Mk3 Rear brake Problems

So the honeymoon is over and after 150 trouble-free miles my rear brake stuck on. I let out some fluid and let it cool down and it freed off. It had got so hot, the disc (a new Girling drilled version) had gone gold.

I removed the caliper but found nothing wrong. I renewed the seals anyway and fitted new brake pads and all seemed okay. Bled the system, checked it was working with the brake pedal and went for a short test ride and it stuck on again.

Pulled everything apart again, refitted and this time I went for a 3 mile ride but didn't use the rear brake. When I got home the disc was hot, caliper body was cold. So something appears to be touching even though I can rotate the rear wheel easily.

The rear brake system is almost completely new from end-to-end including a new master cylinder, new AN brake lines and the new disc. The only original part in the system is the caliper and I have chamfered the edges to allow for the floating disc bobbins.

Reading other posts, one of the suggestions is to check that the master cylinder is alligned properly, but It's new, been fine for 150 miles and the brake had real bite. I can't imagine why it would change? I've also checked that the vent hole in the cap is clear.

One thing I have noticed is the caliper is not sitting square over the disc. It appears 'cocked' from top-to-bottom? I've double-checked the rear wheel alignment and it's bang on.

What could cause this?
Why was it fine for 150 miles if something was wrong from the start?
Is it crucial for the disc to be centered/square with a floating disc?

Not sure what to do next but the Sun is shining for a change and I'm stuck in!

Bilko said:

Reading other posts, one of the suggestions is to check that the master cylinder is alligned properly, but It's new

Click to expand...

Being new does not make it correct so it needs checking to make sure the piston inside is clearing the port to relieve pressure when the pedal is not pressed.

Bilko said:

I let out some fluid and let it cool down and it freed off.

Click to expand...

If the brake frees off after momentarily cracking open the bleed nipple then the cause is almost certainly due to the master cylinder piston (cup seal) not fully retracting so remains blocking the vent port as kommando said.

L.A.B. said:

If the brake frees off after momentarily cracking open the bleed nipple then the cause is almost certainly due to the master cylinder piston (cup seal) not fully retracting so remains blocking the vent port as kommando said.

Click to expand...

Is there a way to check this without pulling it apart? I'm struggling with the fact that it was perfectly fine for 150 miles. Could it move/change position?

I've just removed the caliper and the outer pad was not straight in the bore and was stuck.

Bilko said:

Is there a way to check this without pulling it apart? I'm struggling with the fact that it was perfectly fine for 150 miles. Could it move/change position?

Click to expand...

Bet it was on relatively hot day or a ride you used the brake more than previous rides and the heat build up was enough to take a partially open port to a fully closed port. Mk1 eyeball is required to check the piston position.

Bilko said:

Is there a way to check this without pulling it apart?

Click to expand...

I don't think so or not without removing the 'bolt-on' reservoir.

Bilko said:

I'm struggling with the fact that it was perfectly fine for 150 miles.

Click to expand...

It all depends on how much heat, therefore, expansion builds up in the system. Maybe you were pushing it a little harder so using the brake more? Plus the warmer weather (kommando just beat me to it).

Bilko said:

I've just removed the caliper and the outer pad was not straight in the bore and was stuck.

Click to expand...

Sounds like the pad/s could be undersized for that to happen.

Bilko said:

a new Girling drilled version)

Click to expand...

There were a few reports of problems with the Girling disc buttons clashing with calipers so I'm wondering if that could be something to do with it although the problem was supposed to have been corrected.

L.A.B. said:

There were a few reports of problems with the Girling disc buttons clashing with calipers so I'm wondering if that could be something to do with it although the problem was supposed to have been corrected.

Click to expand...

The bobbins were fouling the caliper, and I had to take some material off of the edges to get clearance.

When you rebuilt the caliper did you clear all of the fluid passageways and clean the machined seal recesses?

Bilko said:

One thing I have noticed is the caliper is not sitting square over the disc.

Click to expand...

You could check if a washer has been inserted here...:


...because the parts drawing shows a washer on the head side of the '32' (drawn as a hexagonal) bolt but that is a 'special', with flats under the head, therefore, there can be no '33' washer in that location.

cliffa said:

When you rebuilt the caliper did you clear all of the fluid passageways and clean the machined seal recesses?

Click to expand...

Yes it was spotlessly clean, but there were a few signs of wear in the bores. I'm wondering if the caliper is the problem.

The AN piston seals were surprisingly easy to fit and the stainless pistons slid in easier than I had ever experienced in the past.

After the lock up I tried to fit a pair of AP seals I had on the shelf and could not get the pistons in no matter how hard I tried, so I used another AN pair I had and the pistons dropped in no problem.

Looking at the bores this morning, they look wet so I think fluid is getting past the seals.

L.A.B. said:

You could check if a washer has been inserted here...:


...because the parts drawing shows a washer on the head side of the '32' (drawn as a hexagonal) bolt but that is a 'special', with flats under the head, therefore, there can be no '33' washer in that location.

Andover Norton Parts Drawings - Genuine Andover Norton Factory Parts - Commando Parts

Andover Norton Parts are the only Norton spare parts in the market manufactured to original drawings, within the factory-approved tolerances, from the correct materials, and with the correct finish.

andover-norton.co.uk

Motorcycle parts and spares for Norton motorbikes - Genuine Andover Norton Spare Parts - Andover Norton Motorcycles Parts - Original Andover Norton Parts

Motorcycle parts and spares for Andover Norton motorbikes: Andover Norton manufactures and supplies all Genuine Factory Spare Parts for Norton motorcycles built before 2002.

andover-norton.co.uk

Click to expand...

No washer there Les, plate is directly on the swingarm.

Just some thoughts.

The rear wheel locking implies master cylinder to me. The linkage may not be allowing the master cylinder to release fully. Much less likely, but might be possible is that the brake pads are very loose in the caliper and twisting sideways.

In one test you didn't use the brake, the disk was hot, and the caliper was not. That implies to me that one or both brake pads are lightly rubbing on the disk which is not a big deal unless the disk is very hot.

Bilko,
Had the exact same problem… turned out to be the hose that had an internal collapse that could not be felt by squeezing it…it would allow brake fluid to pass the collapse but not allow the fluid to return. I would try replacing the hose with the old hose and give it a try…quick and easy check. I chased that issues a long time.
Rod

Just pulled the master cylinder to check the alignment and discovered it has fouled the kickstart lever? Wrecked the rubber boot and took a small chunk out of the m/c body. I've only kicked it half a dozen times as I prefer the green button method.

Couldn't have done it any good but probably not the culprit to my sticking brake.

Now I need to find out why it's so close as there's no way I can adjust either of them..

I have never been impressed with the disc rear brake. The drum brake is just about as good and a lot less trouble.

While it's apart, back the barrel out a turn or two.

You can put a 5/16" flat washer under the rear footrest /peg bolts to make it clear. It's a common thing with MK3s Or shim it in the direction you need with washers.

Just bench tested the master cylinder by blowing air through the outlet as per the instructions and got nothing.

I screwed out the body a few turns until no free-play in the pushrod, screwed it in another full turn, lined it up and put the grub screw back.

Did the air test again and bubbles now coming out of the reservoir. I'll put it all back in the morning and see if it works properly.

Bilko said:

Just pulled the master cylinder to check the alignment and discovered it has fouled the kickstart lever? Wrecked the rubber boot and took a small chunk out of the m/c body. I've only kicked it half a dozen times as I prefer the green button method.

Couldn't have done it any good but probably not the culprit to my sticking brake.

Now I need to find out why it's so close as there's no way I can adjust either of them..

View attachment 109178

Click to expand...

All the mk3s i've had and seen did this

dobba99 said:

All the mk3s i've had and seen did this

Click to expand...

Mine doesn't.