SMC Rebuild

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Can anyone clarify what the little white plastic barrel is inside the SMC under the banjo union with two bolts or whats its function is, im guessing its a one way valve or a small filter possibly. Mine popped out along with a O ring ( I took the two bolts and union off to clean and blow out with air line) and I'm not sure which way up it goes back in as one end is slightly tapered and you can see a little red tab inside it through mesh. Can anyone send some pictures of it in situ or close ups of which way it goes in.

I'm wondering if its a one-way valve and if in upside down this may be the reason rear brake still drags a bit even after fitting new SMC plunger kit (the upgraded one from Honda). if its not allowing brake fluid pressure to return. Any help would be much appreciated.
 

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I'm wondering if its a one-way valve
Yes, and no. The plastic bit does contain a check valve, but also contains an incredibility small passage that bypasses the check valve. When I disassembled my secondary master cylinder, the plastic bit with the end cap (looks a bit like the 60's 'peace symbol') was the end inserted into the SMC, with the O-ring next, it just sits on the plastic bit, and lastly the threaded flange assembly.

When the plastic bit is installed in this manner the check valve allows brake fluid to enter the SMC with minimal restrictions, while brake fluid is still permitted to flow out of the SMC, it's rate of flow is restricted by the incredibility tiny passage. I imagine the intent of the design was to keep a ready supply of brake fluid at the SMC (remember the SMC does not have a brake fluid reservoir of it's own), so that the rear brake caliper always has a supply of brake fluid. That incredibility small passage should allow enough brake fluid to return (to the rear brake master cylinder reservoir) due to expansion of the brake fluid when needed.

The above description of operation is only my interpretation of the brake system. As I've worked on the brakes on my ST1300A, I've disassembled most of the components in an effort to understand the designer's intention of operation.

Here is a link to a drawing I made of the linked brake system used on the Honda ST1300.
As well as a drawing/cartoon of the secondary master cylinder used on the Honda ST1300.
 
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Wow those drawings are awesome, must of taken some time to produce them. Thanks for the info I can rest assured that i did put it back in the correct way, I guess if put in wrong way it could cause SMC to fail. I'm still trying to find out why rear brake still drags some what and have noticed rear disc gets excessively hot, its like something is stopping brake pistons in returning that fraction when pedal released. I have bleed complete system, twice in fact, which was quite easy following the bleeding sequence correctly and replaced all the seals and dust seal with genuine Honda items and even replace the three rear piston with new as were pitted quite bad plus I've got a new rear EBC disc. The strangest thing is when cold with bike on center stand the rear wheel turns freely almost spinning two revolutions but when I've got back from a ride and disc is hot and caliper is warm wheel drags when turned by hand, this suggest that the fluid has expanded with heat causing the brake pressure. If I move caliper as hard as I can by hand back and forth I can make it free off until rear brake pedal is depressed again .
 
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Wow those drawings are awesome
Thanks.

this suggest that the fluid has expanded with heat causing the brake pressure
Yes, the brake fluid volume does increase with heat, however as long as the compensating port in the secondary master cylinder remains clear, the brake system should not see an increase in brake line pressure due to heat build up. When retracted, the master cylinder piston must be clear of the compensating port, otherwise brake line pressure will increase due to heat build up.

When the secondary master cylinder piston fails to retract sufficiently to clear the compensating port, the usual culprits are:
  • Dirt, and corrosion at the piston end of the secondary master cylinder, physically preventing the piston from retracting fully.
  • Air trapped in the brake lines. Some of the brake fluid pressure will be expended compressing the air, rather than returning the piston to a fully retracted state.
Keep in mind that brake line pressure is responsible for returning the master cylinder piston to it's fully retracted position. If the spring in the secondary master cylinder should push the piston faster than the brake fluid can return, then the potential of creating a vacuum in the brake line occurs.

A member here, Mr. Igofar has a considerable amount of practical knowledge regarding the Honda ST brake system. Oh and he likes white telephones.
 
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Thanks for that, you may be right about air still trapped in system, think I might have to purchase that vacuum bleeder after all and re-bleed complete system.
 
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Sounds like you rebuilt the rear calipers. Did you look at the bore of the calipers to make sure there was no corrosion in there that might hang up the piston? Of course you did - you said you replaced the rubber seals. Never mind, this was a dumb thing to say.
 

ST Gui

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A member here, Mr. Igofar has a considerable amount of practical knowledge regarding the Honda ST brake system. Oh and he likes white telephones.
He might also have some insights about using a Motion-Pro vs. a vacuum bleeder.
 

woodybelle

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Did you check the compensating port while you had the cylinder apart? It looks like it would be hard to see and or check as it is down in the SMC bore. If it still has some debris in there could it be causing your problem like Anna's Dad said?
 
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Did you check the compensating port while you had the cylinder apart? It looks like it would be hard to see and or check as it is down in the SMC bore. If it still has some debris in there could it be causing your problem like Anna's Dad said?
I know this topic has been discussed many many times before even on other forums but I think sometimes you can end up going in circles with it. I don't think my SMC is a fault here as I serviced it with new plunger kit & spring, the updated one too, and cleaned SMC thoroughly and blew it out with air line and bleed correctly to sequence. To try to solve the problem I pushed all three rear caliper pistons in against the caliper so wheel spins really really free, I then manually pumped SMC untill felt pressure build, it seem to activate fine and return ok, I got my dad to spin wheel while I did this and sure enough I got braking pressure and when SMC released down wheel spun just as freely again. I then pushed rear brake pedal till pressure came up and released and this is when turning the wheel there was a lot of brake drag. It seems that maybe the rear brake master cylinder is faulty but without dismantling it I can't see how it would cause such drag as its only a plunger and return spring in there and even if spring was broken with out foot pressure on pedal there still would not be that amount of brake drag so it like fluid pressure build but does not release almost like there a non return valve. I've been bike mechanicing for 30 years now and this is the only problems thats had me scratching my head alot. So I'm trying to pick everyone elses brains before I start pulling it apart again and ordering parts I don't need to see if others have the definite answer for this.
 

woodybelle

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I am not sure but in Anna's Dad description of the SMC circuit, in the last paragraph it tells how the rear brake circuit goes through the SMC. Once the pressure is built up by the pedal and transferred to the SMC it will apply the two outer pistons. After that the pressure in the SMC to outer piston circuit needs to be bled off, especially after the brake fluid heats up but maybe somewhat to release the piston to not drag. Thanks for telling what you and your dad went through, it helps,thanks. I hope you get if figured out.
 
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To try to solve the problem I pushed all three rear caliper pistons in against the caliper so wheel spins really really free, I then manually pumped SMC untill felt pressure build, it seem to activate fine and return ok, I got my dad to spin wheel while I did this and sure enough I got braking pressure and when SMC released down wheel spun just as freely again. I then pushed rear brake pedal till pressure came up and released and this is when turning the wheel there was a lot of brake drag.
I think you're on to something here.

Indeed I'm chomping at the bit to get out of work (nothing unusual there), and try this as well as a couple of ideas that have sprung to mind, perhaps allowing me to pinpoint which piston(s) are not retracting.
 
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I use this simple one pic version...may help (click to enlarge);

brakes.png
Thanks for that, its much easier on the eyes with the lines color coded. The problems still seems to be rear caliper and centre piston sticking out and causing the harsh drag. Even though I rebuilt it only 2 months ago with new genuine Honda seals and new pisions I decided to pop pistons out again. Before I blew them out with air line I manualy check to see how well they slide in and out, outside ones (the smallers ones) seemed fine but middle one just really hard to push in easily. It took higher pressure on my air line to pop it out and the dust seal looks torn just round the very inner lip, Ive had this problem before where the dust seal lip gets dragged down with piston and jams piston from returning. So I tried manually again all pistons by hand without the dust seals in just pressure seals and they are really smooth and easy to move in and out. So I'm gonna try another caliper on albeit a second hand one to see if it works better with no drag. As everything so fa points to rear caliper. Have bleed brakes correctly in sequence and tilted caliper etc etc, in fact bleeding is quite easy, I even tested SMC activates and realeses ok which it does, its only binds on and stays on with pressure on rear brake foot pedal, which is center piston. So if another caliper doesn't solve it then bikes up for sale and think I'll buy a BMW. lol
 
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20140702_201845.jpgDrastic measures now, decided to take all the Tupperware off and repaint headlight/clock bracket and tip over bars. Plus now I've found oil weeping out of water pump inspection hole. I wonder if I can get away with just doing the oil seal as manual says to replace pump shaft, the mechanical water seal and the two rubber O rings, more expense. Anyway I've got another rear caliper coming so gonna try that first to cure brake dragging problem plus I manged to get a nearly new complete front left brake caliper with SMC so can swap both over.

20140702_201913.jpg
 
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Soz for late response but been unwell: Anyway just to update this thread I finally cured my rear dragging brake by replacing front left caliper and bracket/SMC and rear brake caliper with some second hand ones off a fairly new bike. My conclusion is that the rubber dust seals on front and rear caliper were getting pinched by piston so not allowing piston to retract once pressure released, and the SMC plunger was just getting stuck in the bore and not returning again due to the rubber seal getting jammed. Seems to me there possibly a design flaw on earlier ST1300 brake calipers as ones I have replaced with are from a later model and seem really smooth to push pistons in by hand and only need low air pressure using my air compressor to push them out. Its almost like the piston bores in caliper and not machined out enough causing the seal to be a tighter fit, or the only other thing I can think of is that due to previous owners neglect in servicing brakes calipers they have got that hot and distorted, which would only need to be a minute fraction as very close tolerances. I also notice the plunger on the newer SMC I brought was slightly different in design to the new one I ordered from Honda which failed. Anyway its nice to ride bike now and not feel that rear brake drag and have awesome working brakes plus my MPG has gone up.
 

woodybelle

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The Goldwing SMC was recalled and it looks similar to the ST. I have always thought that the ST one should be recalled too.
 
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The Goldwing SMC was recalled and it looks similar to the ST. I have always thought that the ST one should be recalled too.
On inspection it seems the SMC bore is fractionally under machined, ie the bore diameter is too small for the piston/plunger and rubber seal. I experimented with both by lubing both with brake fluid first and pushing plunger/piston in by hand and found the newer (later one, think from 2008 on) is really smooth and pops right back with the spring pressure and the early 2003 one just gets stuck each time and I had to use my air gun to pop it back out, if you get me. (I had even used a new plunger seal kit from Honda too).
 
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To be honest with you I would have preferred the ST1300 without ABS and Linked brakes, it just more things to fail, which it does.
 
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I can confirm you are spot on, as I purchased a complete front right caliper with bracket/SMC. That little white plastic barrel filler looking thing indeed does go in exactly as you describe. I've tested to find out how it works and assume it allows fluid under pressure from rear brake pedal which stops the SMC plunger from being pushed in so you don't get excessive braking force on rear wheel when using rear pedal only. (Ive noticed that getting someone to press rear brake pedal and hold it I can no longer push SMC up by hand, this makes sense now)
 
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Thanks for that, its much easier on the eyes with the lines color coded. The problems still seems to be rear caliper and centre piston sticking out and causing the harsh drag. Even though I rebuilt it only 2 months ago with new genuine Honda seals and new pisions I decided to pop pistons out again. Before I blew them out with air line I manualy check to see how well they slide in and out, outside ones (the smallers ones) seemed fine but middle one just really hard to push in easily. It took higher pressure on my air line to pop it out and the dust seal looks torn just round the very inner lip, Ive had this problem before where the dust seal lip gets dragged down with piston and jams piston from returning. So I tried manually again all pistons by hand without the dust seals in just pressure seals and they are really smooth and easy to move in and out. So I'm gonna try another caliper on albeit a second hand one to see if it works better with no drag. As everything so fa points to rear caliper. Have bleed brakes correctly in sequence and tilted caliper etc etc, in fact bleeding is quite easy, I even tested SMC activates and realeses ok which it does, its only binds on and stays on with pressure on rear brake foot pedal, which is center piston. So if another caliper doesn't solve it then bikes up for sale and think I'll buy a BMW. lol
I just wanted to add that my rear brake dragged but not severely. Caliper pistons all in great shape,brake pads fit loosely sliding well , SMC working correctly when manually activated. The drain back port in the rear master cylinder is very tiny (approx.017") I opened mine to .022" and it helped to make the bike easier to roll around in the driveway . I would like to unlink the brakes since a weak brake pedal and way powerful front brake lever reminds me of a Buell lol.
 
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