Can you fix a slow return SMC?

OP
OP
T_C

T_C

Joined
Mar 8, 2012
Messages
4,341
Location
St. Louis, MO
Bike
2005 St1300
STOC #
8568
We'll have to agree to disagree on this point.
and you are simply freeing the stuck piston.
You seem to be confused on what/why the delay system is/does
I'll agree. I'm more thinking a clogged return port. After awhile the rear brake releases. Either way, the bike is in the garage and not moving except to go up on a lift. With a 90 mile commute each day I need to get it fixed asap.
 

dduelin

Tune my heart to sing Thy grace
Site Supporter
Joined
Feb 11, 2006
Messages
9,640
Location
Jacksonville
Bike
GL1800 R1200RT NC700
2024 Miles
006739
STOC #
6651
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.
Yes there is a delay valve to help settle the suspension and reduce fork dive when using rear brake alone. The proportioning valve also helps distribute braking force but I don't believe it's possible to use much rear brake and not apply some front brake. It doesn't take much pressure to activate the SMC......during a diagnostic test hand pressure on the caliper is enough to compress it enough to stop the rear wheel.
 
Joined
Jan 8, 2011
Messages
7,064
Location
Arizona
Bike
2007 Honda ST1300A
I'll agree. I'm more thinking a clogged return port. After awhile the rear brake releases. Either way, the bike is in the garage and not moving except to go up on a lift. With a 90 mile commute each day I need to get it fixed asap.
I suspect the reason that the rear brake releases after awhile is that the pressure slowly forces the piston back through all the corrosion slowly over time.
Check out the photo's of the bore and piston that John posted for me on another thread, maybe you'll get a visual picture of what your piston is up against.
 

jfheath

John Heath
Site Supporter
Joined
Mar 18, 2006
Messages
2,786
Age
69
Location
Ilkley, W Yorkshire, UK
Bike
2013 ST1300 A9
2024 Miles
000679
STOC #
2570
Just a bit of additional information to illustrate what has been said already.

I would suggest that the SMC piston is not able to return under pressure of the spring due to contamination / rust / crud at the top end of the SMC, or due to the cylinder bore becoming rounded.

When the brake pedal is pressed, some of the pressure goes direct to the rear centre piston. Another line goes to the SMC. The fluid enters the inlet port of the SMC which is in between the primary and the secondary seal. When the bike is in the garage and not moving, the pressure from the fluid flows past the outside of the primary seal and applies the rear brake by extending the two outer pistons in the calliper. This is normal.

However, if the SMC piston has not returned properly under spring pressure, then when the rear brake pedal is pressed, the pressure going through the SMC will be able to provide a little extra push against the secondary seal (the one closest to the rubber boot), and push it back - ie in the direction of the rubber boot. This may happen if the extra pressure required to push the SMC back is less than the pressure required to apply the brakes. But by this time, the blockage in the SMC is already enough to prevent the SMC piston returning under pressure of a fairly strong spring.

The symptoms of this are that on the first press of the brake pedal, the braking feels a little soft, and there is too much travel on the pedal. After that, the brake pedal feels normal again. However, each time the SMC moves, the SMC plunger is still not returning and the rear brake will be dragging. The rear pedal will feel soft again when it is able to push the SMC back.

Applying the rear brake pedal also activates the centre pistons of the front callipers, which in turn will cause the SMC to activate. So you cannot prevent the SMC from operating whichever you use - the pedal or the lever.

However, your quick dab of the pedal is probably allowing the SMC piston to be pushed back to its normal position. It doesn't have to be a quick dab - it can be a gentle press - as it has nothing to do with the delay valve. This state of affairs may last for a short while before the extra pressure from the brake pedal returns the SMC to its proper position. The first use of either brakes to slow the bike down, however, will put things back as they were with the SMC not being returned.

The symptoms are a sure sign that there is something wrong with the SMC and it is about to fail completely - resulting permanent brake drag, very hot callipers and brake disc and rear wheel locking. While it is true that you can get home by gently depressing the brake pedal after each use of the brakes, this situation mustn't be allowed to remain unaddressed.

These observations are taken from a friend who had this problem. He reported that the rear brake pedal would go 'long' after riding about 20 miles. The first press of the pedal cleared things up - he heard a clunk as the SMC returned to its proper position - and the brakes worked ok after that and the cycle would repeat. He did not complain of rear brake drag however, and he was not losing fluid. My suspicions in this case were as described above, but that the SMC cylinder bore was so oval that the primary seal was failing and fluid from the rear calliper could squeeze past it to relieve the pressure in the rear calliper.

I dissected his SMC after he had replaced the whole SMC assembly - the photos are here - Post #1. This is little over one year after fitting a new service kit !
 
Last edited:

Gug

Joined
Mar 6, 2005
Messages
564
Age
70
Location
Trenton, MI.
Bike
18' Goldwing Tour
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).
+1 on that! Not to mention Larry was instrumental in helping me with my SMC.
 
Top Bottom