Revisiting rear brake lockup

[Atabeach]

Ok So my rehab continued with bench checking the rear brakes. The Master cylinder seems to have been replaced. Everything worked flawlessly. Upon installation the rear brake locked up. I had very little free play ..removed adjustment screw and reinstalled WITHOUT set screw nut as there was not enough room for free play. Lockup. Replace brake line. Lockup
  The brake pedal starts  out with 2 mm of free play and gradually none until I crack the bleeder valve.
Q: what relieves the pressure from the MC? It should be the retraction from piston spring. I looked at the post about the upside down Chinese “replacement “ and the schematic of  an MC. Could the intake port be allowing fluid in and not out? 

[Laitch]

Q: what relieves the pressure from the MC?

The fluid compensating port of the master cylinder equalizes the pressure in the front cylinder; perhaps the rear cylinder has a similar pair of ports beneath the flange attached to the reservoir hose and one of them is clogged. It doesn't matter whether a master cylinder is upside down if all its components are in good shape. Somebody has replaced your OEM cylinder with a "sidecar" style cylinder like this EMS offering.


A few things seem likely to me:
A brake piston will seize if its bore is corroded. Yours seems to retract when pressure is released at the bleeder but it doesn't retract enough to reset the piston and equalize fluid pressure.
A brake piston could seize if its cylinder has a fluid compensating port and the port is clogged.
A brake piston could seize if it's forced by excessive pressure into its stop screw. The screw is located to the left of the 16 cast into the cylinder illustrated above. Yours doesn't have a hex head screw like the example, maybe it has a screw with a proprietary head, or doesn't have screw at all. I'm too far away from it to see it clearly.
You could attempt to discover what's going on within it by disassembling it. You could just replace it with an Amazon Chinese Special. Mighty Gryphon and Martin have created posts that will give you all the help you need; they seem satisfied with their outcomes. The prices are attractive.

[Atabeach]

OK, so my MC is indeed a Magura with the side stop screw. Looks brand new. This screw limits advance and when backed out allows piston to be removed…I took this puppy apart and that revealed the two holes. The smaller hole seemed clear, I used an angle awl  to remove any debris that may have been present.
   I am not getting travel and reflex to the cylinder more than 2 mm with pressurized oil in system.
I did a test. Pressure on the cylinder, crack the bleeder valve… 1/2 inch of travel nice squirt of fluid. Close bleeder valve piston backs into cylinder nicely.…apply pressure to piston ( as if braking )  Back to 2 mm of travel. This may be normal. The photo shows the cylinder removed. The space between the two rubber seals shows the travel allowed without brake fluid pressure….about 12 mm.

[Laitch]

I am not getting travel to the cylinder more than 2 mm. I did a test. Pressure on the cylinder, crack the bleeder valve… 1/2 inch of travel nice squirt of fluid. Back to 2 mm of travel.

Is this term "travel" referring to pedal free play until pressure starts building? If it is, it sounds typical. If it is no longer seizing, what do you believe was the remedy?

[Atabeach]

Yes Laitch…travel being from non pedal pressure to pads closing on disk. (2 mm ) Keep in mind, I’m not expecting 1/2” of travel. It seems 2mm is a bit small…not really allowing for normal heat expansion as the fluid get hot. Now that the system is cleaned and back in place…I don’t really know why she locked up the first time. Free Play has not been added. We will see…

[Martin]

A mates  Brick was suffering from occasional rear brake locking up. The pedal had to be manually pulled up. On investigation it was found that the pivot bolt was suffering from corrosion. Once cleaned and lubed all is now fine.
Regards Martin.

[Atabeach]

Thanks Martin, the pedal and all moving parts are free and clear…spring action working nicely. I will be using the rear brake more aggressively in the next few days in 90 plus degree heat. If she locks up again, I may try to force fluid backward through MC. and reservoir …..I read somewhere some holes were clogged from factory….Another point would be I removed the nut BEHIND the adjustment screw to allow more clearance…

[The Mighty Gryphon]

I suspect that the problem is that your master cylinder has too big of a bore and it is pumping too much fluid.

The stock master cylinder has a 12mm bore and yours has a 16mm bore.  That extra 4mm of bore is going to push a lot more fluid and may not stroke to the right spot to allow pressure to be released when the pedal is released. 

I have no idea on how you can make that oversized master cylinder work.

[Atabeach]

Griff, This part is marketed as an “upgrade” to stock MC. I am hoping that another bricker that has installled this part with success to chime in.  I am well aware of the success you and others have had with the Chinese MC….Im a week or two away from that option. ( Where are we buying those lately? ) 😜😜 ….on a positive note the bike goes FORWARD nicely!

[Laitch]

I don’t really know why she locked up the first time.
I will be using the rear brake more aggressively in the next few days in 90 plus degree heat.
Another point would be I removed the nut BEHIND the adjustment screw to allow more clearance…

*Based on what you've indicated, it locked up because the compensating port was partially or fully clogged. It's possible that metal fines from production machining were the culprits. Dirt from sloppy brake maintenance also is a usual suspect. Hopefully you didn't damage the ports probing them with an awl, if that's what you used. A narrow diameter angled pick would be the thing.
*Using the rear brake aggressively is a fool's errand and better left to those who have deep experience in its technical use. If it works, enjoy its functioning. You aren't on long, steep, twisting mountain roads where it can be really needed. Aggressive use of the rear brake leads to lockups and low sides when normal mortals choose to do it.
*Is the nut indicated on the attached photo the one you removed?


Regarding Mighty Gryphon's observation: I suspect the stroke length and relative positions of the delivery and recovery ports to the piston seal would be the same. I view the difference in bore diameter of this cylinder would be the amount of effort needed to push the fluid, and cylinder's length perhaps reducing the effective length of the adjustment screw.


Regardless, Atabeach is selflessly offering to be the test pilot. Godspeed, Atabeach!
 

[Atabeach]

Laitch, If she locks up again I will re visit the clogged port hole. I used a fine scratch awl and should not have damaged or bored it open further.  The brake system was fully serviced just prior to my purchase. The MC could have been clogged from old brake line debris.
I will heed your advice and use the rear brake normally…
The nut removed was the adjustment at the switch. I used it on the other side of brake lever to lock it in place…. Please see photo.
Stroke and bore notwithstanding… these are sold to k bike owners….for $200!
Surely the other k bike owners would not be buying these things if there were issues.
I am going to remove caliper & cap and gently squeeze caliper pistons to see if fluid is free flowing. ( if and when she locks up again)

[Laitch]

I've got no issue with this cylinder in your Brick but you should realize it takes more force to get the same effect as with the 12mm. The brake will work and serve its purpose but it will feel stiffer. It will brake the wheel. This cylinder also has a greater volume of fluid which will take a longer time to boil under extreme application, like braking with a sidecar attached rolling down Pike's Peak.

🔥🔥
If the new cylinder is longer than the stock cylinder's length, that means the piston stroke length may have changed. 🔥🔥 Changing length of the stroke creates a couple of possibilities—a stroke too long might not open the compensation port enough to equalize fluid during piston retraction and allow full brake pot retraction, a stroke too short might release hard enough to jam the piston against the stop screw. There are also fines and dirt that could have their own effects on fluid movement.

I believe you've corrected the problem. If you want more length of adjustment for personal or esthetic reason, use a jam nut as the nut on that adjuster rather than the standard nut that's on there now. Jam nuts aren't as thick as standard nuts.

I doubt that it will lock again but if it does, that would indicate a need for you to closely review your karmic debt and make amends where necessary. Fergawdsake, go enjoy the Brick sensibly, and clean the garage again if that will help.
 

[Atabeach]

Ok to be clear the photo was taken without spring attached to frame. It would appear that a nut…any nut would fit behind head of bolt….thats not the case. For some reason that gap since my ownership seems to have decreased….the nut was on back of bolt and there were some adjustment threads. I don’t see that happening anymore 😳😳😳 The gap between the frame stop and top of pedal bolt when brakes are applied is about 2 credit cards…( hand pressure ) ….It is currently working fine.
  As for the length of the MC in correlation to an original…not sure. I will be taking a closer look at my other K75 which has all original equipment….I really don’t care about the aesthetics or feel of pedal…
  I hope you are correct….that the problem may have been solved. Rear lockup is a problem. Thanks for the input…

[Laitch]

Reviewing your photos, I realize I've misinterpreted them so I've burned down some text.  I'll tell you what I've got on my Brick. Pedal free play = 5mm–7mm  You might try loosening the piston adjuster bolt to allow the piston to retract ≈4mm more if brake action tightens up again. You don't have any more pedal bolt adjustment.

[Laitch]

This bolt; loosen locknut, turn clockwise.

[Atabeach]

So we wrap up this post by thanking those who replied with useful insight. After all possible adjustments the rear brakes still have very little free play but have not locked up since I changed to stainless line and flushed system again. The brakes don't stop bike without front brake "help" but this may actually help keep her upright in emergency braking. They seem to work fine... Rehab will be completed shortly on this daily commuter..Photos to come. Thanks to all...

[The Mighty Gryphon]

If you are talking about free play in the pedal, the fact that you are using a 15mm master cylinder in place of a 12mm will reduce the free play significantly, somewhere around 40%. 

It will also increase the required pedal pressure by the same percentage.  Pedal stroke will also be reduced by 40% which will reduce pedal feel to the rider compared to the stock 12mm version.

Master cylinders are matched to the caliper piston to get optimum performance.  Diverging from the stock diameter, either up or down will result in degraded brake operation.  I suspect that the larger diameter may result in a more rapid onset of lock up as the caliper pistons will move more for a given amount for pedal travel making brake operation more touchy than with a smaller master cylinder diameter.

[Atabeach]

Ok Just when I thought it was over…I ordered stainless brake lines for a K75s..Never bothering to check handlebar rise. The top banjo line is too short. The rise is about 7” meaning they are probably RT?....I think I’m changing them just in time. See photo!

[Laitch]

FERFECKSAKE!