Looks Like SMC Failed Again

While practice riding yesterday I checked the rear brake rotor for heat (something I do periodically) after using only front brakes. The rear rotor was very, hand burning hot even though the front brake was easy to touch warm. The rear brake was dragging so hard the tire spun only 1/8 turn. The SMC assembly moved its 1/8 inch easily. When I was checking the tire pressure before the ride it turned 1 turn. I road it the 2 miles home. This morning I check the rear tire spin and it spun 1 turn again, huh? It used to turn 1 1/2 turns.

I also checked the SMC this morning. It moved easily. I had the good wife spin the rear wheel and activated the SMC, nothing happened. The rear wheel continued to spin like nothing was on it for the rest of the 1 turn spin. I believe the SMC piston is stuck in the down/pressed position. I've had an SMC failure before and revived it two years ago. I'm just going to replace it with the whole assembly, just don't trust this SMC any more.

I double checked John's "avoiding the pitfalls" article and saw something I didn't do. When I put the new rear pads on I just pressed the pistons in without the outer nipple open. I guess this action started the failure I'm experiencing with an already weak SMC. Thanks for the great article John.

Question for those that have replaced the SMC before. How did you relieve the pressure in the brake line without spraying brake fluid every where? I'm thinking that opening the PCV nipple will work.

Hope I get the part, fixed, and tested before TentSTOC! All my sources for discounted OEM parts use Ground FedEx and no overnight, 5 to 10 days starting Tuesday. Think I'll go by the shop and pay more for a 4 day ship. Any sources that can do it quicker?

If you get stuck I have a rebuild kit and my old SMC bracket. It was fine but I replaced it anyway.

Throttlejockey said:

If you get stuck I have a rebuild kit and my old SMC bracket. It was fine but I replaced it anyway.

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Thanks, but I just committed to the assembly thru the dealer here with 2 day shipping, so with the holiday that means Thursday. I'll have next weekend to get things straight. The dealer closes early today so I took a heavy expense for speed. Dealer mark ups are steep!

Again thanks, I'm grateful for the offer. Any advice on pulling the old SMC assembly off that's not in the manual?

fnmag said:

I'm beginning to dislike the SMC on the ST1300s.

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Yeah, it's my ST's Achilles heel. :censored

I've rebuilt, and replaced the SMC (Secondary Master Cylinder) on my '06 ST1300/A three or four times. The last time I installed a SMC (rebuilt I think), I packed the pushrod area, as well as the rubber boot with a silicone brake grease, in an attempt to prevent moisture from seeping into the SMC cylinder bore. I am currently in the longest period between SMC replacement, so I think the silicone grease packing is a useful procedure.

Reginald said:

The rear brake was dragging so hard the tire spun only 1/8 turn.

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Classic symptom of a SMC failure.

Reginald said:

The SMC assembly moved its 1/8 inch easily. When I was checking the tire pressure before the ride it turned 1 turn.

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If the SMC piston was stuck (pushed IN), then this SMC motion was merely moving the pushrod into contact with the SMC piston.

Reginald said:

This morning I check the rear tire spin and it spun 1 turn again, huh? It used to turn 1 1/2 turns.

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When the SMC piston fails to fully retract, it blocks the Compensating Port, thus preventing the return of brake fluid (under pressure) from the rear brake caliper (and Proportional Valve) to the rear master cylinder. Over night, the pressure of the "trapped" brake fluid, may have bled down, due to minuscule leakage around the SMC piston.

Reginald said:

I also checked the SMC this morning. It moved easily. I had the good wife spin the rear wheel and activated the SMC, nothing happened. The rear wheel continued to spin like nothing was on it for the rest of the 1 turn spin.

Click to expand...

That sounds like confirmation that the SMC piston is stuck, and has not returned to it's "at rest position". Note: If you applied the rear brake, the rear wheel will stop spinning, and probably not be able to spin (easily) a second time.

Reginald said:

I believe the SMC piston is stuck in the down/pressed position.

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Agreed.

Reginald said:

When I put the new rear pads on I just pressed the pistons in without the outer nipple open.

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This is generally (not just a ST thing) not a good idea, as it tends to move any debris in the caliper back towards the components with a greater susceptibility to clogged passages.

Reginald said:

How did you relieve the pressure in the brake line without spraying brake fluid every where?

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The easiest place to relieve brake fluid pressure due to a failed SMC is at the Rear Brake Caliper. Slip a bit of hose over the rearward bleed screw, and gently open the bleed screw, this will relieve brake fluid pressure in the brake line from the Proportional Valve (which is fed solely by the SMC).

Here's a sketch of what (I think) occurs when the SMC becomes contaminated due to water ingress.

Anna'sDad said:

The easiest place to relieve brake fluid pressure due to a failed SMC is at the Rear Brake Caliper. Slip a bit of hose over the rearward bleed screw, and gently open the bleed screw, this will relieve brake fluid pressure in the brake line from the Proportional Valve (which is fed solely by the SMC).

Click to expand...

Excellent explanation of my ramblings and great conformation for me. I'm little more than a shade tree mechanic. I've fixed the SMC before when the dealer failed to flush the brake fluid from the PCV and maintain the SMC for a service. I got it working again thanks to Larry (Thank you Larry). I've packed the SMC boot yearly since. As you and John described I probably caused this failure pushing in the pistons.

Thanks for the advice on releasing the pressure, easy to do. Also liked the SMC diagram, and believe you're right about the water corrosion.

Reginald said:

Thanks, but I just committed to the assembly thru the dealer here with 2 day shipping, so with the holiday that means Thursday. I'll have next weekend to get things straight. The dealer closes early today so I took a heavy expense for speed. Dealer mark ups are steep!

Again thanks, I'm grateful for the offer. Any advice on pulling the old SMC assembly off that's not in the manual?

Click to expand...

No problem.

The removal is pretty straightforward, nothing tricky about it.

It's odd that some have failed multiple times and others continue to remain in service to very high mileages.

Not really, some bikes sit for a month then go on long trips, some commute daily, some are in garages and some not, some are exposed to coastal effects and some not, some are in hot climates, some cold.. I do wish Honda would have designed this differently.

Mellow said:

I do wish Honda would have designed this differently.

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A poor design by the Honda engineers is not a good thing, but even worse is that fact that, according to a mechanic at a Honda dealership in North Bay, Ontario, who tried to repair my failed brake system, the tech people at Honda are "totally worthless and clueless" (his words) regarding the issue.

I feel our ST1300 should be recalled like the GOLDWING was.
Here is 2 pages out of 5 pages on the GOLDWING SMC recall in 2011.
I'm not sure on how to put the PDF on the forum from an iPad.
If any one knows how to, please let me know or I can send you the file and you can post it on here.
It's a really good read with schematics.



Ms. Nancy Lewis. December 1, 2011. Page 1
Associate Administrator for Enforcement
NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION Attn: Recall Management Division (NVS-215)
1200 New Jersey Avenue, SE
Washington, DC 20590
Re: Recall Notification
2001-10 and 2012 Model Year GL1800 Secondary Master Cylinder

Dear Ms. Lewis:
American Honda Motor Co., Inc. 1919 Torrance Boulevard
Torrance, CA 90501-2746
Phone (310) 783-2000
11V-567 (5 Pages)

On November 25,2011, Honda Motor Co., Ltd. (HMC) determined that a potential defect relating to motor vehicle safety exists in the secondary master cylinder on certain 2001-10 and 2012 model year GL1800 motorcycles, and is furnishing notification to the National Highway Traffic Safety Administration in accordance with the requirements of 49 CFR Part 573 Defect and Noncompliance Reports.

573.6(c)(1)
Name of manufacturers:
Manufacturer's agent:
Honda Motor Co., Ltd. (HMC)
Honda of America Manufacturing, Inc. (HAM)
Jay Joseph
American Honda Motor Co., Inc. (AHM) 1919 Torrance Blvd.
Torrance, CA 90501-2746
573.6(c)(2)
Identification of potentially affected vehicles:

Ms. Nancy Lewis December 1, 2011 Page 2
573.6(c)(3)
Total number of potentially affected vehicles: 126,000

573.6(c)(4)
Percentage of affected vehicles that contain the defect: 4%

573.6(c)(5)
Defect description:
Some vehicles may experience rear brake drag after the rider releases the rear brakes. The combined brake system utilizes a secondary master cylinder mounted on the left front fork that is actuated by a torque-reaction of the left front brake caliper. As with a typical master cylinder, the secondary master cylinder includes a compensating port to facilitate expansion of brake fluid as the brake temperature increases. Due to production tolerance stack-up, the secondary master cylinder may be mounted at an increased angle compared to the design specification. Swelling of the secondary master cylinder pressure cup may also occur after prolonged exposure to brake fluid and high temperatures. These factors in combination may result in the compensating port becoming blocked. If the compensating port becomes blocked, the rear brake will drag and the brake fluid temperature will increase, causing the fluid to expand and the braking force/drag to increase. Unexpected braking/dragging increases the risk of a crash and continued riding with the rear brake engaged/dragging may generate enough heat to cause the rear brake to catch fire.

Add my name to the list of ST-Owners who have experienced SMC failure and rebuilt, then suffered another failure.

BTW, not my screen name--My real name.

Thanks.