Front disc brake binding on

I have a standard disc set-up with the original disc and caliper. When I first got the bike running I found that the brake would bind on after a few applications of the brake. I thought the problem was with the caliper, so I had it rebuilt and the disc ground by Old Britts. I put it all back together and all was fine for a while (i.e a few short, around-the-neigbourhood rides), but last night it bound on again.

I had been doing a series of stops from about 40-50mph to bed in new pads. The caliper seems to get sort of pressure locked. I struggled home with the brake dragging badly, and released the pressure by slackening the bleed nipple.

As the caliper is newly rebuilt I now think the problem lies with the master cylinder. Anybody had a similar problem? Any ideas about which bit inside the master cylinder isn't working right?

Piston sticking? Perhaps disassemble and clean, check piston for any nicks etc and lightly sand to smooth. The simply assembly and disassembly may do the trick.

Corona850 said:

As the caliper is newly rebuilt I now think the problem lies with the master cylinder. Anybody had a similar problem? Any ideas about which bit inside the master cylinder isn't working right?

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It is highly likely that the small bleed port that allows excess pressure out of the system when the brake is released has blocked, because it is a very small hole (it's the hole that is furthest away from the lever if you look inside the reservoir) both holes look roughly the same size but the upper part is a wider counter bore.

If you have rubber brake hoses that are over 10 years old then I would replace them, as the inner walls of the hoses can disintegrate and act like a one-way valve?

I agree with LAB, I had this problem with a CB750 an American import with 20k on the clock. That little return hole hadn't been drilled. I wondered why the rear brake had had heavy use.

Another cause might be the lever isn't letting the M/C piston return fully. This can happen when using the longer better leverage type of lever (know what I mean I hope) or a sleeved M/C when the sleeve isn't fitted correctly.

Cash

cash said:

I had this problem with a CB750 an American import with 20k on the clock. That little return hole hadn't been drilled.

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I found the same thing on a friend's US import GS1000, the tip of the machine tool used to drill the hole had obviously broken previously, as the hole didn't quite reach the cylinder bore. This was the front master cylinder, and every so often the pressure in the system would cause the pads to drag and the extra heat build up would make the system go critical! The result being that the front twin disc brakes would apply themselves of their own accord! He had a few scary moments until I fixed it by fully drilling the bleed hole.
The thing is, it could have been like that from new? As that bike had around 28,000 miles on it!

I have experienced these symptoms and for largely the same reasons as Cash mentions. I had a re-sleeved master cylinder (done by RGM) but did not find the bite point consistent, particularly after several hard stops. No problems whilst braking normally but after a few rapid stops all lever free play disappeared and the front caliper started to bind. The caliper pads are too small really and soon overheat if punished.
I eventually gave up with this system, particularly after a few weeks of winter inactivity caused the master cylinder to pack up completely.
I fitted a Norton upgrade kit part number 13-1600 from Mick Hemmings plus a Lockheed CP3697 racing caliper (44.5mm piston diameter - same as original). Now braking is progressive consistent and mighty powerful. I can chirp the front tyre but lever bite point never changes. It costs money,though!

Thanks for all the info.

I have just bought a resleeved master cyclinder, which I am about to fit this afternoon. It's a MkIII master cyclinder, and my bike's a MkII, but it will work OK, I hope. Ill then disassemble the original one and see what I can find wrong with it.

scim77 said:

I fitted a Norton upgrade kit part number 13-1600 from Mick Hemmings plus a Lockheed CP3697 racing caliper (44.5mm piston diameter - same as original). Now braking is progressive consistent and mighty powerful. I can chirp the front tyre but lever bite point never changes. It costs money,though!

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scim77 -- I just looked in the Mick Hemming's online catalog. I can't find the part number you quoted (13-1600). Does this cylinder fit up with the Lucas switchgear?

I have also thought about the Grimeca cylinder that is available from Andover Norton and is modified to fit the Lucas switches. It isn't cheap, though. Anyone used this item? Is it worth the money - £170 I think?

It's pretty easy to get carried away and end up with a front brake that's worth more than the rest of the bike!

Corona850 said:

I have just bought a resleeved master cyclinder, which I am about to fit this afternoon. It's a MkIII master cyclinder, and my bike's a MkII, but it will work OK, I hope.

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But a MkIII master cylinder doesn't fit the earlier switchgear?

L.A.B. said:

But a MkIII master cylinder doesn't fit the earlier switchgear?

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Yeah! I just found that out - bugger it!! Boy, do I feel stupid!

Oh, well - on to Plan B - whatever that is/was.

Corona850 said:

Plan B

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Buy a Mk III to fit it to?

I think I've found the problem

I just disassembled my master cylinder. All the ports and weep holes are clear. LAB - I checked the bleed port you mentioned.

The workshop manual says: "Place this primary cup washer "hump" upwards ... The effect of the dished cup washer is to close the primary cup during the return stroke, allowing oil to by-pass the cup and replenish the pressure cylinder."

Well, the cup washer in my master cylinder appears to be flat, with no cup to it. I laid it on glass and used a flashlight to try and see the cup, hump, what-have-you. No hump!

This bike was an ebay bike. As such, I have found, it was more than a little hyped-up. Not really a good example of truth in advertising! :roll:

It came with some receipts, including one for a master cylinder rebuild. But, I have found that some of the other work it had receipts for was a bit suspect. Seems like this master cylinder rebuild might be yet another example.

So I guess on Monday I'll ship it off to Fred Eaton to be re-sleeved. Might as well do that now.

By the way - anybody want to buy a MkIII master cylinder? :lol:

Re: I think I've found the problem

Corona850 said:

The workshop manual says: "Place this primary cup washer "hump" upwards ... The effect of the dished cup washer is to close the primary cup during the return stroke, allowing oil to by-pass the cup and replenish the pressure cylinder."

Well, the cup washer in my master cylinder appears to be flat, with no cup to it. I laid it on glass and used a flashlight to try and see the cup, hump, what-have-you. No hump!

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The early style washers were flat I believe? They were replaced by a dished washer for a while, but that later changed to a wave washer.

Has the valve (the part that goes in the cylinder first) actually got the small hole in it?

With the master cylinder assembled, can you blow air backwards through it (with the reservoir cap off)?

So I might not have the answer after all??

All the ports seem to be clear. I've blown it all through with compressed air. The trap valve seems to be OK as far as I can tell. The bleed port is clear; small, but definitely not obstructed.

What do you think? Put it all back together and see what happens? Old Britts charges $200 for resleeving, plus $62 for a braided steel hose. Andover Norton sells the Grimeca replacement for £170 (i.e. $340), which includes the hose and stoplight switch, if I'm reading it correctly.

Any advice?

What is important is that the excess pressure can escape out of the system when the brake is completely released?

If you are able to blow air, NOT compressed air, back through the assembled unit (reservoir cap off) then the master cylinder must be venting correctly?

So, what about the brake hoses, are they rubber and if so, how old?

I can blow air through it, i.e me blowing, not compressed.

The hose is rubber, but relatively new. I replaced it when I put on lower, standard-european handlebars.

So the master cylinder seems clear, and the caliper has been rebuilt.

Re : Mick Hemmings. I agree that, at the moment, the 13-1600 Grimeca master cylinder kit is not listed in his catalogue, but he has stock of them nonetheless and I suspect it is the same kit as Andover Norton are listing, especially since the price is the same.
The kit comprises a 13mm bore Grimeca master cylinder with built-in brake-light micro switch plus steel braided dedicated hose with appropriate unions to fit the original Norton-Lockheed front caliper. A CNC machined alloy adaptor block is provided that accommodated the original Lucas switchgear and the mirror stem, thus retaining some semblance of originality.
I agree with your sentiments about originality and costs and I, too, have tried to keep things original. But modern traffic has such stunning braking ability that I reluctantly concluded that eventually, in circumstances of the unpredictable, I simply might not be able to stop the bike. The subsequent damage to me and the bike would then seem paltry compared with the price of a reliable braking system.
The original system was only just ok in the early 70's. It definitely needs some method of upgrading for todays traffic.
My system also benefits from a CP3697 racing caliper and larger front disc. I felt that the original caliper itself suffered from expansion and dimentional changes when things got really hot, thus reducing pad to disc clearance and then also reducing lever free play etc.

Corona850 said:

I can blow air through it, i.e me blowing, not compressed.

The hose is rubber, but relatively new. I replaced it when I put on lower, standard-european handlebars.

So the master cylinder seems clear, and the caliper has been rebuilt.

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In that case, if you reconnect it all, and after bleeding the system, it should be possible to push (lever?) the caliper pistons back into the caliper without too much force, which should result in the the fluid level rising slightly in the reservoir?
If the brake pistons will not return into the caliper or a large amount of force is needed to make them do so, then you still have a problem somewhere?

I know this is stupid but, you're not over filling the reservoir, or is the vent blocked?

Cash

cash said:

... you're not over filling the reservoir, or is the vent blocked?

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Cash - I don't think I am overfilling the reservoir. I fill the reservoir about half full so that the bellows just contacts the fluid level when I put the cap on. The vent in the cap is clear. I poked a bit of wire through it.

I've put the master cyclinder back together and bled the system. It SEEMS to be OK at the moment. That is, the brakes apply and release OK cold, in the garage. I suppose I cleared a blockage somewhere by disassembling and cleaning everything. Now I just don't trust it to stay fixed. I'll find out more when I take it for a run in a bit.

I think I might have to break into the piggy-bank and upgrade the whole lot, for all the reasons that scim77 said, which I fully agree with.

Digressing from the original topic: Everyone praises Brembo calipers. RonL posted some pictures of his set up with a four piston brembo caliper. Very impressive.

I looked on Vintage Brakes website and they recommend a master-cylinder to caliper ratio of 27:1 for opposing pistons. The best fit for this seems to be the Brembo F09 with 48mm pistons (used on Guzzis once upon a time, I believe) and a 13mm master cylinder. The Lockheed gives about 24:1 with a 13mm master cylinder.

I know there are as many opinions as there are people, but what is the consensus for the best value option? CNW's Brembo kit is $1095, Norman White's twin disk Brembo set-up looks really nice but it's £930! That's a lotta dough!

I think that it is really worthwhile to go for a high hydraulic ratio. I also think that Brembo make the best stuff (even though my own is Lockheed and Grimeca). If you can get a ratio of 24:1 or greater and have the budget for it then CNW is probably the best bet. Norman White must be better balanced and less prone to fork twist but it is even more money.
Remember to also get a curved mudguard stay if you are in Roadster trim.
I very carefully rebuilt my original Lockheed/Norton caliper but am reluctantly still convinced that, when worked hard, too much heat gets into the caliper and pads and causes all manner of variables at the lever. Looks pretty, though.