lost rear brakes then SMC lost fluid any ideas?

I am brand new to the forum. just picked up a new to me 03 st1300 abs. rode it 35 miles home with no issues. last night went out for a ride and 5 miles in I hit the rear brake and the pedal was dead. nothing there. it wasn't a fast stop so I applied the front and stopped fine. I started to head back and a couple miles as I was coming into a turn the front brake made a loud knocking sound. needless to say it got my attention. I slowly downshifted and pulled over. the top of the SMC had blown off and fluid was all over. I could pull the piston and spring out. towed the bike to the shop. there it sits waiting for diag. any ideas what happened?
thanks in advance for any insight or advise.

Maybe a frozen SMC that just blew apart. Replace it and do a complete check on all brakes, clean and lube all moving parts and then do a proper bleed on them and you will be good to go.

Here's a pictorial of the linked brake system.

The SMC piston doesn't pull out without removing a snap ring iirc and it's bolted to the fork mount.
Sounds maybe a casting failure? Or the snap ring a/o piston corroded spit out the piston and then the calliper rotated until it hit the fork.

This may help with what parts to get.

https://www.st-owners.com/forums/showthread.php?146908-Another-SMC-tale-of-woe-and-Igofar-shout-out!!!&highlight=

Rod

thanks guys. I know the front tire was just replaced before I got the bike. not sure if that matters. love the bike but not a great start. the bike only has 32k. the bike was from texas so hard to find any service history.

Was or is the clevis still attached to the left fork leg? It connects a short push rod from the fork leg to the SMC. If the tire tech did not insert and tighten the retaining bolt in the fork leg lug the clevis probably came detached under braking and broke off and/or levered the retaining circlip in the SMC bore out of place thus releasing the piston and spring.

sidebike1010 said:

thanks guys. I know the front tire was just replaced before I got the bike. not sure if that matters.

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It matters if was not fully put back together! Like not finishing the clevis bolt install.

ok the dealership called and they are rebuilding the front and rear calipers and bleeding the entire system. they cant pin point the problem so they said to be safe they are doing it all. I feel really good about that answer. I was worried I wouldn't be comfortable with their repair.

sidebike1010 said:

ok the dealership called and they are rebuilding the front and rear calipers and bleeding the entire system. they cant pin point the problem so they said to be safe they are doing it all. I feel really good about that answer. I was worried I wouldn't be comfortable with their repair.

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I would also hope they rebuild the SMC or better yet replace it??

I hope it's a different dealer than the one that mounted the new tire.

dduelin said:

If the tire tech did not insert and tighten the retaining bolt in the fork leg lug the clevis probably came detached under braking and broke off and/or levered the retaining circlip in the SMC bore out of place thus releasing the piston and spring.

Click to expand...

This is a very plausible scenario, especially if it can be determined that the SMC clevis is no longer attached to the lug on the left fork leg.

On the other hand, what would be a plausible scenario if the SMC clevis is discovered to be still firmly attached to the left fork leg? I'll venture an idea.

As the Honda ST1300 does have a history of contamination entering into the SMC at the cylinder opening, and causing corrosion to develop in that area:

• The corrosion can eventually lead to a failure of the rear brake caliper not releasing properly.
• A non-releasing rear brake caliper will create massive amounts of heat, that will damage the brake rotor, the brake pads, and cause the brake fluid to boil inside the brake caliper.
• The effect of the boiling brake fluid would appear quite differently on the two rear brake caliper circuits.
• The rear master cylinder brake fluid circuit would probably experience a loss of pressure when the brake pedal is depressed, due to brake fluid vaporization. The expanding brake fluid (due to heat) would merely pass into the rear master cylinder reservoir, with no increase in brake line fluid pressure.
• The SMC brake fluid circuit would experience a build up of pressure (due to heat), and with the path to relieve pressure blocked (corrosion of the SMC bore), it is possible the accumulated pressure could force the SMC piston out of the cylinder at a high rate of speed.

I think that a close examination of the condition of the circlip retaining boss on the SMC, as well as the condition of the rear brake rotor (for signs of over-heating) is warranted. If the SMC piston was forced out of the SMC due to high pressure, I would seriously consider replacing the entire SMC unit as the internal plastic check valve may have been compromised. Also I think the replacement of the rear brake pads should be considered.

For the difference in cost (about $65), I would definitely replace the whole SMC sub assembly rather than just the piston rebuild.

Rod

As previously said, I think you need to get involved with your brakes. I see no reason why this should have happened with properly maintained brakes. Learn what the procedure to bleed them is and what a seated pad looks like and then inspect the work performed by the shop. Also spin the wheels periodically while it's on the stand to make sure they spin freely; this is where the shop should have caught the problem. My shop was not maintaining my brakes properly and they started to seize; the linked nature of the ST1300 brake system is different than other bikes. Thanks to this forum and support I solved all the issues and my brakes work fine now for only the cost of materials.

yes it s a different dealer.

turns out it is just as Anna's Dad described. rear brake corrosion. they are rebuilding calipers replacing pads fluid and the sub assembly like for the smc like was suggested.

only downside is the bike is down for a week and its sunny and 85 here.
thanks everyone for your help