Motorcycle front calipers locking up.

tyr_shadowblade

tyr_shadowblade

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I am ashamed to admit that I am too inept to do any but the most basic of mechanical repairs, but I do recognize my limitations and seek advice from those who are knowledgeable about this sort of thing.

Front brakes are locking up for unknown reason. Last weekend I rode for over a hundred miles with no problems. Today, within about 10 minutes both front calipers locked up and I hadta ride it home in rush hour traffic with smoke pouring out of them . . . but they didn't catch on fire, so that's good.

Forks are from a 78 Honda 750 Super Sport. Master cylinder was modified by a local shop to be pedal activated and brake lines needed some modification. Did this shortly thereafter, they bled the lines again and road tested it and it was fine. I drove it a few times with no problems. Never touched the brake pedal again as it was intended solely as an emergency backup. Then today they lock up.

It is my understanding that hydraulic brakes are simple devices. Am not sure what is causing the backup in pressure. Shop says they're closed tomorrow and I should bring it by Saturday, but I'm about ready to just unbolt those calipers and toss them in the trash rather than risk it happening again. No front brake is better than a front brake with a mind of its own that wants to kill me.

Any ideas as to what might be causing this and how it might be fixed?
 
crack the bleeder and see if that releases the calipers. if it does then you could have a stuck plunger or the piston in the caliper is sticking. pm me your number and i'll talk you through a few thigs you can check out.
 
Opened both bleeders, drained a bit of fluid, and everything seems to be working fine right now. Drove around town a bit, using the front brakes to stop, and they are releasing fine.

Master cylinder and calipers were both filled with crud when received and cleaned out thoroughly and soaked in solvent then were completely rebuilt. It may be possible that something is sticking, but what troubles me is that the brake pedal hadn't been touched at all -- it just locked up while I was driving -- and it seemed like it happened gradually, like pressure was building up for some reason. Brake line goes up about 20" from the master cylinder pedal then through the triple trees and down to the calipers if that makes a difference (physics is not my friend).

Too dark to look at it right now and I will probably be working all day tomorrow. May contact you this weekend if it acts up again. Will probably take it across town to the shop where they have a better idea of what they're doing from now on.
 
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did you put the right fluid in? some systems require a silicone base fluid. the seals could have swelled causing the problem. not too much can cause the caliper to lock up.
 
richard j said:
did you put the right fluid in? some systems require a silicone base fluid. the seals could have swelled causing the problem. not too much can cause the caliper to lock up.
Click to expand...

Original specs call for DOT 3, they used DOT 4, but the system was completely cleaned out and rebuilt and the fluids are supposed to be interchangeable. Both are glycol base.
 
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richard j said:
not too much can cause the caliper to lock up.
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Heat can. Is any part of the sytem running near anything hot?
If you have a sticking plunger, I wonder also if vibration couldn't be building pressure? How big/heavy is the brake pedal and how is it mounted? The only reason I ask is that I had a similar problem on one of my old Harleys. Large rear brake pedal bouncing on a sticking master cylinder cooked a set of pads and almost caused a lock up of the rear tire.
 
I'm no mechanic, just a part-time motorcyclist. Doesn't stop me from having some opinions though:
1) I wouldn't ride the bike until the problem is diagnosed and repaired.
2) I wouldn't ride without a front brake; most of a bike's stopping power comes from the front brake.

Good Luck!
 
Bob W said:
I'm no mechanic, just a part-time motorcyclist. Doesn't stop me from having some opinions though:
1) I wouldn't ride the bike until the problem is diagnosed and repaired.
2) I wouldn't ride without a front brake; most of a bike's stopping power comes from the front brake.

Good Luck!
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IIRC he has a V8 trike.
 
m1marty said:
IIRC he has a V8 trike.
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Yep. Two wheels in the back with disc brakes. Had a leak and lost pressure once, and that was uncool, so I had the lines and seals replaced and haven't had any issues since, but it would suck to have brake failure coming down the mountain so I decided to have dual discs installed up front as a backup emergency brake. I don't use the front brake when I'm riding, it's just there in case the primary system fails.

It was under 90 yesterday and the brake system is far removed from the rear mounted engine & exhaust. It is possible vibration may've caused something, but there doesn't seem to be any play in the pedal at all. It is a modified hand lever attached direct to the master cylinder. Hand lever is cut to half length and a small square piece of steel is welded in place. Doesn't look very heavy at all, and it is depressed horizontally rather than down into the floorboard. I'll try to post pics this weekend.

Had a hell of a time getting one of the bleeders open yesterday -- even after I got it home and let it cool down. But now that is fixed, and I'm making sure to carry the proper sized wrench for the bleeder in case it happens again. Pliers weren't doing it.
 
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I'd change the brake fluid,try a silcone fluid that's made for racing applications,it sounds like your fluid is heating up,once it does,it will expand and do exactly what your saying.
 
check and see if the brake pedal is sticking before tearing anything apart my motorcycle does that from time to time , either the spring is weak or the pedel has a binding spot . so now after stopping i simply put my foot under it and pull up to unstick it .
 
Well, I don't know why the fluid would be heating up if I'm not using the front brake system, and the pedal seems to work fine (until pressure builds and it freezes up and cannot be depressed), and I'm also not using it.

Pressure is gradually increasing even though the brake is not being used.

After bleeding it, had the trike out today. Ran fine, but after a while noticed that the pressure was increasing again, just enough to make the pads touch the rotors without locking up the wheel. Got it home just before it clamped down. Brake was hot, but not as bad as yesterday when smoke was pouring out.

Opened the bleeder and fluid squirted out. Caliper instantly released. Opened both bleeders and pumped the brake a few times. Now there isn't enough fluid to close the calipers and that is fine with me.

Also realized that the genius who fabricated and installed this braking system also installed the directional switch upside down, so the blinkers are opposite from what they should be.

So, I'll be sharing my feelings about his quality of workmanship with him tomorrow. I'm thinking I'll leave the brake system empty until I can meet with someone who knows what they're doing -- who can also double check that the front end isn't going to detach while I'm riding down the road.

1978 brake system is not rated for DOT 5 fluid, which will coagulate if mixed with DOT 3 0r 4. It is possible that DOT 5.1 could be used, but I don't know enough about it and don't want to risk it. Do not understand why pressure is building up if the brake is not being used. Stupid question: any chance of a caliper locking up with no fluid in the system?
 
A couple of additional things from an ex-M/C tech:

1. The makeshift pedal/handle thing: Not a bad idea, but if there is not full return on the lever pedal (actually on the piston plunger in the m/cyl), for whatever reason, the bleed port in the master cylinder will not be exposed to allow extra pressure (without depression) to return to the reservoir. All it takes is an a couple of millimeters of piston travel to block that off. ALL japanese M-cyls have this port.

It's modified. I'd be looking at that first. That couple of millimeters may also be slowly pressurizing the caliper. Now see #2.

2. If there is corrosion in the bore that the caliper piston travels, then it could not release fully or easily. Enough to have the pads touching and increasing friction and then heat into the caliper--which will expand the fluid and make things worse...

Talk to your tech guys before blasting them No one wants a liability.

Dot 4 is perfect. Nothing else needed or advised.

Coop
 
SharpByCoop said:
2. If there is corrosion in the bore that the caliper piston travels, then it could not release fully or easily. Enough to have the pads touching and increasing friction and then heat into the caliper--which will expand the fluid and make things worse...
Click to expand...

That sounds about right. Genius Wrench did say, "There's a little bit of brake noise from the pad touching the rotor but my bike does that too and it is nothing to worry about." When I pointed out that the pedal was unusually stiff, that was "okay" too.

Yesterday I was unable to loosen the bleeder on the road and whenever I hit a patch of dirt or needed to turn the wheel at low speed on the way home it would lock up HARD.

The first time it happened he thought his assistant didn't bleed the lines properly, and it has been bled 3 times since. Now it is empty. Don't think I'm going to let Genius Wrench work on the brakes again. I'll save my pennies and go across town to the guys who know what they're doing.
 
Hi tyr_shadowblade -

I had something similar happen on an old truck that had been sitting for a while.

Check the rubber brake lines. They can become de-laminated internally, and then create an unintended "anti-reversion valve" that will allow pressure to be applied through the system, yet will not allow reverse flow, i.e. release of the hydraulic pressure.

At any rate, it would all be coming off and get a good inspection - I would look for any swelling in the rubber components, any "crystalized" gunk from old dry brake fluid, and check out the lines (or better yet, just replace them if they are old).

Good luck and best regards -

mqqn
 
Genius Wrench was out on a service call today, so I'll wait to discuss this with him on Monday.

I'm getting a little bit of brake noise up front, but the rotor isn't heating up at all. Draining the fluid seemed to "fix" it, but now no front brake and that is okay for putting around town for now.

I'm thinking it is probably a combination of things. I will probably replace the calipers, master cylinder, and cut a hole through the floorboard to run the brake line through so the fluid doesn't have to move against gravity so much. Good idea, poorly executed with substandard parts.
 
Edit: sorry I misread the part about the old honda forks and thought the brakes were old Hondas too. I had an old 75 super sport that had a sticky front brake pad piston. Corrosion problem according to the mechanic.
 
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I had a Honda of a similiar size and vintage to your front forks that did the same thing. In my case, the sun would heat the master cylinder on the bars, and would lock the front brake up tight. When I took it in, they changed the master, and I never had anymore problems with it.

Until I was able to get it in, I parked in the shade. :D
 
shunsui said:
Edit: sorry I misread the part about the old honda forks and thought the brakes were old Hondas too.
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Forks, wheel, and brakes are stock 78 Honda Super Sport CB750 with minor modifications.

Ran great with no fluid. Finally got ahold of Genius Wrench last night. Fixed the directional switch and removed the brake lever/pedal thing and ground off a bit of metal where it comes in contact with the plunger. Rode it for an hour afterwards with no issues whatsoever, and hopefully it is now fixed.
 
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