Back caliper locking up

[bplandis]

Started on this a little while back with Larry, just before he went to doc. I have left it sit since and dug in today again. Is my dads bike that was shipped to me. Sat outside under a cover for two weeks. Brakes worked ok prior to it sitting under the cover outdoors. I only rode it a few hundred yards down the driveway from the truck to the house before it went under the cover. Did not notice them dragging or anything but could have been.

After the two weeks under cover, rear was 90% seized. Cracked the rear bleeder on caliper and it would spin. Still dragging though.

Pulled front and rear calipers, cleaned them up and reinstalled. Proper bleed, but still dragging front and rears. Can hear them on rotor. Has been sitting inside garage for about 3 weeks, pulled it out today and rode it down driveway and back. Rear is locked up again. Cracked rear bleeder and frees up, but still not all the way.

Any idea what the cause(s) of that rear locking is? Or is it the old 20+ step process to try and narrow the exact cause down?

Regardless of cause:
Going to rebuild back caliper as it needs it regardless of issue. Rear caliper pistons have some pitting in them. I used para cord and brake fluid to clean them up. Will also replace the rear bracket sub assembly and new caliper bolt (14 mm head) and retainer clips and spring pad as well.

SMC was replaced in 1/21, as well as rear pads , rear bracket (was spiraled) and all clips/retainers. All Honda parts. Brakes bled last season.

 

[TerryS]

The main reason for the rear calliper seizing on (specifically: the outer two pistons) is a problem at the secondary master cylinder on the left fork leg. When you use the front brakes these rotate the SMC bracket and that moves the SMC piston which drives fluid to the rear calliper. The SMC can suffer from corrosion-induced seizure, or it can have some internal blockages; either will stop brake pressure releasing from the rear calliper. Many will advise just to bite the bullet and buy a new SMC assembly from Honda.

Take a look in the Articles tab above; plenty of brake guidance in there,

 

[SMSW]

There are more articles but these will get you started. The second one, by John Heath is a must read.

Article [13] - ST1300 - Secondary Master Cylinder Rebuild- SMC | ST1300 Articles

Secondary Master Cylinder Rebuild- SMC This is a job that probably all ST1300's over 5 years old need to consider. A regular addition of silicon grease into the outer boot might be excellent preventive maintenance. Forest had warned me of this problem but I really didn't understand the...

www.st-owners.com

Article [13] - ST1300 Maintenance - Brakes- Avoiding the Pitfalls | ST1300 Articles

A close hands-on look at the Honda ST1300 braking system. These are personal observations about the things that can go wrong, why they go wrong and what can be done to keep things working sweetly. Updated April 2020 and to address more recent observations, to include information about the SMC...

www.st-owners.com

Article [13] - ST1300 Maintenance - Replacing the Secondary Master Cylinder (SMC) | ST1300 Articles

The usual format from me - Description, some useful photos and a few tips and tricks to help you to avoid the normal 'Doh!' moments - Like emptying all of the fluid out before realising that you wanted tomove the pistons out as far as possible; Like reftting everything before realising that you...

www.st-owners.com

 

[bplandis]

Appreciate the help and links. Tons of great info in those articles.

A first for me today the rear locks as soon as you hit the rear brake. I had it on center stand today, pulled wheel/caliper and cleaned it. Reinstalled and pushed rear brake. Wheel locks. Crack rear bleeder and it releases. Never touched the front brake. Never took bike off center stand.

Went back down after I posted. Pulled caliper and pads again. Using C clamp and old pad, have to crack the rear bleeder to get cylinders to compress. Both covers off reservoirs (front/rear).

 

[Catmandu2]

Appreciate the help and links. Tons of great info in those articles.

It's got all the earmarks of a bad smc, Brad. As so,,, it is not safe to ride it. You don't want the rear locking in traffic,, or at an intersection. Follow the repair process as if your life depends on it,,,, cause it does !! cheers,, CAt'

 

[bplandis]

Catmandu was hoping that was not the case. Has been setting, no plans to ride it. Going to sell it, so will get it sorted.

Weird thing for me was I only pressed the rear brake pedal and rear wheel locked. Bike is on center stand. If you press rear pedal, SMC would not be activated would it? That is what has me perplexed. That and the pistons can not be compressed without cracking the rear bleeder.

7K miles, 2 seasons riding and a flush each season on the current SMC. If it is a bad SMC... it is and I will get it replaced. Just want to make sure that is the issue.

 

[SMSW]

Read John Heath's article describing how the brake system works. That should explain everything.

 

[Grand Rouge]

Could the return port in rear master cylinder be blocked? Tiny tiny hole. Without going through brake diagrams, fluid routing etc......With bike stationary as bplandis describes.......SMC plunger is not activated.....

Weird thing for me was I only pressed the rear brake pedal and rear wheel locked. Bike is on center stand. If you press rear pedal, SMC would not be activated would it? That is what has me perplexed. That and the pistons can not be compressed without cracking the rear bleeder.

7K miles, 2 seasons riding and a flush each season on the current SMC. If it is a bad SMC... it is and I will get it replaced. Just want to make sure that is the issue.

 

[bplandis]

https://www.st-owners.com/forums/attachments/st1300-removing-air-from-brakes-pdf.253443/

Appreciate the help and feedback.

I dont have an issue replacing the SMC if that is the issue and will rebuild rear caliper. Just found it odd and trying to find the "why" this is happening.
SMSW have been looking at those articles as well as the document above which color codes how system is linked and flows. My wondering is that the bike is on the centerstand and not moving... may or may not be relevant. Red text below is what has me curious. And not being able to compress the pistons without cracking that rear bleeder (linked to SMC). Could bad SMC cause both of those? Or could there be another issue?

The article above is what has me thinking. Below is what it states under misconceptions.

Pressing the rear brake pedal only activates the rear centre piston.
Although there are two scenarios here - when the bike is moving, and when the bike is stationary. If the bike is stationary, the rear brake activates the centre piston of the rear caliper - as shown by the pink lines in the diagram in Step 4. It also pushes fluid down the purple lines shown in step 7 of the bleeding cycle to the rear outer pistons. The rear brake pedal also applies the front brakes after a delay via the centre piston. So if the bike is moving, then when the rear pedal is pressed, the SMC plunger will be activated, which in turn activates the two outer pistons on the rear caliper. Either the pressure from the pedal or the pressure from the rocking of the front caliper will result in the two outer p

 

[Andrew Shadow]

I didn't refer to the service manual so be advised that this suggestion is from memory of how the brake circuits work.

If fluid flows the way that I remember, when stationary applying the rear brake pushes fluid to the rear centre piston. It also pushes fluid past the seal of the SMC piston and then on to apply pressure to the two rear outer pistons. If the SMC piston is stuck/seized far enough in to the SMC bore to be blocking the return port, the fluid can not return from the two rear outer pistons. When you crack open the rear bleeder you are releasing the pressure from the two outer pistons that can not return through the SMC.

Perform the SMC functional test to see how much SMC piston movement there is to get an idea of its condition.