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weird brake issue

76Twin

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Seattle/Puget Sound area
So yesterday it was nice out and I decided to dust off the bike for a ride.
Got it started and idling just fine and was about to take off and realized the front wheel wouldn't budge.
Sure enough the front brake is locked.

Took the caliper apart and inspected everything. Piston still in great condition with no pittings whatsoever. The bore is in great condition as well. Pads aren't new but is hardly worn either and have a lot left. Disassembled the caliper arm to clean and lubricate and now it moves like butter. Bled the brake line completely and refilled using the reverse bleed method. No bubbles. Check! Well.... Not so fast. as soon as I tightened down the caliper bolts, the wheel is again locked up tight. What the hell...

If i loosen the bolts and only hand tightened them the wheel moves fine and the brake will grab as expected.
It's as if there is not enough of a free gap between the brake pads and the disc to allow the wheel to turn when I tighten down the caliper bolts. Can brake pads swell due to moisture? I didn't think this was possible.

The bike didn't have this issue when I parked it for the winter.
 
When the piston is completely retracted how much gap between the rotor and pad?

is the mc fluid return plugged?
 
My first thought is the master isn't releasing. Squeeze the master lever a couple of times and then open the bleeder w/o touching the master. if fluid squirts out then the tiny pressure relief port in the master is plugged up.
 
My first thought is the master isn't releasing. Squeeze the master lever a couple of times and then open the bleeder w/o touching the master. if fluid squirts out then the tiny pressure relief port in the master is plugged up.

I will check this tomorrow. Hopefully it won't rain as the bike it outside right now LOL
 
When the piston is completely retracted how much gap between the rotor and pad?

is the mc fluid return plugged?

with the piston completely retracted (used a c-clamp to push it back in all the way) and without even touching the brake lever as soon as I tighten the caliper bolts the wheel is locked. It is as if there is no gap at all.
 
And the strange thing is if I only hand tighten the caliper bolts the wheel will spin free and the brake will grab and release as expected. Obviously I am not going to be riding around with the caliper being hand tightened so....
 
Did you try and remove the pad and see if anything got behind it and the piston?

worst case take some sandpaper and shave it down a mm or two?
 
Did you try and remove the pad and see if anything got behind it and the piston?

worst case take some sandpaper and shave it down a mm or two?


yep did that.
As for sanding it would take a lot more than sandpaper to shave this down. The pads are Ferodo and look to be made of an iron composite of some sort. I know they can rust.
 
You didn't mention it - did you check the clearance adjustment screw?

I did check that and I tried adjusting to allow maximum clearance and it made no difference. Also in the past when adjustment was needed the worse it got was some dragging when the wheel is turned. This is the entire wheel locked in place and not budging at all from the brake pads. Like I said with the piston fully compressed/retracted there seems to be zero clearance between the pads and the rotor disc. But how can that be? It's not like the pads are brand new and maybe the manufacturing defect caused them to thicker than required. I've had these pads for a couple years now and this was not an issue when I parked the bike for the winter.

I really am stumped on this one.
 
My first thought is the master isn't releasing. Squeeze the master lever a couple of times and then open the bleeder w/o touching the master. if fluid squirts out then the tiny pressure relief port in the master is plugged up.


Juts tested this and nope the fluid did not squirt out. Only weeps out when I opened the bleeder.
Brake is definitely grabbing and releasing properly.
 
That's a good thing, one potential problem eliminated which leaves you with something weird going on with the caliper. I'm not familiar with that particular one so no help there.
 
Can brake pads swell due to moisture? I didn't think this was possible.

I didn’t think so either, but you mentioned they could rust. Do you have a spare/extra set of pads you could try? Hate when this stuff happens on a nice sunny day. Good luck with it.


Tom - 1982 CM450E
(“Noob”, but learning fast!)
 
I didn’t think so either, but you mentioned they could rust. Do you have a spare/extra set of pads you could try? Hate when this stuff happens on a nice sunny day. Good luck with it.


Tom - 1982 CM450E
(“Noob”, but learning fast!)


Yeah besides doing the brute force method of grinding down the current pads I'm probably going to get some new pads to see if that solves the problem.
Definitely a head scratcher for sure.
 
It's starting to sound like the caliper holder pin has rusted in place, or (worse) the caliper holder has somehow gotten twisted between the pin and caliper. When you had it apart, did you remove and lube the holder pin, flex the joint? Are all the mounting points coming into flush contact with the fork tube?
 
Yes I removed the pin and lubed it as well. There is no flex in the joint. All mounting points are flushed with the fork. I will snap some pics in a bit
 
Hard to see anything amiss there, all looks pretty normal. And you're sure the piston is bottoming in the caliper when pushed back?
 
Hard to see anything amiss there, all looks pretty normal. And you're sure the piston is bottoming in the caliper when pushed back?

As far as I can tell. I mean the piston isn't completely flushed and there's maybe 2-3 mm lip sticking out but I think that's expected.
I pushed it in as far as I could with the c clamp without damaging the caliper body.
 
Certainly looks all the way in, but I'm not the best judge as I haven't touched one of those in decades
 
Isn't the pad supposed to be pushed into the cylinder to beyond the engraved red line (the wear indicator)?


I don't see a red engraved line anywhere. As you can see there isn't much of the pad sticking out when the piston is fully retracted.
There is maybe 3 mm if even that.

PXL_20210505_012458841.jpgPXL_20210505_012413491.jpgPXL_20210505_012408772.jpg
 
Well I happened to still have the old pads so I swapped out the backing pad with the old one which is half as thick due to wear.
Now the brakes work perfectly and no longer stopping the wheel from spinning.

For now I can ride it again temporarily but this isn't right. I guess I'm going order new pads and see how those fare.

Very strange problem.
 
It's very strange indeed. The horizontal cracks in the pad look suspicious though. Is there a chance that the bike has had some frost during the winter period? Humidity could have gone into the cracks and widened them with temperatures below 0°Celsius.
 
It's very strange indeed. The horizontal cracks in the pad look suspicious though. Is there a chance that the bike has had some frost during the winter period? Humidity could have gone into the cracks and widened them with temperatures below 0°Celsius.


The bike was parked outside for most of the winter. The coldest it got was maybe 30° F which is probably around -1°Celsius. In any case I'm going to order new pads and see how those fit.
 
The bike was parked outside for most of the winter. The coldest it got was maybe 30° F which is probably around -1°Celsius. In any case I'm going to order new pads and see how those fit.

Hey, are you using DOT 3 fluid? If so, and you know it absorbs moisture, I wonder if it's possible it started some corrosion behind the piston and it's expanded a bit pushing the piston out to where you bottomed it? I mean, I can't recall if the piston should go down to the flush position or not but yours doesn't go that far back in
 
I measured an old brake piston and the respective housing this afternoon and calculated that it should probably stick out approximately 1 - 1,5mm. I couldn't put it in because the housing was too dirty. It is the same housing from the late 450s (not the one with the finns) which is identical to the 500T.
 
Hey, are you using DOT 3 fluid? If so, and you know it absorbs moisture, I wonder if it's possible it started some corrosion behind the piston and it's expanded a bit pushing the piston out to where you bottomed it? I mean, I can't recall if the piston should go down to the flush position or not but yours doesn't go that far back in

I am using DOT 3 and yes I am aware the it does absorb moisture hence why I bled the entire system and replaced the fluid.
Inspecting the caliper bore I do not see any corrosion whatsoever. It's actually very clean and in great shape. The bore feels completely smooth to the touch.
 
I am using DOT 3 and yes I am aware the it does absorb moisture hence why I bled the entire system and replaced the fluid.
Inspecting the caliper bore I do not see any corrosion whatsoever. It's actually very clean and in great shape. The bore feels completely smooth to the touch.

I figured you knew how DOT 3 reacts, it was just a thought... shooting in the dark here, such a weird problem.
 
In any case I'm going to order new pads and see how those fit.

Fingers crossed the new pads solve your problem. Sure seems you’ve checked everything else it could be. Frozen calipers, stuck pistons, etc. yes, but I haven’t heard, read, nor experienced a swollen brake pad.


Tom - 1982 CM450E
(“Noob”, but learning fast!)
 
Fingers crossed the new pads solve your problem. Sure seems you’ve checked everything else it could be. Frozen calipers, stuck pistons, etc. yes, but I haven’t heard, read, nor experienced a swollen brake pad.


Tom - 1982 CM450E
(“Noob”, but learning fast!)

Yeah I can't think what else it could be but the pads themselves. I must have disassembled, inspected, lubed, and reassembled the calipers and all the associated parts at least 7 times now.
 
Yeah I can't think what else it could be but the pads themselves. I must have disassembled, inspected, lubed, and reassembled the calipers and all the associated parts at least 7 times now.

I believe that’s what is known as due diligence! [emoji3166]


Tom - 1982 CM450E
(“Noob”, but learning fast!)
 
I may be talking out of turn here, but it worries me when I read that a C clamp was needed to retract the caliper piston.

On my CB600, which has twin pot sliding calipers, if I can't push the pistons back in just using finger pressure something is wrong.

I routinely service the calipers by removing the pads, pumping the pistons out as far as possible without ejecting them, then clean the pistons, lube with a smear of silicon caliper grease, then push them back in.
 
I'm not really surprised to see Danh mention that amount of force necessary. The single piston calipers from back then are more difficult to work with, the larger piston is always harder to manually retract. They have a pretty thick o-ring seal on them and a snug fit, and of course his is original so I'm sure there's at least a little bit of corrosion involved which is why I wondered about a corrosion ring behind the piston from sitting outside all winter.
 
I'm telling ya. The piston and the caliper bore are in great condition. The bore ifels very smooth and has no pittings whatsoever.

PXL_20210505_213745332.jpgPXL_20210505_213745332.jpgPXL_20210505_213315613.jpgPXL_20210505_213312135.jpg
 
Just adds to the weirdness of the situation...

I ended up taking the back pad to my buddy and used his grinder to grind off about 1.5-2mm or so.
No issues now. This is still very strange though.

New pads are also on the way so I am curious if they'll fit without issues when they get here.
 
I ended up taking the back pad to my buddy and used his grinder to grind off about 1.5-2mm or so.
No issues now. This is still very strange though.

New pads are also on the way so I am curious if they'll fit without issues when they get here.

Did you happen to measure the thickness before you ground it down? Curious to know the difference between old/new. And yeah, that’s one clean caliper bore and piston!


Tom - 1982 CM450E
(“Noob”, but learning fast!)
 
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