no free play but the clevis pin is free to rotate so assume no load is being applied to the lever / thrust bearing when the clutch pedal is released.
Just checking as my master cylinder is a TRW!!
Both the clutch and brake pedal pushrods are set to give 1/32” clearance to the stops.
Thanks
Les Robinson

A couple of questions:-
Alan Anstead stated that the pedal pushrods will need to be reduced in length to 3.75” when using 3/4” master cylinder - why? According to Moss catalogue the 7/8” and 3/4” pushrods are same part number.

How far does the pushrod screw into the fork? - mine is about 1/4”

The bolts that act as stops for the pushrods- how far do they protrude from the pedal box?

Thanks

Les
Les Robinson

When Alan said the pushrod length needs reducing for 3/4” master cylinder , does he mean screwed into the fork so it measures 3.75” from the pushrod end to the clevis pin hole ? How long is the 7/8” pushrod? With 3/4” master cylinder my pushrod only screws into the fork 1/4” - I’ve no idea what the pushrod length is.
Les Robinson

Les
The suppliers sell one length of pushrod making no mention that 3/4 and 7/8 dual master cylinders require different length rods, between piston & pedal, because the piston in one or other is hollowed.
It should become apparent whether you need to trim the length of the new pushrod when you come to fit it. If you still have the two differing masters then compare the pistons. One should present as solid and the other as hollowed.



Alan Anstead

Alan
Thank you.
I’ve no idea what rods I have, been in for years. What caught my attention is that they only screw into into the fork by 1/4”, which is fine - no idea why was even looking!
I have a 3/4” master cylinder, although have a 948 clutch arrangement. Might be worth looking at modifying it to give 7/8” for the clutch. Can’t find anywhere local to me (Lancashire) but can send it to PastParts if needs be - looked at their website and seem a good place, just wrong end of the country!
Les Robinson

This thread was discussed between 02/05/2026 and 17/05/2026

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.