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MG Midget and Sprite Technical - midget 1500 brakes

Hi my midget has excessive brake pedal before the brakes work. Car has new/very recent cyls, calipers,discs, servo and master cylinder, with the plastic resevoir. After bleeding i have this problem so i asked the garage to pressure bleed before the mot. No air they say and passed the mot as pedal hard albeit low down. Any ideas????
Thanks in advance, tony
a boyle


Are you sure it was the correct replacement master cylinder?

The different midgets had different master cylinders over the years with different internal bores to make life difficult. If the bore is too small the piston in the master cylinder will not move sufficient fluid to actuate the brakes until much further through the pedal stroke.

Just a thought anyways, worth checking!

D Prince

wrong sizes pipes anywhere
Nigel Atkins

This can happen if the rear brakes are not adjusted properly. They still work fine, giving a hard pedal, but give excessive travel. The rear brakes need to be adjusted so that they actually drag lightly. lf the shoes don't just scuff the drums, then they are probably too loose.
The other thing to check for is that the pedal pivot hasn't worn oval.

What year is your 1500 - mine is 1979 and it has the twin type M/cyl. There would have been only one size ( bore ) - so the only thing is if its a near equivalent part ( for example i have seen someone advertising an MGB part as fitting midget). I don't think pipework will affect outcome - its master cylinder bore cross sectional area and slave cross sectional area that matter regards presssures exerted and pedal travel.

Are all of your bits absolutely correct spec?

wheel cylinders could be bigger bore more likely than master cyl being wrong I would have thought.
calipers - what piston diameter do you have - if they are lareger than std - then that also may explain your problem.

But first thing is to make sure brakes are all adjusted up properly as mentioned above - and maybe pivot wear /lost travel before the master cyl piston starts to move...

Where did you get your parts?
John Barber

Now, I'm not a gambling man. And I think the 'Moody Blues' are still number one in the hit parade (pop pickers) - but I would 'stick a score' on Mr Boyle having been sold rear wheel cylinders suitable only for pre-1974 Midgets.

Pre-'74 Midget rear wheel cylinders have a 19.00mm bore. Post '74 models have cylinders of bore 17.5mm (also fitted to some Mini Coopers). Outwardly, the cylinders do look the same, I believe.

I haven't done the maths, but it seems to me that an oversized bore diameter difference of 1.5mm would give a longer pedal stroke (as well as brake efficiency difference) compared to the 17.5mm bore.

And I'm sure the Moody Blues are still at #1.
Andy Hock

Many thanks all your replies..The brake pipes are all copper and the flexis s/s all supplied by mgb hive.
The servo came as an eu sourced good quality one. I feel there is not enough fluid displacement, but the m/cyls all have been superceeded by the last one fitted so in effect there is now only one type master cylinder for 1500 ...this i have supplied by jag abingdon . { not dual circuit}.The only thing here is if the bore was smaller for a vehicle with drums all round that could be a problem. I will contact abingdon and ask the question.I would be amazed if there was a larger bore caliper piston but the large bore rear cyl theory is very interesting. i will try to see if there are any markings
cheers for now tony
a boyle

sorry I missed the servo and it's pipework added in to the mix possibly something to do with this(?)
Nigel Atkins

I just can't remember the details but a mate who knew a thing or two about brakes I sure told me at one time that the larger bore wont give more pressure
Nigel Atkins

Hi nigel larger bore does not give more pressure.....but disc brake calipers displace more fluid than wheel cylinders so therefore need a master cylinder displacing more fluid ie with a larger bore.
Befor i wrote the initial thread i was thinking master cylinder but thought i would leave the comments open for other ideas.
Many thanks again for all you replies tony
a boyle

<<<there is now only one type master cylinder for 1500 ...this i have supplied by jag abingdon . { not dual circuit}.>>>

don't quite understand this, what year/VIN is your car and what m/cyl do you have? go here
to identify what you have and /or what you should have.
Why do you have a servo, no Midget *needs* a servo noe was supplied with one new. Once we have confirmed you have the correct hydraulic parts, you could do worse than bypass the servo and see if the problem changes or goes away.
David Smith

I was going to suggest a bypass of the servo but wasn't sure how much work that would now invovle, it would eliminate one element

personally I agree with David I don't think a standard road going Spridget needs a servo or much in the way of braking upgrades but it's your car and you want a servo and upgrades that's all that matters
Nigel Atkins

A brake servo (working or not) won't make any appreciable difference to brake pedal travel. In effect, a servo is simply a thin membrane that interfaces/sits between the (under bonnet) end of the brake pedal arm and the master cylinder piston.

Excessive brake pedal travel can only be caused by any/all of the following:

Excessive (wheel cylinder) piston movement (included here is badly adjusted brake shoes).
Excessive (caliper) piston movement.
Excessive brake master cylinder piston movement (possible causes include internal leak-by of piston seals, as well as external leaks).
Ballooning of brake hoses.
Leaking hydraulics (eg corroded pipes, split hoses etc).
Faulty brake-line pressure compensating valves (none in the Midget).
Worn/loose clevis/hinge pins (eg brake pedal linkage).
Incorrectly supplied parts.
Badly designed brake system (hopefully corrected before the car goes into production!).

Andy Hock

A good list, Andy.
But given that Tony gets a firm pedal and good braking, some on your list can be ruled out.
I still think the likely cause is a worn pedal pivot. Part of the pedal movement that should be pumping fluid is taken up in unwanted movement at the pivot.

The other thing - although the calliper pistons are large, the movement is small so in normal situations they don't need a great volume of fluid to bring them on. Poorly adjusted tears will take up much more fluid and therefore pedal movement.

hi guys many thanks your thoughts
I am a senior car owner and was brought up on drum brakes and the problems of getting a 'high' pedal.
Hurrah when discs arrived. My car is a 1500 GAN6=173606G chassis with the type one m/cyl. I think i mentioned i replaced this with one from jag and that has a laqrger plastic resevoir Not oe but ok for me. btw the servo was to keep the wife happy, she has only driven with them!!!When i bled the brakes, i wound the adjuster right up and even in that state the pedal was low. I also put new front discs and have visually looked for run out. I can see nothing and if there was a lot it would be felt on braking thru the pedal. Brake hoses was ones, stainless i think from bhive. No movement in the pedal or the cylinder push rod pin The mot last week showed the brakes to be well ok for brake force and balance..I still have a gut feeling not enough fluid
displacement and with the non oe m/cyl, probably made in china, is the problem there
thanks again tony
a boyle

Hi, Tony. My Midget is an early 1500 version. It is GAN6-173244G, so yours rolled off the production line approx 360 cars after mine. I've owned it since 1978.

I (again) recently did some work on its brakes, namely new caliper pistons/seals. New rear wheel cylinders and a brake master overhaul.

Can I ask why you replaced your OE master cylinder with one of the plastic reservoir replacement types? Replacement is only really needed if the original's internal bore is scored or corroded. Inspecting and overhauling it is a very simple (and cheap) job.

I know replacing it is a PITA, but if you really don't like your 'low brake pedal', you could overhaul your original master cyl (assuming you still have it) and refit it.

At least that will eliminate/prove the cause of your low pedal.
Andy Hock

hi andy that sounds like a good idea. I have eventually worked out what a pita is!!!!!!!!!!!
I thought it was some sort of bread.Apart from having to bleed the system again i thought changing the cylinder was easy, anyway first thing monday order some genuine seals thanks
rgds tony
a boyle

Have you managed to eliminate out Andy's other good suggestion relating to the rear wheel cylinder size. It would be a fairly quick task to take a wheel off and measure the ID of one of the cylinders.


I think i will check if there are some casting marks before i strip one down. According to a parts book i have, the 1500s areall the same and i believe the cylinders came from a reputable dealer so they should be correct. The casting numbers may well be the same for different bores. The easiest job is the master/cyl seals, and the cheapest so i will do that asap
a boyle

I was thinking that one could remove the drum, flip the dust seal up on one cylinder and measure the ID across the end of the bore with a dial calliper - or piece of bent wire if you don't have callipers.
Shouldn't need any dismantling.

Regarding confirmation of rear wheel cylinder identity. Tony - didn't you keep the boxes they were supplied in? Nothing stamped on the boxes?

Did you say you bought them from Abingdon Spares? If that is the David Manners Group (Birmingham), they are bound to have a record of what they sent you!

True, the box contents may not have tallied with the box labels, but at least it's a start. If the boxes are labelled 'LW 11396' or 'GWC1102' (the wheel cylinder part numbers for pre '74 Midgets) you know you have the wrong cylinders! 1500 rear wheel cylinders are GWC1129 or LW11193.
Andy Hock

attn your comments on rear cylinders. It woul seem that lockheed used the same wheel cylinder casting for 4 different bore cylinders, from approx 15 up to 20 mm. You may have noticed the little roll pin in the rear of the cylinder that ligns up with a hole in the backplate. I am told these are in different places for the different bores so to fit the wrong cylinder would mean drilling the backplate or removing the pin..Curiosity is getting the better of me,,tomorrow i will remove a drum and measure a bore size, should be 17.8mm, If it is different, i will check out the rear pin
Allegedly the new style master cylinder rubbers may have been less than satisfactory, so i have a set of seals coming for my old master cylinder..If nothing else, if the problem remains, i will know it is not the master cylinder
rgds tony
a boyle

as you may have seen I've had problems with rubbish rubber in parts (coolant and fuel hoses, TRE boots, steering rack gaiters, wiper blades) so I'd be wary of any rubber seals at the moment
Nigel Atkins

Thanks for your reply, Tony, and the concept of the roll pin being used as a cylinder bore identifier. Sounds very feasible. And, if it's true, it might go some way in dispelling the rumours that the the roll pin is an essential part of the rear wheel cylinder anchoring design. The large circlip and bleed nipple does seem to locate the wheel cylinder very well.

As with many other owners, I have removed the roll pins and drilled/tapped the hole to accept an M5 bolt. I can't see the 'play' inherent in the roll pin design being very important.
Andy Hock

I have tapped and fitted my rear cylinders with setscrews too. But actually I think that the degree of "float" when just fitted with the correct circlip does help to allow the shoes to align properly.

This thread was discussed between 14/04/2012 and 16/04/2012

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