What to suspect first

I had my MKIII out today. It is hot for southern California this week. I have a standard front brake with the sleeve in the master cylinder. Lately, I have found when it gets hot out, or the bike has been sitting in the sun, something sticks in the front brake system after I apply the brakes. Today I had to pry the pad off the disc before I could ride home, and heaven help me if I had to use the front brake. It would pinch tight and not release. Usually, after the system cools down, the brakes release and everything is fine.

I thought I read somewhere on this forum about something similar, but couldn't fine the posts.

So, what do I suspect/repair first, the master cylinder or the caliper?

I have read of several re-sleeved master cylinders exhibiting the same symptoms. My guess is that the sleeve and MC housing are expanding at different enough rates that the holes are getting blocked off for fluid return. But that;s just a guess on my part. Might be worth noting the source of the sleeve job, so we can begin to narrow down cause & effect.

I have a re-sleeved MC. done by Miles Classic Brake, that hasn't had any problems. But I've not owned it long enough (only a hundred miles or so) to make any definitive statement.

calbigbird said:

So, what do I suspect/repair first, the master cylinder or the caliper?

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First try a thorough bleeding of the system. Enough to flush and replace the fluid. Then try again.

Has this problem started when the master was sleeved. ? When were the caliper seals last replaced ?

calbigbird said:

So, what do I suspect/repair first, the master cylinder or the caliper?

Click to expand...

Very likely to be the master cylinder. If opening the bleed valve immediately stops the pads dragging, then the problem is almost certainly caused by the master cylinder and not the caliper.

Any excess system pressure should be able to vent to the master cylinder reservoir from the bleed port which is uncoverd when the M/C piston returns to the fully "brake off" position.

This locking on of the brake during hot weather seems to be a problem with some of the sleeved master cylinders, probably because the bleed port in the sleeve has no open pathway to the original vent port, or the piston is not retracting sufficiently to uncover the port?

Been there and even crashed at 2 mph with locked front d/t fluid heat expansion. Solution is gradually shorten the new lever plunger til it allows piston to move back enough to dump the excess pressure at caliper. We are talking a few 1000th's at a time and testing in hot sun or hair drier, to just get enough off w/o loosing much to matter in pressure stroke lever squeeze. it took me a while to understand what was happening as prior had no issue even with race level contests in tights for half an hour of no fade tire squeal. I did Peels on a rare side walk in old Ozark town. Worked fine rest of summer heat wave trip over 1000 miles. But I only took off the minimum so next few stops would throw a cap over master cylinder or parked in shade.

hobot said:

or parked in shade.

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Heard of a shade tree mechanic, but didn't connect it with this problem !!??

Inneresting but....

Thanks for the feed back. I purchased the master from Fair Spares in 2008. It has worked well until the last year or so. The first time it happened, the bike sat in the sun for at least an hour or two. I was picking it up after having some work done on it. I rolled it onto a trailer and found things frozen while securing it. By the time I got home, with air flowing across the cylinder, it was free. Today, I parked in the shade and when I got on it to ride home, the front brake locked up. I had to get a screw driver out and pry the pads away from the disc.

The brakes have been fine since I got the bike in 07, so I haven't done anything to the caliper. Maybe it's time to rebuild that as well.

I am going to call Fair Spares on Monday and see if they have any suggestions. In he mean time, I think I'll try opening the bleeder valve and see if that does anything.

Thanks again.

Another thing to consider, but I think not much related to temperature - that is the failure of the inner wall of the rubber lines (assuming you still have them). I had the front brake lock up on me back around 1990 (with stock MC and calipers) due to some sort of failure mode that would allow pressure to be put against the disk, but not to release it (except really slowly). If your brake lines are relatively new (or teflon lined stainless lines?), then this issue can probably be discarded.

Rohan the sidewalk machinist work was on sun lighted egg frying hot cement, the shade parking aways from home was afterwards in parking lot of a mall Wes and I sipped a mint Julip and snacks to recover from not having our handy shade tree while working feverishly on mysterious show stopper 50 miles into 1000 m saga of much more of the same. Real Englishmen may not mind the sun but but us red necks don't really enjoy getting that way. Much as flame attacks moths to their doom so do the curves here in heats.

Near Lo Gap, Mt Sherman on route called Jasper Disaster. Its where i tested Trixie's factory disc to brake fade till air layer in pores got evaporated out to get back lock up too easy again.

Don't take it to heart Steve, it was just a funny connection I saw.
At least, I thought it was funny....!

Pretty obscure problem when a few thou can make such a difference ?
I've seen some crud cause similar problems, semi-blocking up that little port, in several makes of bikes. Change that fluid, quite regularly.

P.S. When I think about it, it didn't cause the brake to lock on, it gave no effect pulling on the lever...

I'm just enjoying a distraction on a rainy day we needed badly, but this is not a new issue and discussed prior with similar solution done inside a shop. It can happen to rear brake m/c too even if not re-sleeved I've read here prior. There is a tedious measurement way to find how much to relieve the plunger length if ya the luxury quality fettering time.

The bike has now been sitting for several hours. Nothing seems overly hot at this point. In the past, the issue would have resolved itself, but tonight the master seems frozen tight. As suggested, I opened the bleeder valve. I got just a drop or two of brake fluid and no relief at the brake lever. With a screwdriver, I was able to pry the brake pads open enought to roll the bike around he garage, but the lever is still stuck.

I'm still not sure which end the problem is, so I don't know whether to take the caliper off and rebuild it, or send the master back to Fair Spares and see if they can find an issue.

Guess I will just have to ride anoher bike for now. Luckily the Triumph didn't sell when I had it listed on ebay.

I went through hell with mine after a crash and replaced the brake lever with what I thought was stock dimensions. I warped two rotors, rebuilt the caliper and bled a quart of DOT 3 through the system before figuring out lever was out of spec and when hot the brake return hole didn't open fully. :evil:

calbigbird said:

The bike has now been sitting for several hours. Nothing seems overly hot at this point. In the past, the issue would have resolved itself, but tonight the master seems frozen tight. As suggested, I opened the bleeder valve. I got just a drop or two of brake fluid and no relief at the brake lever. With a screwdriver, I was able to pry the brake pads open enought to roll the bike around he garage, but the lever is still stuck.

I'm still not sure which end the problem is, so I don't know whether to take the caliper off and rebuild it, or send the master back to Fair Spares and see if they can find an issue.

Guess I will just have to ride anoher bike for now. Luckily the Triumph didn't sell when I had it listed on ebay.

Click to expand...

Have you checked your brake line? Mine was almost blocked from age on my Triumph's rear disc brake. Similar symptoms as you have. New brake line solved my problem!

My brake line is braided stainless I changed out in 2009. My brake fluid appears pretty dark, I am beginning to suspect dirty fluid. I will probably take the master off and send it to Fair Spares for inspection. I will probably rebuild the caliper while it is apart. Not sure what else I can do.

Thanks for all your thoughts.

The necessary clearance for a caliper to release a rotor after the brake is no longer being applied comes first hydraulically, as the pressure decreases to a point it is no longer pushing the pads sufficiently against the rotor to bind it, and then mechanically, as the stretched caliper seals exert a minute pull on the caliper pistons, thus allowing the pads to just back off the rotor enough to free it. A sticking brake has to result from one or the other (ignoring out of the ordinary items like wildly warped rotors or thrashed wheel bearings, etc).

So, if when you open your bleeder while the brake is stuck hard, you don't get a spurt of pressurized fluid which results in the brake freeing up, then it isn't a pressure problem and is not your master cylinder or brake hose. The only thing still holding your pads against the rotor when the bleeder is wide open (or, if suspected of being clogged, removed entirely) has to be the caliper and its pistons/pads. This is not to say you can't also have a problem with the MC or hose, just that in lieu of pressure - the primay culprit has to be the caliper.

What doesn't compute is that you said the brake lever stayed tight, as in you couldn't squeeze it to the bar (??), even with the bleeder open? Perhaps you do also have a hose problem. We see a lot of that but usually where the lining develops a flap inside which acts like a valve in a vein, allowing the flow of pressure/fluid in one direction when subjected to the effort through the MC, but not in the other direction when the pressure is left off. Unusual to have it block up in both directions.

The piston in the MC seems to be stuck in the full in position. The brake handle flops around because there is no return pressure. The last time I applied the front brake (fortunately while in my garage) it felt a bit mushy, and the MC piston stuck in the full in position. I can take the brake handle completely off the bike and the piston remains pushed in. I suspect that is the primary problem. Why it is not pushing the brake lever to the at rest position is a mystery to me.

I think my best course is to rebuild the entire system, from top to bottom. Not knowing how to disassemble the sleeved MC, I will probably send it back to Fair Spares. If there is an issue, perhaps this will help them for future reference.

What confuses me is the brake has worked well since I put on the sleeved MC. It is just in the last year that I have had issues I have always associated with external heat, but it always recovered on its own after everything cooled down. (It actually only acted up two or three times and never this bad.) So far, cooling off has not done anything. The joys of owning a vintage bike I guess.

The master cylinder scared me off first time in but its actually one of the simpler items to service and can be done on bars if only some rust or crap to flush out. Beware of brake fluid drips to jets up on tank if doing on bars. The spring is inside m-c is what pushed back not fluid pressure and should be plenty enough to push piston and lever back, so where I'd look first, but can't ignore old hoses can clog up inside like a one way check valve. If it was just the lever plunger length then the caliper should of released as soon as lever removed. Bleeding off at caliper should also release pad grip so might have rust in m-c and caliper bores. Air flow at speed should prevent discovering lock up on the fly, so main issue is the slow speed take offs with a touch of brake to stay safe that instantly takes front out if turned much. Rust pitted piston is a non issue as long as not raised rust left on it as the rubber seals are only thing that should touch bore wall or outer end low pressure sealing.

Went to the garage this morning and found everything back to normal. MC piston released overnight and the brake if functioning as expected again. I am still going to contact Fair Spares and try to figure out why this is happening.

You could get the CNW Brembo kit and never have this problem again. Just a thought.