St1300 rear brake issue

[jfheath]

Going back to things you have said that may mean something.

i) Corrosion under the rubber boot.
The rubber boot is there to keep water out. If you have corrosion there, then you also have corrosion behind the circlip and washer and spreading into the bore where the secondary seal moves.

ii) The rear outer pistons and the rearmost bleed valve on the rear caliper.
While it is true that the rear outer pistons are operated by the SMC, when working on the bike in the garage, you will note that the brake pedal also results in the operation of the rear outer pistons. The fluid flows from the rear reservoir to the front centre pistons, SMC and on to the rear outer pistons via the Proportional Control Valve. It is able to flow past the primary seal which actually seals only in one direction (like all primary seals in master cylinders).

I mention this in case that you draw the wrong conclusions when you open the rear most bleed valve on the rear caliper.

iii)Regarding the Rear Master Cylinder

You ask whether or not it could be the rear master cyclinder ? The same master cylinder has two lined connected to the Banjo bolt. One goes direct to the front left centre piston before continuing to the SMC. (It also goes to the front right, but it is subjected to a pressure delay by the delay valve). The other goes to the rear centre piston. If the centre pistons release after applying the brake pedal repeatedly, then it isn't the rear master cylinder that is causing the problem. You can also try pushing in the rear centre piston or the front left centre piston to see if fluid returns to the rear reservoir. (The rear centre is the better one to choose, as the fluid can go only in one direction. From the front left, the fluid could go in two directions).

But check out the documents - Going on an Airhunt and Avoiding the pitfalls will give a pretty good idea of how the brakes work and where the fluid goes from where.

Of course, your rear brake problems could always be due to a badly fitted front wheel.
What ? How?

Badly fitted front wheel = caliper offset in relation to the brake disk rotor = one pad of the left caliper catching on the rotor (especially if new) = SMC is applied = dragging back brake.

So see how the front wheel should be fitted. Here is one source.

Article [13] - ST1300 - Maintenance - Front & Rear Wheel Install and Front Axle Animation | ST1300 Articles

Animated sequence to show the correct procedure when installing the front wheel into the forks of a Honda ST1300. The right hand side of the bike is the side with the brake lever. Left is the clutch lever side. This is the convention used in the Honda manuals. The video and the photo below...

www.st-owners.com

Do the research, go through the brakes systematically, don't dismiss anything. See what you find.

 

[Arturs]

Going back to things you have said that may mean something.

i) Corrosion under the rubber boot.
The rubber boot is there to keep water out. If you have corrosion there, then you also have corrosion behind the circlip and washer and spreading into the bore where the secondary seal moves.

ii) The rear outer pistons and the rearmost bleed valve on the rear caliper.
While it is true that the rear outer pistons are operated by the SMC, when working on the bike in the garage, you will note that the brake pedal also results in the operation of the rear outer pistons. The fluid flows from the rear reservoir to the front centre pistons, SMC and on to the rear outer pistons via the Proportional Control Valve. It is able to flow past the primary seal which actually seals only in one direction (like all primary seals in master cylinders).

I mention this in case that you draw the wrong conclusions when you open the rear most bleed valve on the rear caliper.

iii)Regarding the Rear Master Cylinder

You ask whether or not it could be the rear master cyclinder ? The same master cylinder has two lined connected to the Banjo bolt. One goes direct to the front left centre piston before continuing to the SMC. (It also goes to the front right, but it is subjected to a pressure delay by the delay valve). The other goes to the rear centre piston. If the centre pistons release after applying the brake pedal repeatedly, then it isn't the rear master cylinder that is causing the problem. You can also try pushing in the rear centre piston or the front left centre piston to see if fluid returns to the rear reservoir. (The rear centre is the better one to choose, as the fluid can go only in one direction. From the front left, the fluid could go in two directions).

But check out the documents - Going on an Airhunt and Avoiding the pitfalls will give a pretty good idea of how the brakes work and where the fluid goes from where.

Of course, your rear brake problems could always be due to a badly fitted front wheel.
What ? How?

Badly fitted front wheel = caliper offset in relation to the brake disk rotor = one pad of the left caliper catching on the rotor (especially if new) = SMC is applied = dragging back brake.

So see how the front wheel should be fitted. Here is one source.

Article [13] - ST1300 - Maintenance - Front & Rear Wheel Install and Front Axle Animation | ST1300 Articles

Animated sequence to show the correct procedure when installing the front wheel into the forks of a Honda ST1300. The right hand side of the bike is the side with the brake lever. Left is the clutch lever side. This is the convention used in the Honda manuals. The video and the photo below...

www.st-owners.com

Do the research, go through the brakes systematically, don't dismiss anything. See what you find.

So I bled the system again and the drag on the rear caliper has now reduced. When I bled it the second time there was some air in the system, which i was surprised about as there was no air when I bled it previously. When I bled the last rear brake end valve, some really old fluid came through (even though it was flowing clear when I bled it previously), so I presume there must be a blockage in the system somewhere that is causing this. Any tips on how I can investigate this?

 

[jfheath]

Air in the system doesn't cause brake drag, but gunged up old fluid will.

Read through this - it may help - download the pdf.

Article [13] - ST1300 - Brake Maintenance - Getting all the air out of the brake system | ST1300 Articles

A detailed look at the process of getting the air out of the Honda ST1300 brake lines. A sequence of diagrams of the brake circuits, colour coded to show which parts of the system are being accessed when each bleed valve is opened and explanations of where the trouble spots are. These are just...

www.st-owners.com

 

[Arturs]

So today I took the SMC apart and checked the view valve and the small circular plastic bit was clogged up. I used a string of metal from the gear cable to clear it out. I put it back together and there is a massive improvement, but there is still a little bit of a drag and takes a second for the caliper to completely release. So at this point I'm thinking I should maybe try to make that hole bigger? What are peoples thoughts?

 

[Arturs]

Stand by

I am running out of ideas why the rear brake is still drugging

 

[Igofar]

The tiny return port that you just cleared, is not the only one in the system.
Inside the SMC bore is another one

 

[Arturs]

The tiny return port that you just cleared, is not the only one in the system.
Inside the SMC bore is another one

I done both

 

[Igofar]

Have you inspected and cleaned the return ports in the handlebar master cylinder housing (small port directly under the spoon) and gone through the rear master cylinder assembly that is behind the side step on the right side of the bike (foot pedal master cylinder)?
If it were me......(and you probably don't want to go there) I would start from the beginning....
Remove ALL brake pads, make sure they are OEM and not aftermarket.
REPLACE the SMC assembly with a new one, inspect and clean all calipers, clean behind all the pistons, clean/rebuild ALL the master cylinder bore assemblies, make sure all the ports are clear, inspect/lube all guide dowels, make sure all the hanger pins are clean/smooth, and the o-ring on the end of them is in good condition etc.
Inspect the rear brake caliper bracket for damage (see photo's in prior post) Flush and bleed the entire system using a check valve (do not use a vacuum).
And take several close up pictures of what you find, what parts look like, etc.
This may help members spot something that may help solve your issues.

 

[Arturs]

Have you inspected and cleaned the return ports in the handlebar master cylinder housing (small port directly under the spoon) and gone through the rear master cylinder assembly that is behind the side step on the right side of the bike (foot pedal master cylinder)?
If it were me......(and you probably don't want to go there) I would start from the beginning....
Remove ALL brake pads, make sure they are OEM and not aftermarket.
REPLACE the SMC assembly with a new one, inspect and clean all calipers, clean behind all the pistons, clean/rebuild ALL the master cylinder bore assemblies, make sure all the ports are clear, inspect/lube all guide dowels, make sure all the hanger pins are clean/smooth, and the o-ring on the end of them is in good condition etc.
Inspect the rear brake caliper bracket for damage (see photo's in prior post) Flush and bleed the entire system using a check valve (do not use a vacuum).
And take several close up pictures of what you find, what parts look like, etc.
This may help members spot something that may help solve your issues.

Any reason why I shouldn't use vacuum pump instead check valve? I've been using check valve, however one of the posts on here stated that I should use vacuum pump for the best results so I orderd one online.

 

[OkieRider]

Any reason why I shouldn't use vacuum pump instead check valve? I've been using check valve, however one of the posts on here stated that I should use vacuum pump for the best results so I orderd one online.

I think IGF said vacuum can damage one-way passages/valves.