Weak rear brake function

[Superdirk]

After renewing the pads and bolt pins on my rear caliper last winter, I notice weak function of the rear brakes. 2003 model ST1300 ABS with around 70k miles.
When driving, the rear brake doesn't bring much deceleration, some slight diving of front fork. Front brake results in good deceleration combined with diving of the front fork. Pedals feel solid (don;t think air is the issue).

I thought the rear caliper may be seized mechanically, but all is free to move smoothly. When testing the functions on the center stand, I found following:
- When applying rear brake, rear wheel not turning. Front wheel has resistance, but I wouldn't say the brake is firmly engaged.
- When applying front brake, rear wheel is completely free to move. Front wheel locked firmly.
- Front left caliper : All moving freely.
- SMC : Seems seized, even when removed from the caliper.

I had a similar issue 3 years ago, where it seemed the SMC was locked, but when I removed the hoses and cleaned it, it was working fine.

Before ordering parts and taking everything apart, I wanted to consult the collective wisdom of this forum. Does it sound like a SMC issue ? Should I look at other parts (e.g. PCV or delay valve) as well ? I have more money than time, so like/need to get everything fixed correctly in one go..

 

[Mellow]

- When applying front brake, rear wheel is completely free to move. Front wheel locked firmly.

You're not going to get the rear wheel to lock by simply applying the front brake - sitting in your garage.

The smc has to grab the front rotar and move upward triggering the SMC function. I think some have used a board to push it or a 2nd person to do that and you may get some rear wheel brake function.

When was the last time you changed the fluid?

 

[Fawlty]

Pressing the rear brake pedal when on the centre stand has no effect on the front wheel. Others will be along shortly, but if the SMC is seized you will probably need a replacement. Good luck.

 

[Mellow]

1 - Zip tie your front brake lever, lay on the ground and with one foot turn your rear wheel and with your hands push up on the SMC..

2 - make a chiropractor appointment.

 

[jfheath]

The SMC should move only around 1-2mm when operated by hand pressure only. Within that short movement the rear wheel should be held firm by the brakes. When you let go of the SMC it should move back the 1-2mm by itself and the rear wheel should release instantly.

If that doesn't happen then you have two possible scenarios: Either

* the SMC is not working as it should. It is a 20 year old bike and that is extremely likely to be the case.
* The rear caliper is not behaving as it should.

You have to accept that there is a realistic possibility that you will need to replace a number of components. I am thinking:

• The front left caliper / SMC.
• The rear pistons and seals.
• Brake pads
• Rear Caliper bracket

But unless you want to bite that bullet and get ordering now, we could look at finding out what the problem is !!
Some questions:

1. Does the rear pedal by itself operate the rear brakes to lock the rear wheel and release them when you release the pedal ? (You will need help with this one, but we need this answer first))
2. Check your rear reservoir fluid level.
3. Get a non-return bleed tube (or two). Attach one to the rear centre piston bleed valve (the most forward of the two valves), open the valve slightly and press the rear pedal. Does fluid come out easily ? Close the valve without letting any air into the system
4. Fit the non return bleed tube to the rear outer piston bleed valve - the most rearward of the two bleed valves. Does fluid come out easily when the rear pedal is pressed ?
5. Same as above, but operate the SMC. Does fluid come out easily. Can you move the SMC through a full stroke. Does the SMC return when you let go. Close the bleed valve. Do not let air into the system.
6. What colour is your brake fluid ? Lemon tea, Weak tea, medium tea, weak coffee, strong coffee. Anything except lemon tea is not a good sign !!

At this point I am going to recommend reading this article:

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

And then remove the rear caliper to investigate what is happening there.

1. Remove the pad pin and slide the pads out of the rear caliper. Turning the back wheel will help them to slip out. Keep them face to face safely on one side.
2. The large caliper from which you have just removed the pads should now be able to move in towards the brake disk and outwards. This should be an easy sliding movement requiring very little effort. When you let go, it should stay where it is. Note how this behaves.
3. Remove the large rear axle nut on the left hand side, and take the washer off as well so that you don't lose it.
4. Undo the caliper bracket stopper bolt, holding it by the hexagon head remove it and inspect the round shaft for marks, grooves, marks, abrasions. Take photos if it has any. The manual says to replace this with new. Some clean them up and re-use them with Loctite. I'd suggest getting a new one
5. It is possible to manoeuvre the caliper and caliper bracket as one away ftom the wheel and the brake disc. The hoses are still attached to the caliper, so don't strain them. When it is clear, slide the caliper off the bracket. The bracket will be free. The caliper needs to be rested where it will remain without putting strain on the brake hoses.
6. Take a look at the oval shaped hole where the stopper bolt located. Marks, grooves, spiral marking, fretting.
7. Note the condition of the slider pin and the chrome locating clip where the rear brake pads rest. Not this has a ridge on it.
8. Go and look at the brake pads. The thick bronze coloured backing plate at the opposite end from the hole where the pad pin fits. There is a tab. It should have a square notch in the end. Does it have this notch - that corresonds with the ridge on the retaining clip ?
9. Take a look at the caliper - the slide pin that is attached to that. Note its condition. It should be smooth and shiny.
10. Note the state of the pistons. Clean them up with a shoelace and brake fluid or a toothbrush or the ehm of an old towel. Don't use anything other than brake fluid to clean them.
11. Put a bleed tube on the front left lower bleed valve. Yes - that is correct FRONT LEFT LOWER. open the valve slightly and pump a little fluid through with the brake pedal - just to prevent air getting back in. Then with no air in the tube likely to go back into the valve, go to the rear caliper and apply thumb pressure to the two outer pistons simultaneously. If you don't do it simultaneosuly, then pushing one in, will push the other out. If this is too hard, find a piece of wood that will stop one of the pistons from being completely ejected, and use two thumbs to press in the other. Fluid should come out of the bleed valve on the front left centre piston. This should not be particularly difficult.

OK enough. The answers comemnts and photos should give us what we need to know to advise you further. If you want to stop before (say) undoing the back wheel and report back on what you have found so far, that migght be constructive.

 

[SMSW]

While I agree with John, the cheapest and easiest thing to try first is a fluid replacement/flush, especially if you have not done this in the last two years.

 

[Igofar]

Crack the PCV bleeder with a hose on it, and while the bleeder is open, manually push the SMC piston in. It should move almost a half inch if the piston is moving correctly.
If it does not move, you have verified that the piston is stuck in the bore.
Your cleaning and attempt to rebuild it before may have cleared a path in the rear cartridge, but use and dirty fluid probably clogged it up again.
Replace the entire unit, do not attempt to rebuild it.
Also inspect the rear caliper bracket for wear and damage.

 

[Superdirk]

Thanks for all the tips. Obviously the front-to-rear link only works when the SMC is tilted, 20-20 hindsight vision enabled.
Rear caliper looks fine, the slide pins, rubber sleeves and brake pad-pin were replaced a few months ago, pistons move just fine, fluid comes out without air..
Fluid is not too bad, weak tea colour... shouldn't be the cause of anything, but up for renewal..

Did some more tests, confirmed the SMC was not moving / working. Then used some pliers to compress the SMC, then used a lever to uncompress and repeated a few times. Now it's moving again, and braking seems back to normal !

Obviously ordering a new SMC and fluids... but at least I can drive until the parts arrive !

 

[Igofar]

Thanks for all the tips. Obviously the front-to-rear link only works when the SMC is tilted, 20-20 hindsight vision enabled.
Rear caliper looks fine, the slide pins, rubber sleeves and brake pad-pin were replaced a few months ago, pistons move just fine, fluid comes out without air..
Fluid is not too bad, weak tea colour... shouldn't be the cause of anything, but up for renewal..

Did some more tests, confirmed the SMC was not moving / working. Then used some pliers to compress the SMC, then used a lever to uncompress and repeated a few times. Now it's moving again, and braking seems back to normal !

Obviously ordering a new SMC and fluids... but at least I can drive until the parts arrive !

Just as a word of caution, what you've done is to force the piston through rust/corrosion by using your pliers, then again forced it back out with hydraulic pressure a few times.
This also probably forced all the crap into the return port hole and screen cartridge, and its just a matter of WHEN it locks up on you again.
All its going to take is one hard stop, and without warning, you can jam the piston into the bore far enough to lock it up where you won't be able to move the bike at all.
Please be careful, these bikes have been known to toss folks off without warning when the brakes lock up.
Riding it in that condition can/may also cause very expensive damage to the rear rotor/lines/caliper/bracket.
I would suggest not to operate your bike until the SMC is replaced, and the system flushed correctly.
NOTE: the front to rear link has nothing to do with the SMC being tilted. The SMC is tilted 15 degrees during the flush/bleed.

 

[Superdirk]

Just as a word of caution,

Thanks, for the advice ! Wife took the car & kids for couple of weeks,, so bike is the only more of transport.. Will have to be careful until the SMC arrives in a few days, will have to limit driving until then..

By tilted, I meant "tilted by compressing it"...

 

[jfheath]

If you insist on riding it like that, then it is a potentially dangerous situation. I would recommend not to. If you choose to go out on it, then at least take the precaution of regularly pulling the clutch lever in to see if your rear brake is dragging - and certainly when getting back up to speed after every application of brakes, front or rear. On a level road, the effect of pulling the clutch in on the road speed should be negligable. If the rear brake is dragging, the bike will slow down immediately. If that is happening it will get worse very quickly as the heat builds up. Time to stop the bike before it stops you.

You might think that you would be able to tell. No, you won't - unless you are particularly looking for it.