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MG Midget and Sprite Technical - Brake Disaster - Gaydon now in Jeopardy!

Just had the brake/clutch master cylinder replaced on my early MK2 midget by an mg specialist garage. Picked up the car from the garage, I'm told they took it for a 5 mile test run without any problems. I drive the car away from the garage - the brake pedal feels a lot harder than before, with less travel. I get 2-3 miles down the road, and the car seems very sluggish - 40 mph in 4th gear with my foot flat to the floor. The brake pedal is rock hard, and there's a burning smell. I pull up on the side of the road, and there's smoke coming from the wheel arches! The brakes are stuck on, both front and rear! Called out the RAC, who arrived promptly. On examining the master cylinder, there is a pool of brake fluid at the union of the brake pipe and the master cylinder.
Loosening this union, a little more fluid spurts out, and this frees up the brakes, enabling the car to be towed home. I'll be calling the MG garage first thing
in the morning, but does anybody have any ideas?
On the invoice, the new master cylinder is the 7/8 inch type, but has been fitted with pushrods described as '948/1098 M/CYL P/ROD'.


Adrian Jones

wouldn't be Beech Hill by any chance?
David Smith(davidDOTsmithATstonesDOTcom

Sounds like the pedal free play needs to be adjusted correctly?
Richard Wale

No. I'm refraining from naming the garage for the time being, to give them a chance to rectify the problem, at no additonal cost to myself.
Adrian Jones

948 and 1098 brake pushrods are not the same. How long are the pushrods ?
Spyros

Yes simple problem to sort out the master cylinder piston is not returning to fully clear the vent hole in the cylinder to resovoir. Surprised the RAC man did not realise this! If it is due to the actuating rods being too long then a short road test would not have highlighted the problem, that is in defence of the garage so they should immediately be able to put it right for you.
Bob (robert) Midget Turbo

Spyros,

I'm not really able to measure the pushrods.
I would expect an MG specialist garage to know what they were doing in this respect. When I booked the car in for the work, they did mention an issue with the pushrod lengths, and that they would need to fit the ones of the correct length.
Only the brakes are a problem, cluth appears to be fine.
Adrian Jones

Adrian,

I think the mk2 Midget uses disc brakes on the front, is that correct in your case. If so the 7/8" bore master was intended for drum brake cars and will give a heavier pedal and shorter travel compared to the 3/4" master used for disc brake cars. I have used a 7/8" myself with disc brakes but have read subsequently that there is a valve that should be removed for disc brake use, IIRC not removing it can lead to brakes locking but I never had a problem I was aware of. The pushrod length issue between disc and drum front brake cars is well known and should be checked as I think that leads to locked brakes also.
David Billington

Bob (robert) Midget Turbo, England ....Is exactly correct....been there bought the yellow T-shirt in XX-large, and even got the mustard stains for free...lol.


the mistake I made was not reassembling the MC correctly and blocking that vent hole that allows the fluid to return correctly....blocking that little hole will definatly lock up all four wheels...takes about 30 miles to get the full effect or alot of pedal braking....

most likely the push rod is to short and cant move those inner ring parts past the vent hole....

Sorry I dont know the correct names of the MC brake parts...but you probably know what Im talking about.


should be a fairly easy fix.

prop
Prop

David,

Yes, the car has disc brake on the front.
I searched the archives for problems like this, and there were numerous hits, but I must admit I was litle confused in relation to removing that valve you refer to - what purpose does that valve serve for drum braked cars, and why does it need to be removed for a disc braked car?
Adrian Jones

Adrian,

I can't help with that question. I fitted disc brakes to my frogeye and used the 7/8" master without any apparent problems and left the master cylinder as it was as I didn't know the valve existed, after a few years the master cylinder needed replacing so I fitted the 3/4" for disc braked cars. I did experience the disc locking on occasionally with the 3/4" master but think that was down to the very worn discs. I think the valve in question is a residual pressure valve but you would have to research it more.

If your garage have swapped a 3/4" for a 7/8" then that would explain the pedal feel and travel, maybe they have not gotten the pushrod length right or not adjusted it correctly.
David Billington

It will be as Bob suggests
At the front of the pedal box you can look down at the pedal and pushrod under the bonnet

the pushrod has a locking nut and should be set to allow 1mm ish / 0.035" ish) of free play before it starts pushing the rod.

All that happens is that the failure to let the piston come back far enough leaves pedal pressure against the piston which then holds pedal pressure against the relief hole inside the master cylinder

this holds the discs slightly on and in my case set fire to the track rod end rubber gaiters due to the discs glowing almost white hot when I finally gave up trying to drive home

I threw the contents of my Astra type washer bottle over them to put out the fire

I really do not expect there to be anything wrong inside the cylinder, it will be certain to be a simple adjustment to be performed. No extra valves to put in or take out
Bill

Bill,

There was an article in Mascot earlier this year about the importance of pushrod lengths with the two diameter m/c and someone having this same problem - can't find them now - must get a filing system other than the bin !

Richard.
richard boobier

I remember seeing that Richard, though having fitted two by now and not having the relevant pushrod length issue, I had forgotten it.

I'll be surprised if it isnt just adjust-outable, though.
Bill

Thanks for all the responses.
Bad news is that the garage in question can't look at the car until Friday at the earliest. My chances of making it to Gaydon are not looking good.
Adrian

Adrian

You need to assert yourself with the garage. This is not a new job they are taking on, it's a job from earlier in the week that they haven't finished amd they have returned a car to you in a dangerous state.

Make it clear you are not going to back down.
Jeremy Cogman

It might be useful to tell the garage that there will be 999 other Spridget owners expecting them to give a full explanation of why they weren't able to get it right first time and why you have to wait till Friday to have the problem rectified.
Just a thought
G Lazarus

I'm with Jeremy on this...insist that they sort it straight away.
Dave O'Neill 2

Ok, the car is now back with the garage that carried out the work. I asked about the pushrods, and they showed me one that are identical to the ones they fitted to my new cylinder. I measured them: The rods are 105mm in total length, with the smooth, unthreaded section of length 74mm. The rest of the length is taken up by a hexagonal section and threaded section. They showed me pushrods that were identical to the ones previously fitted to my old master cylinder, and they were 90mm in length with a smooth section length of 63mm. I asked for, and was given, my old master cylinder, but they couldn't find my original pushrods.
Adrian

Hi Adrian,

I'm the MASC member featured in the article. It's in the January '08 issue (and a follow up in a later issue). I have a GAN2 MK1 with disc brakes and the 3/4" master cylinder. Did they give you any reason why the master cylinder was replaced in the first place? (I'm assuming yours was the 3/4" one as well).

The rods in the 3/4" cylinder are about 1/2" shorter than the 7/8" one. I have it on good authority that a 7/8" master cylinder shouldn't be used in a disc braked car (although I know some people have used it).

If they have replaced the master cyl and the push rods they will be correct for one another because you can't get the short rods any more. My garage (MG Barn in Shedfield nr. Southampton) were great and spotted the problem immediately and had to cut my new rods down to suit. It didn't even occur to me to check the length, they were just listed as the correct part for the car.

Ali.
Ali

Ali,

The master cylinder seal for the clutch pushrod on my old cylinder was leaking - they recommended replacement of the entire master cylinder, rather than just attempting to replace the seals.




Adrian

Adrian,

I'd be inclined to get a second opinion. We just re honed the bores and replaced all the seals in mine and my garage were more than happy with that when they has to open it back up to fix the rods.
Ali

UPDATE:

The garage says they've fixed the braking problem.
Seems it was the only the front brakes that were stuck on, so they say, and it was the hoses that were the cause of the problem. Changing those has, allegedly, cured the problem. An 8-10 mile test run revealed no brake problems, and after the run, the car could be pushed easily by hand with the handbrake off. So Gaydon, here I come - assumimg the weather doesn't decide to mis-behave.
Adrian

I'm half happy for you.
And I'd recommend to be cautious with the clutch.
The difference of fluid volume between the 2 types of brake/clutch cylinder is actually big enough to throw the slave clutch piston out of his bore when fully declutch.

My Sprite is currently out of the road, clutchless. This because of this issue after having tried to replace the 3/4" by a 7/8".

So, I took the only correct road to fix this : Resleeve the original 3/4" cylinder. I hope you kept yours
Spyros

Spyros,

Thanks for warning, I'll beat it in mind.
I have the original master cylinder, but not the pushrods.
Adrian

This thread was discussed between 19/05/2008 and 23/05/2008

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