master cylinder-removal of residual valve- no top hat seal

Rebuilding, adjusting, maintaining & alternatives
The master cylinder on my disc braked car was leaking so i got a new one from a well known supplier .As my car is disc br1braked my mechanic took the MC to bits only to find there were no residual valve components to remove as shown in the inststructions I gave him which were from ESM whose discs are fitted to my car. So he fitted the MC to the car onlybto find that the discs started to bind after a short distance . We are now mystified. Is there perhaps a version of the MC which has a different arrangement for the residual valve ? If so details wouldbe welcome as indeed would be any comments /suggestions . Cheers ian brawn
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ianb875
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I would think it has either remained in the cylinder when everything else was removed, or it has been removed and the master cylinder was sold as ready for disc brake use.

If the latter it is possible with manufacturing tolerances that the piston is not coming all the way back because there is no free play in the pedal, in which case the operating rod on the pedal has to be adjusted. Done through the inside of the car with a couple of spanners after removing the cover over the master cylinder.

Sean.
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Thanks.There is 3/4 inch free travel on pedal. I like you thought the seal must havecjammed but the mechanic says there was not a seal and I had the impression there was no metal cup and no spring either,He even got another mechanic to check the MC. Clearly however something in the MCis causing the problem hence us thinking maybe the residulal valve has been changed and maybe is tied up with the main seals.
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ianb875
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does it look like this,

MasterCylinderBreakdown.jpg


the seal to remove is the Valve Cup 2nd closest to M/C

Alan,
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Thanks for the diagram.This is the diagram axtually that i gave to the mechanic but he insists and so does another mechanic that the valve is not there.! So how come the brakes are sticking think L As previously said I wonder if the residual valve arrangement is different on my .m.c, altho I think it unlikely . I think .i am going to have to say to the mechanic look again or .i just give in and buy an already modified .MC. thanks for your input
ianb875
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Have you still got the old one?

Could be worth taking them apart as per diagram to compare, or if you only have the new one open it up again and post a photo.

I wonder if someone took too many parts out of it. All that should be removed is the little rubber seal.

It definitely needs the spring to push the master cup back, if it is missing I would ask them to replace the master cylinder, or I would reuse the spring from the old one if I was pressed for time.

Sean.
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Thanks. Ineed to check wth mechanuc again cos I had the impression that the spring ,the metal cup and the seal were all missing, i do have thecold one so i . can do what you say. Hopefully the mechanic is back at work wednesday . Thanks again L will keep you posted.
ianb875
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3/4" of free play is a bit much. If the internal spring is in fact missing the piston will not return completely. If the piston does not return to the stop then the brakes will not release.
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geezer
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He did say 3/4 inch at the pedal, so at the master cylinder it will be much less.

Definitely don't want zero at the master cylinder.

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Ian,

Where do you live? There maybe someone 'knowledgeable' who could come and look at it?

I don't understand why there is no spring or other parts still there. If it was supplied like that, then the first thing should be a new master cylinder and start the 'remove residual pressure valve' process from the beginning.

Looking at the BMC Workshop Manual there is also a 'dished washer' that plays a part in fluid transfer:

Master Cylinder Dished Washer.jpg

The Manual also says (including the words in bold at the end):

Later master cylinder assemblies are fitted with a dished copper washer between the piston head and the main cup, to ensure that the transfer holes in the piston are kept clear.

The washer must be assembled with its concave side against the main cup and its convex side in contact with the piston, if it is to function correctly. It is imperative that this washer should be fitted to all assemblies.

Although there are 2 versions of master cylinder fitted to the Minor, but the only difference is the cylinder bore being larger on the early 803/948 engine cars with 7" front brakes, and smaller with the 8" front brakes - the residual pressure valve arrangement is the same in both versions.
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You did purchase a proper Lockheed type master cylinder did you? I know there is a cheaper pattern alternative and maybe that has a different build up?
Just a possibility to throw into the mix as I cannot, like everyone else, understand what has happened. I'm at a loss......
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Spoke to mechanic today on phone. I am to see him next week with the diagram again so we can both be sure what he says is missing. I have established the spring IS there. Thanks to all of you for your contributions thus far..I will keep you posted.
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Had similar issues with M/C's in the past. Production methods changed a year or so ago, and a thick waxy fluid was used during some part of the production. This fluid can go hard in the tiny hole from the main reservoir into the piston cylinder, effectivly blosking the system giving no residual back pressure. You need to remove the cylinder, once empty of fluid, remove the piston seals etc Then look through the top hole to see the tiny 1mm hole, use some solid wire or the like to push into the hole, use a torch as its easy not to clear properly. Once the 'slug' is out bench test the mc .
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Thank you so much . I will show the mechanic what you have written and hopefully we can crack this problem .
ianb875
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Brake hoses? Are they becoming restricted.

The purpose of the dish washer is to break the surface tension between the piston and the seal in order to allow fluid to flow into the main cylinder before the piston is fully home. But when would that happen? The spring is always compressed against the seal, so a vacuum is needed in order to deform the seal enough to allow fluid to flow, But again when would that happen? If it's to compensate for lowering temperature, the main port would allow fluid to flow from the reservoir to compensate for any contraction. It's not all that critical how that spring washer goes in, if it is needed at all. It will break the surface tension either way. If it was a pump, I can see the value, so it's only use may be when the system is being filled and bled, and pumps the fluid into the system. During that process when the bleeder is closed, the system will be at zero pressure, then the pedal is released with the bleeders closed and a vacuum will be created during the return stroke and it would be desirable to replace that vacuum with fluid.
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