1975 MK3 front calliper problem

Hi again everyone I took my 1975 Norton commando on its maiden voyage today and after 10 miles of calm riding the front disc and calliper locked up, I called the AA who got to me in approx an hour by this time it had all cooled down and freed off so the AA man kindly offered to follow me home and as I turned into my road history repeated same distance each way, anyway the bike has a new master cylinder and hoses also disc and wheel and seeing all this was not expecting any trouble from that end but obviously the calliper has not been touched, so I have ordered a rebuild kit with new stainless pistons and started to gather the tools for the job, then I saw the bleed nipple which is obviously a different metal to the ally calliper and looks to be corroded and ridiculously weak I have cleaned it off with a brass brush and given it a squirt of WD40 but I honestly think as soon as I put pressure on it to release it will break has anyone a foolproof method for removing these so I can enjoy what little summer we have left Many thanks Chris

The brake lock-up was almost certainly due to the master cylinder vent port either being blocked or the piston not retracting sufficiently to uncover the port causing a build-up of pressure in the system.
Momentarily opening and closing the bleed nipple should have returned things to normal until the system overheated again. The pressure not being able to vent into the reservoir instead pushes the pistons and pads against the disc.
The heat from the friction causes even more pressure until the brake finally locks up.

chrisf1 said:

I saw the bleed nipple which is obviously a different metal to the ally calliper and looks to be corroded and ridiculously weak I have cleaned it off with a brass brush and given it a squirt of WD40 but I honestly think as soon as I put pressure on it to release it will break has anyone a foolproof method for removing these

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Heat cycle the calliper with a blow torch or heat gun in the area around the bleed nipple with applications of WD40 in between as it cools, and the bleed nipple should eventually free off without too much force. Probably better to disconnect the brake line or completely remove the calliper before doing so.
If you do that then you might as well overhaul the calliper.

The previous owner said he had adjusted the master cylinder because the lever was travelling to far before activating the brake he also told me it is a new master cylinder with the reduced bore could this be the cause and how do I rectify the problem My intention was to overhaul the calliper so I will be removing it anyway by the look of it the nipple problem is probably a common one is there a fix for a sheared off nipple or new calliper required thanks Chris

If the nipple shears the you use the central hole as a guide and drill using a drill the same diameter of the root dia of the thread. Then you use a thread tap to remove the remains of the nipple and restore the caliper thread.

chrisf1 said:

The previous owner said he had adjusted the master cylinder because the lever was travelling to far before activating the brake he also told me it is a new master cylinder with the reduced bore could this be the cause and how do I rectify the problem

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Yes, that would almost certainly explain it as he has overdone it.

Is it the RGM sleeve conversion with the adjustable pusher? If so, then the pusher can probably be adjusted until the vent is open with the piston fully retracted.

I have never dismantled it but from what I can see that bears a resmblance what and how do I need to do with or to it many thank LAB for your knowledgable help Chris

chrisf1 said:

I have never dismantled it but from what I can see that bears a resmblance what and how do I need to do with or to it

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You need to remove it, slacken the locknut and back off the threaded adjuster until when replaced with the brake lever, fluid will flow in the reverse direction (from the master cylinder to the reservoir) with the piston fully retracted.

I will get on it in the morning so just slackening off will cure the problem is there any guide for the adjustment settings thanks Chris

chrisf1 said:

so just slackening off will cure the problem

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I would expect it to cure the problem.

chrisf1 said:

is there any guide for the adjustment settings

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Nothing more than I've already mentioned as far as I know.

Hi again everyone you were all on the money with no return to the master cylinder and when playing with the adjustment to get a return it did not happen so after much advice and help from a NOC man Peter Shand with no positive results I have sent him both calliper and master cylinder for refurb

chrisf1 said:

Hi again everyone you were all on the money with no return to the master cylinder and when playing with the adjustment to get a return it did not happen so after much advice and help from a NOC man Peter Shand with no positive results I have sent him both calliper and master cylinder for refurb

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AKA dobba99