1975 MK3 Commando front caliper problem

I suspect the new master cylinder has a sleeved bore to improve brake performance, but in doing this there is a risk of the piston seal not completely clearing the pressure release hole at the fully off position. The pressure is not released after applying the brake which stays locked on. It works when the master cylinder is cold but out in the sun the master cylinder heats up and whatever small gap there is closes up completely, in your case the seal may be always too far in as yours locks up after 10 miles.

The fix is to get the piston to move further back when the brake is not applied so the seal clears the hole (you can visually see this with the cap off and the fluid reservoir empty), take the hand lever off and see if the piston moves back far enough. If it does then file the end of the lever that touches the piston so with the lever in place the piston is not pushed inwards. If the piston cannot move back far enough with the lever not there then its more complicated so come back if that is the problem.

That nipple is easily sheared off, it corrodes in the alloy easily. Can only suggest lots of heat and if necessary enough to damage the seals, you have new ones on order anyway but that will take you off the road. 

Hello Chris

From your previous posts about the front brake did you find out what kind of master cylinder had been fitted? A small bore AN one or an original sleeved down master cylinder? This very much sounds like a sleeved down cylinder that has not been set up correctly. If it has been sleeved with the RGM kit, it is possible to adjust the piston pusher between the lever and the piston itself. You must be able to see a small squirt of brake fluid from the holes at the bottom of the reservoir when the brake lever is squeezed (reservoir cap and bellows seal removed) This proves that the reservoir holes are venting the brake system when the lever is at rest (brake not applied) This is essential to prevent exactly what is happening to your brake. Can you take a photo of the mastercylinder. The attached photo is of the sleeved master cylinder on my Mk3

Don't apply seal melting heat to the caliper, only use a hot air gun or similar. Seal melting heat will ruin the heat treatment that the caliper had during manufacture. I have removed loads of broken bleed screws and the like by drilling and grinding the center of the screw out. A battery die grinder from Lidl that cost less than £20 and a pack of the diamond bits is ideal for jobs like this. Ive even removed a broken easyout from a broken bleed screw and left the threads undamaged using this gear.       

.. as though it's been set up with the M/C piston covering the tiny bleed hole that allows fluid back into the cylinder when everything hats up a bit. This applies the brake resulting in yet more  heat till the whole things comes to a halt. I had that problem on my Commando when I fitted a different - Guzzi I think - master cylinder.

But as said, if you're not absolutely confident in your abilities get someone who is to sort it out.

I'm sure that Peter can fix it, but Andover Norton have new calipers. Personally, I use a Lockheed 'variable ratio' master cylinder....the parts are out there and a decent usable brake is quite possible.

Had similar front brake problem on both my 850s , a Mk 1a and a Mk3 , and finally gave up trying to fix after several attempts which brought only small improvements , fitting upgrade disc and caliper (Lockheed Racing) kits from Norvil and RGM respectively which sorted everything. Kept original Norton master cylinders with standard bores in both cases but fitted new hydraulic pipes. This was five years ago and both front brakes still excellent. Definitely worth the money.