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MG Midget and Sprite Technical - Brake Problem- Master Cylinder?

I have an early 1964 MK 1 that I have worked on here and there for the last four years, and I finally got it started and around the block for a test drive. I've spent many hours here reading good advice, and I was hoping to report I am a new member of this group with a midget that moves under its own power. Unfortunately, I need your help first.

I have installed a new wiring harness; new brake pipe;, rebuilt calipers and new rotors; new slaves, shoes and drums at the rear; and a resleeved master.

I have bled and bled the system using silicone fluid and finally got some good pedal feel when parked. As I drove around the block, the brakes slowly tightened up until they were fully locked. The brake lights were on and the pedal was rock solid without me in the car. After an hour or so the pedal was back to normal and the lights were off.

Since all the brakes were effected, I assume it is a problem with the master? Any thoughts?
TS Dougherty

Do you have discs or drums up front?
Dave O'Neill 2

On aonother point - I might be reading this wrong but I thought you had to change all seals when switching to silicone brake fluid from ordinary

You could also check out the (many) threads on bleeding brakes as it's a usual problem (assuming there's no difference on 1964 USA brakes) - you're not alone

get the car finished and enjoy it :)
Nigel Atkins

OK - this is really obvious, but, trust me, I've done it, and in the most embarrassing places too - you DID have the handbrake off didn't you? And/or, the cables were straight and not caught up?

Like Minnesota - lovely state :) Hoping to drive my Midget up through there to Winnipeg next year for a blast around the roads with a friend in her bright green one!

Is this the adjustment on the shaft from the master cylinder. If I remember correctly if not adjusted correctlt the seal does not return enough to allow the fluid to leak back in to the resevoir, which means the brakes just get stuck on getting hotter and hotter making the situation worse and worse. Hopefully an easy fix and then you can start to really enjoy your great little car.

C Bintcliffe

My similar problem turned out to be the result of contamination
or mixture of DOT3 brake fluid and synthetic. The line just below the
master cylinder was fouled with a snotty substance, which prevented
fluid from returning to the M/cyl. After sitting a while, it would ease
off. I wound up flushing the entire system with alcohol and used DOT3
No problem since.
D Ramsay

I think Carl is correct. The pushrod is not returning far enough to allow a complete pressure relief.
Trevor Jessie

Thanks for all the quick response. The problem cannot be from fluid contamination because all parts were new/rebuilt and have only seen silicone.

The car does have disks in the front, and I know the handbrake wasn't on. I haven't adjusted it yet, so it doesn't grab anything.

Rach- Minnesota is great, especially this time of year. If you are coming through the Twin Cities next year, let me know.

Carl - I'll double check the adjustment and see if that helps. Thanks

TS Dougherty

Assume you mean rebuilt for silicone (unless I'm wrong about this if not you could have problems with the seals latter)

If I've got the wrong end of the stick just ignore me (most do :) )
Nigel Atkins

I did the same with the same car and had the same problems.

2 causes

1 In the masterbrake cyliner there are 2 holes going into the bore. The one towards the front of the car starts as a big 3 or 4 mm hole but ends as a very tiny hole before opening up to the bore. That one is smaller then 1 mm. I managed to open it up with a single thread from a bicycle brake wire. Thats how small the opening is. This hole is to release the piston.

2 There is a difference in the masterbrake cilinder between the earlier drumbrake cars and the later ones with diskbrakes at the front. Not only the diameter is different (but that doesnt realy is a problem) but there is a little plastic anti return valve included in the rebuild kit. I am sorry but forgot if its needed for the drumbrake cars or for the diskbrake cars. But I do know that if its wrong the brakes dont release and the calipers wont return. There is no such thing as springs on calipers. They back off just a little because the pressure is released. This is also the reason its good to adjust the drumbrakes at the back correct so there is no slack. Otherwise they wont do their trick properly.

Hope this helps.

BTW I do also have silicone fluid in mine for over 10 years now and I dont regret it. The only thing I had to change twice since then is the brakelight switch. I found out it helps if you have those fine smallbore braides brakehoses fitted. They wont expand as much as the normal ones so no braking effort is lost. Do bear in mind that pedal travel with silicone can be a bit more and you will have a little more difficulty in bleeding the system because it easily catches airbubbles when stirred in the bottle. So rebleeding the day after is a good thing.

Last tip after bleeding adjust the pedals so there is only 1 or 2 mm play before they catch. Thats a little less then per Haynes.
Bas Timmermans

I guess I need to pull the master to check that the holes into the brake bore are clear. Is there any way to do this with out contaminating the fluid and still leave the master in the car?

I did put the plastic piece in the brake bore when I rebuilt the master. Haynes didn't say anything about using it for drum or disk brakes. Any idea where I could find this out?
TS Dougherty

You can acces the holes by removing the alu lid. Not only the platsic cap but the whole rectangular lid.

When piercing the hole be carefull not to puncture the rubber cap inside. So if you do make sure the piston with cap is backwards. If its difficult to remove just loosen the cupro/nickel tube and put an airgun in the hole where it went in. The piston and the valve will come out. The best part off it will be that you will love the fact that you used silicone because its not agressive:-)

I am sorry to forgot wether to use or not the plastic thingy but the archives or someoneelse will help. You probably will find a posting of me when searching the archives because I discussed it before. It could be a while back because the first fitting I did 9 or 10 years ago and I refitted everything after putting a new master in 3 or 4 years ago. Just long enough to forget it:-(

Bas Timmermans

TS set the adjustment on the pedal pistons as Bas aadvises

This happened to my car ten or more years ago and it was the very same advice from the very same Bas that helped me fix it

The pushrods on the pedal assembly are reachable from the top of the pedalbox and need no special access just ondo the lock nut on the pushrod and adjust the free play. I forget what Haynes and the manual say but using the amount Bas says gave me free running discs again.

When I finished the long restortaion of my car I took it for its first longish drive to a "team meeting" at work, on a day off, so I could drive the Midget instead of the company van. Driving back afterwards the car went slower and slower until it stalled.

I got out scratching my head and looked at the front discs (rotors in the US) were glowing hotter than red the rubbers covers on the track rod ends were on fire.

I was horified, I had some bottles of water in the boot and used them to spray water inside the wheels from inside and outside.

Car surrounded in clouds of steam, people stopping with fire extinguishers and weird ticking noises from the front end as it cooled down.

After it cooled I drove it home slowly (about a mile and a half to go) and came on here for my first HELP ME message.

Bas advised the possible answer and I have trusted his instincts ever since.

I'd advise doing this first, if it hasnt worked then try a thin wire in the second (transfer) port in the cylinder as advised.
bill sdgpm

I had the same problem on my Mk1. What Bas says is probably the most likely cause but mine was a slightly different problem and might be worth checking. Did you renew the pushrods? There is a slight difference between the early pushrods (948 cars) and the later pushrods (1098 cars) The earlier ones are slightly longer and don't allow the piston to return fully enough to clear the hole to allow the fluid to drain. I had exactly the same symptoms as you. After much headscratching and then admitting defeat the local mg specialist eventually found the problem and after taking about 1/2 an inch of both pushrods it solved the problem. The pistons for each car are slightly different as well. There was an article written about it in MASCOT a couple of years ago by a friend of mine. It gives the correct lengths for each version (sorry I can't remember it off the top of my head).

Hope that helps.

Ali- Midget Mk1 1098

The article is in the January 2008 issue of Mascot and there is another atricle on the early master cylinders in the October 2007 issue. Not sure if I can post a link to the articles here or not. The longer rods measure 4 inches and the shorter ones are 31/2 inches. The picture on the article also seems to have the plastic piece installed on it.
Ali- Midget Mk1 1098

TS I found my own posting in the archives. It said REMOVE the plastic anti return valve otherwise the front diskbrakes will stick.

Hope this helps.

Bas Timmermans

Just set the pedal free play to about a millimetre, see if it helps before you do any messing about inside the hydraulics.

If not, prepare to delve!
bill sdgpm

Thanks for all the great suggestions. It's nice to know I'm not the only one this has happened to, and I'm always amazed at the collective wisdom found here.

I'll let you know what happens.
TS Dougherty

Here's a big thank you to you!
I have just been through the same saga. Stuck in the middle, and i mean the middle of a busy Brussels junction, breaks locked solid.
I too had just changed the master on my mk2 and had some binding. So i took it to my mechanic and thought i might have done a bad job bleeding them. Bled all, did a test driving out, paid him a tenner and drove off.
A mile down the road i felt it again, next thing i can't move.
So i had a cup of tea on a terasse watching the busy rush hour traffic swerving around my abandoned vehicle. Rang the mechanic, he had knocked off of course. So i waited for things to cool down and then drove it home using the handbrake only.
Now i came on here and blimey! It's solved!
I just needed 2mm play extra- and done!
Thanks again
sb blum

Glad it helped you Steve

now you know how good it made me feel

thank Bas, next door in the Netherlands

and if this is your first foray into here, welcome

be aware

you may never go away

bill sdgpm

This thread was discussed between 20/06/2010 and 29/06/2010

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