Can you fix a slow return SMC?

T_C · Aug 24, 2017

Or better question, is it worth it? Time, money, safety?

Back story here. Changed brake pads 2+ months ago. Everything is good. Did full flush, as I do once a year. Tested SMC, all was working.

Rode into work yesterday and the bike seemed to be dragging when I pulled in the clutch on the highway. Really felt it pulling onto base. At my parking area I sit on a small hill, maybe 3Â° incline, I park in nose first. Siting there I put the bike in neutral and it wouldn't roll backwards without a push.

Came out a few hours later and all is fine. Took the bike across base to buy some gas and it starts feeling the same.

My plan for the ride home was to use my eyes, look plenty far ahead and downshift, use the back brake only so limit the SMC movement. Stayed late to avoid some rush hour.

Made it just about home and didn't really feel any drag. Grabbed the front brake going up the driveway a few times hard enough to activate the SMC and then started feeling drag as I was making the turn into the garage.

Now the question?? Do I try to flush the debris out? Can I open the SMC and clean the ports? Or do I just save myself the aggravation and be pro-safety by replacing the 12 year old 119k mile SMC with a new one?

Igofar · Aug 24, 2017

If your operating your bike with a 12 year old secondary master cylinder with 119,000 miles on it, you should run out and buy a lottery ticket!
Once the bores are damaged, the rebuild kit will be useless, and a waste of your hard earned money.
IF your problem is ONLY the secondary master cylinder (you will also need to go through your entire brake system, checking all the guide pins/clips, stopper bolts, etc.) you will need to replace the unit as a whole, and do a proper bleed of the entire system.
2005 Secondary master cylinder - 06454-MCS-G03 (complete assembled unit) and you will also need (4) crush washers - 90545-300-000.
You should probably check out the pictures of the damaged SMC that john heath just posted for me on this thread (2009 ST1300 Sticky Rear Brake).

T_C · Aug 24, 2017

Igofar said:
Once the bores are damaged, the rebuild kit will be useless, and a waste of your hard earned money.

Yeah, re-build kit wasn't really a thought. Heard too much about them. It was either can I clean ports and continue to use original, or replace the whole thing. You always say Larry that properly maintained and flushed the ST1300 brakes will continue to work well and be trouble free for a long time. Here is an example!

Remember I was thinking about being prepared to replace the '05 SMC with an post '09 SMC. But since I have new pads on the '05 SMC, I will continue along the same line.

SupraSabre · Aug 24, 2017

I know Larry likes "new" SMCs, and not rebuilds, but I rebuilt my 2010's SMC, foot MC and rear caliper when I had my rear brakes freeze up on it a few years back. And I haven't had anymore issues from it either. (Although the bikes been in storage for the last three months due to a slipping clutch.

This bike will be next on the new lift to get a complete maintenance done to it after I get back from WeSTOC! I'll probably park the 2012 in my storage unit for a little while!

dduelin · Aug 24, 2017

T_C said:
Or better question, is it worth it? Time, money, safety?

Back story here. Changed brake pads 2+ months ago. Everything is good. Did full flush, as I do once a year. Tested SMC, all was working.

Rode into work yesterday and the bike seemed to be dragging when I pulled in the clutch on the highway. Really felt it pulling onto base. At my parking area I sit on a small hill, maybe 3Â° incline, I park in nose first. Siting there I put the bike in neutral and it wouldn't roll backwards without a push.

Came out a few hours later and all is fine. Took the bike across base to buy some gas and it starts feeling the same.

My plan for the ride home was to use my eyes, look plenty far ahead and downshift, use the back brake only so limit the SMC movement. Stayed late to avoid some rush hour.

Made it just about home and didn't really feel any drag. Grabbed the front brake going up the driveway a few times hard enough to activate the SMC and then started feeling drag as I was making the turn into the garage.

Now the question?? Do I try to flush the debris out? Can I open the SMC and clean the ports? Or do I just save myself the aggravation and be pro-safety by replacing the 12 year old 119k mile SMC with a new one?

If it was me or a friend asking me I'd say first have a look over of the entire system to diagnose the problem. I would want to know what is wrong to cause the problem. Like Larry said if the SMC is damaged then it's probably not worth messing with the old one but it may not be the SMC and it's simple enough to check out the entire system and you and everybody should on higher mileage bikes before yanking and replacing. It is quite possible the problem lies elsewhere, a blocked compensating port or gummed-up caliper slide pin would not be unusual at that mileage. I had a sticking rear caliper at 125K that was caused by a sticky slide pin.

Properly maintained the SMC and associated system is more than able to last much longer than 100,000 miles or so. Mine has 60k more than yours and is in excellent shape. Regular inspection and service combined with a bit of luck to get a good one, who knows?

T_C · Aug 25, 2017

dduelin said:
It is quite possible the problem lies elsewhere, a blocked compensating port or gummed-up caliper slide pin would not be unusual at that mileage. I had a sticking rear caliper at 125K that was caused by a sticky slide pin.

Thanks. Yeah I won't know 100% fer sure until I dig into the system a little more but I'm pretty confident the culprit is up front just to the left of the wheel. Braking system behaves when I use the back brake, it only starts dragging after I use the front brake.

dduelin · Aug 25, 2017

Just be methodical in diagnosis. Both front and rear brake compress the SMC.

Al Gully · Aug 25, 2017

Do you have infra red temp gun. Use your back brakes only and measure the temps on the 3 rotors after your ride. Let them cool down and take your bike on the same circuit and measure again.
This might help in your diagnostics.
Lots of different pistons configuration workings when you use either the front lever or rear pedal by themselves.

Anna'sDad · Aug 25, 2017

dduelin said:
Just be methodical in diagnosis. Both front and rear brake compress the SMC.

Did you know that in the event that the rear brake caliper is not releasing due to hydraulic pressure (not mechanical binding), noting which bleed screw relieves the pressure will help determine if the fault is with the Rear Master Cylinder or the SMC? The center piston of the Rear Brake Caliper receives hydraulic fluid only from the Rear Master Cylinder, where as the two non-center pistons receive hydraulic fluid via the SMC. Here's a diagram of the brake system that I made to aid in my understanding of the Honda DCBS, perhaps it will be of assistance to you.

T_C said:
Do I try to flush the debris out?

Probably not a good idea, even if the problem were flushable the next bleed screw is located just downstream of the Proportional Valve. Don't want to push debris through that!

T_C said:
Can I open the SMC and clean the ports?

I think that's also known as rebuilding the SMC.

T_C said:
Or do I just save myself the aggravation and be pro-safety by replacing the 12 year old 119k mile SMC with a new one?

I think the prudent thing to do if the SMC is the culprit: Buy a replacement SMC, and a SMC rebuild kit. Install the replacement SMC, and rebuild the faulty SMC when you have the time. Now you have a spare ready to go!

This is exactly what I've done....Twice!

T_C · Aug 26, 2017

dduelin said:
Just be methodical in diagnosis. Both front and rear brake compress the SMC.

There is the delay to the front when using the rear pedal. So with quick modulation you can use the rear brakes and not activate the front.

T_C · Aug 26, 2017

Al Gully said:
Do you have infra red temp gun. Use your back brakes only and measure the temps on the 3 rotors after your ride.

Good reminder... thanks!

T_C · Aug 26, 2017

Anna'sDad said:
Did you know that in the event that the rear brake caliper is not releasing due to hydraulic pressure (not mechanical binding), noting which bleed screw relieves the pressure will help determine if the fault is with the Rear Master Cylinder or the SMC?

Thanks for the memory refresh. Helpful to pull back some of the penguins that jumped off the iceberg.

Igofar · Aug 26, 2017

T_C said:
There is the delay to the front when using the rear pedal. So with quick modulation you can use the rear brakes and not activate the front.

This is INCORRECT, you cannot apply the rear brake lever without activating the front brake.
BRAKE SYSTEM INSPECTION - this model is equipped with a dual combined brake system. Check the front and rear brake operation as follows:
- place the motorcycle on its center stand and shift the transmission into neutral. Push the left front brake caliper upward by hand. Make sure the rear wheel does not turn while the left front brake caliper is pushed.
- Jack up the motorcycle to raise the front wheel off the ground.
- Apply the rear brake pedal. Make sure the front wheel does not turn while the rear brake pedal is applied.
* No matter how quick you use the rear brake, it activates the center piston in ALL three calipers.
Refer to brake system inspection in Factory service manual(s).

Igofar · Aug 26, 2017

T_C said:
Or better question, is it worth it? Time, money, safety?

Back story here. Changed brake pads 2+ months ago. Everything is good. Did full flush, as I do once a year. Tested SMC, all was working.

Rode into work yesterday and the bike seemed to be dragging when I pulled in the clutch on the highway. Really felt it pulling onto base. At my parking area I sit on a small hill, maybe 3Â° incline, I park in nose first. Siting there I put the bike in neutral and it wouldn't roll backwards without a push.

Came out a few hours later and all is fine. Took the bike across base to buy some gas and it starts feeling the same.

My plan for the ride home was to use my eyes, look plenty far ahead and downshift, use the back brake only so limit the SMC movement. Stayed late to avoid some rush hour.

Made it just about home and didn't really feel any drag. Grabbed the front brake going up the driveway a few times hard enough to activate the SMC and then started feeling drag as I was making the turn into the garage.

Now the question?? Do I try to flush the debris out? Can I open the SMC and clean the ports? Or do I just save myself the aggravation and be pro-safety by replacing the 12 year old 119k mile SMC with a new one?

It would not matter if you used either the front lever, or the foot pedal, as they both activate both wheels.

Al Gully · Aug 26, 2017

Larry,
I think what Thomas was trying to explain is the delay valve will delay the fluid to the front calipers, so if he does short jants to pedal, it won't be quick enough to apply fluid to the front center pistons which in turn activates the SMC.
I guess Honda put in the delay valve to limit nose diving while braking.

ST Gui · Aug 26, 2017

Al Gully said:
I guess Honda put in the delay valve to limit nose diving while braking.

The question then becomes— what's the duration of the delay? Is it only time sensitive or is it time and pressure sensitive? That is— is the delay shorter with greater pressure?

Like ABS I'd guess that there's a threshold at below (or above?) a human's response time is insufficient to circumvent a device's intended operation.

(Is this the same as a Gold Wing's anti-dive mechanism? My GW buddy had to get his replaced.)

Igofar · Aug 26, 2017

Al Gully said:
Larry,
I think what Thomas was trying to explain is the delay valve will delay the fluid to the front calipers, so if he does short jants to pedal, it won't be quick enough to apply fluid to the front center pistons which in turn activates the SMC.
I guess Honda put in the delay valve to limit nose diving while braking.

The brake system works with pressure.
ANY time you apply either brake lever, the pressure starts to move piston(s). There is NO way a person can operate the rear lever and only activate the rear brake caliper, and not activate both the front and rear brakes.
If you don't believe me, go out in the garage, place your bike on the centerstand, and have someone turn the front wheel, with the wheel moving, TRY to apply pressure to the rear brake lever and watch what happens.
Unless you have superhuman, spiderman like skills, you will find the front wheel with stop as soon as you even begin applying pressure to the lever (if your brake system is working correctly).
There is no way around this, if you grab only the hand lever, you are applying pressure to 6 pistons, if you apply the foot lever, you are applying pressure to 5 pistons.

T_C · Aug 26, 2017

Al Gully said:
short jants to pedal, it won't be quick enough to apply fluid to the front center pistons which in turn activates the SMC.
I guess Honda put in the delay valve to limit nose diving while braking.

Correct Al. Just a quick tap on the pedal and you can activate the rear brake and not the front.

From what I read and the way it works is based on pressure. More push and the shorter the delay.

No need to deny it's existence, but there is a delay built into the system. As was said the lack of nose dive shows it as well as the documentation from Honda.

Igofar · Aug 26, 2017

T_C said:
Correct Al. Just a quick tap on the pedal and you can activate the rear brake and not the front.

From what I read and the way it works is based on pressure. More push and the shorter the delay.

No need to deny it's existence, but there is a delay built into the system. As was said the lack of nose dive shows it as well as the documentation from Honda.

We'll have to agree to disagree on this point.
My guess would be your piston is sticking in the bore, and by tapping the rear lever, (your brakes are not working correctly), and you are simply freeing the stuck piston.
You seem to be confused on what/why the delay system is/does
:shrug2:

Anna'sDad · Aug 26, 2017

Al Gully said:
I think what Thomas was trying to explain is the delay valve will delay the fluid to the front calipers, so if he does short jants to pedal, it won't be quick enough to apply fluid to the front center pistons which in turn activates the SMC.

This is incorrect. The Delay Valve does not inhibit hydraulic flow or pressure to the LH Front Brake Caliper, only the RH Front Brake Caliper is delayed.

ST Gui said:
Is it only time sensitive or is it time and pressure sensitive?

The Delay Valve is pressure activated only. Here's a diagram of the Honda ST1300 Delay Valve Pressure.

Igofar said:
There is NO way a person can operate the rear lever and only activate the rear brake caliper, and not activate both the front and rear brakes.

Perhaps. I imagine it is possible to apply the Rear Brake Master Cylinder (Brake Pedal) such that both the Rear Brake Caliper and the LH Front Brake Caliper are activated before the Delay Valve allows the RH Front Brake Caliper to activate.

Igofar said:
My guess would be your piston is sticking in the bore, and by tapping the rear lever, (your brakes are not working correctly), and you are simply freeing the stuck piston.

I think this is the most likely scenario as well.

At a minimum I would peel the rubber bootie back from the SMC, and look for any signs of contamination.

Additionally, try to quantify the amount of 'free play' of the SMC push-rod. Then the next time the Rear Brake Caliper is locked up, and not releasing, check if the 'free play' of the SMC push-rod has changed. If the SMC piston is not moving freely in the SMC bore the push-rod 'free play' will be greater.

Can you fix a slow return SMC?

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