Recurring rear brake lockup?

On the xs850, the rear brake has a tendency to not release pressure after riding for a while. I only notice when at a stop and I can tell the brake is dragging. It got real hot once and and smoked the pads.. Oops!..

When it happens, I can hop off the bike and crack the bleed nipple at the caliper to release the pressure and get rolling again.

Thinking it must be something in the master cylinder not allowing fluid back into the reservoir.. or something?

After the first instance, using a syringe, I couldn't get fluid to flow from the caliper bleeder back up into the master cylinder and had to pull the unions and do a full re-bleed starting at the master cylinder. On the stand the brake locks up and releases as it should.

Both caliper and master cylinder were completely re-built and using brand new Speigler lines and unions & a brand new stainless piston.

I do have a spare rear caliper that could be re-built and swapped in, tho I dont think the caliper is to blame.

Hoping to isolate the issue, before just throwing parts & cash at it.

Front brakes are unafflicted.

Anyone seen this? Ideas?

 

Blocked return port in the master cylinder l think most likely since you have new brake lines.

 

X2 on master cylinder bleed port blocked. Seen it many times before. When the MC was rebuilt was it throughly cleaned? Make sure all ports between the reservoir and cylinder sections wide open?

 

Yes, it's tiny

 

This one.

 

Very thoroughly, or so I thought. Caliper and MC bodies both hand washed/scrubbed with brushes, picks & carb clean, then boiled in 100% lemon juice for an hour on a Sunday morning.

I guess we go round two, on that whole process..

Some other thoughts after tearing things down this morning.... The little plastic reservoir cup on the MC is basically shot.. its got a hairline crack starting on the outside. To my eye, its not all the way through to the inner wall, tho still not good.
The cup is also BARELY held in by the o-ring and can be lifted out with 2 fingers...maybe I can replace the o-ring for a better fit.
It's still mostly shot and seems its weeping fluid past the oring or at the crack, as the fluid level was quite low even tho i just bled the thing. No obvious fluid leakage on the frame, so dunno for sure..


Everything still looks very clean.. tho I havent yet completely disassembled the MC piston assembly.

Given the reservoir is expired, I hopped on ebay and found an example that looks much healthier, so I guess we try that too...$$$

 

https://www.google.com/search?ie=UT...r&q=motorcycle+master+cylinder+port+operation

 

Yes l am sure that's it. It would have to be a small hole otherwise most of the fluid would go into the reservoir instead of the brake line l think

 

Although I have a pretty good understanding of how all this works.. this link was quite good and gave me a few ideas to chase down.
http://www.st-riders.net/index.php?topic=11931.0

 

I filmed this you can just see the piston moving across the first port.

 

Yes that's it the compensating port has to be smaller.

 

Can you check the small port with compressed air?

 

Are you sure the master cylinder piston is returning fully?

 

Is there a piston adjustment screw on the rear master cylinder? Could it be that for example keeping the pads on the disc when you take your foot off the brake. Or does the pedal need moved round on the splines? Try and turn the rear wheel by hand and if the pads are binding remove the brake pedal and try again.

 

Alright, my dudes... Lots of good suggestions and perspective given above... Thank you, everyone!

I can now report that the XS750/850 rear MC has this integrated plastic reservoir cup setup (not like above, also see below). I haven't checked if some XJs are this way or not, it is what it is on my XS850... The thing with these reservoir cup inserts is, you cant get at the primary and return ports without removing the plastic cup, and usually they are very tightly seated (and by design not intended to be removed or rebuilt).

With the plastic cup removed, and in the photos below, you can see the two ports discussed by members here, above.. The main, large opening, for supplying fluid for actual braking events, and the "return" port (which is TINY by comparison) for allowing rebound of the caliper piston, after brake release, so brakes do not drag.

Finding that my MC was not as clean as purported above. Regrets. On 2nd look, the primary port is obvious & large & I had cleaned it thoroughly. An oversight, the return port, which is RIGHT BEHIND the primary on this MC, is EFFING TINY by comparison & not obvious. Unlike the photos shared by members, above, these ports in the xs7/850 are not plainly visible with just the reservoir cap removed. The XS750/850 rear MC has the ports setup perpendicular to the reservoir cap face....and have a separate plug/bolt for access.....in manufacturing when they drilled these ports, they had to drill them through this access port opening, then plugged the access port with, essentially, an oil pan style drain plug w/ crush washer.. wtf... ¯\_(ツ)_/¯ Remote reservoirs are king now, and apparently for good reason.

My MC is now re-certified clean and I did a full, dodeca-tupple, re-flush on the re-bleed to clear any debris/air.

Shakedown run planned for this evening if it doesn't rain. Photos below.

 

Ok.. Test ride was a failure.. after about 6 miles of in-city riding and the rear brake was starting to lock up.

I think everyone can probably agree, based on the above photos, the MC is pretty clean.

So, I dug out the shop manual & assembled snips of everything on rear brake adjustment below.

The free-play / brake drag warning appears twice....so....yeah...

Heading to the shop now to check & re-check all these adjustments.

 

That looks to be in the same place as my XJ1100 Rear MC. Does your run the rear brake and one of the front brakes as well? The XJ1100 have that. What I ran into was a blocked hose by the MC. As the hoes come out of he MC that make almost a 90 degree bend. This was the block location on both the lines. Just away from the MC on that first bend.

Really nice job on those pics and your added editing with illustrations and lines, well done.

 

Appears the free-play / brake rod adjustment was my real issue. Tho, nice to have the MC so extra super clean now

A PO rounded off the brake rod adjuster lock nut, which was fairly seized. I did a full disassembly and brought everything back to bare metal, and gave a generous coating of Maxima Chain Wax to finish it off.

I then followed the shop manual procedure exactly, especially Step 5, and the special note about the adjuster locknut top and the punch mark on the rod. The PO was WAY off.

Basically, if you can see the punch mark, the rod assembly is too long. Backing off the locknut and twisting the rod counter clockwise will shorten the assembly.

It's my own error for not inspecting this more closely, and sooner.

Test ride was textbook. Zero drag and no heat in the rotor or caliper after several miles of in-city riding.

Thanks everyone for the comments, likes, etc. Case closed.

See also the XS850 Master Thread

 

@slackard well that work on your master cylinder is worth it too should be trouble free for a long time. @Timbox l thought about a blocked lines too initially. I watched a video of a car clutch with a blockage due to incorrect fitting which caused clutch issues.

 

Correction...

It's CLOCKWISE!

When installed on the bike, back off the adjuster lock nut (lefty loosey), then twist the rod CLOCKWISE to SHORTEN the assembly.