Article [13] ST1300 - Secondary Master Cylinder Rebuild- SMC

[C110]

Hi
Is it possible the smc leaks brake brake fluid afther a while?

 

[Obo]

Hi
Is it possible the smc leaks brake brake fluid afther a while?

It could, but it shouldn't.

 

[jfheath]

Yes.

But the same answer would apply to any part of the brake system that you care to mention.
It depends on who services it.

So for my bike for this year, the answer is No.

Probably.

What is your concern. If you have an old bike and you suspect that you have a brake issue, then you need to inspect and evaluate the entire system. In that position I would expect to be needing to replace boots, pistons, seals, in masters and slaves, SMC and rear caliper bracket. Then I'd go round and see which things dont need replacing.

But first of all, you need to know what the problem is. If you have binding brakes then that might be because you put the spacers in the front wheel incorrectly, or because you have started wearing police style riding boots, so you could spend all that time and miney without fixing the fact that you have started resting your new heavy boots on the brake pedal and cannot feel that is what is happening.

 

[C110]

The smc works smooth but i,m affraid the rubber inside that seals the fluid is leaking
When i come home afther a ride i see fluid that probably comes out the tiny little hole from the smc

 

[jfheath]

Any fluid that comes from that hole could be rain water, or it could be brake fluid. It is on the outside of the hydraulic system.

If it is rain water, then that is normal. The hole is there to drain away rain water from the outside of the boot.
If it is hydraulic fluid, then the secondary seal inside the bore is not doing its job.

In if the latter, then something needs to be replaced. Fluid should not get past the secindary seal inside the bore. It is possible to get a service kit to replace the boot, the piston and the spring - but really, if fluid is getting out, then water is getting in, and the inside of the SMC bore will more than likely be corroded. Between us, on the forum, we have seen many examples where a service kit has not resulted in a fix for the problem. The best solution is to buy a new SMC, along with the washers for the banjo bolts (2 per banjo bolt that has to be undone) - they don't need to be Honda, you can buy them from motorcycle parts suppliers. I use copper ones and have had no issues with them for the last 120,000 miles in our salty wet road conditions. (There is an argument that says that copper/alloy create better conditions for 'galvanic' corrosion due to the different electrical properties of the two metals. In practice I have noted that the Honda alloy ones seem to corrode more readily from out salty winter roads - - but I do protect mine with eg ACF50). The service kit is part of the new SMC / caliper bracket and is already fitted, and greased. It is an easy replacement, but can be a bit messy and you will need to bleed the brakes afterwards - so you need non return bleed valves and plenty of brake fluid. I buy 2 litres to do this, as I like to flush the old fluid out before I remove anything.

The peace of mind that you get after doing this is worth far more than the SMC costs. Yours is a pre-2008 SMC. Make sure that you order the correct part. From 2008, the SMC changed - they are not interchangeable.

 

[C110]

i dismantled the smc and one rubber has to be replaced for the best
The service kit i do not need if i can find the rubber(s)
The bore from the smc looks like new
I will measure the bore and search the rubbers

 

[jfheath]

If you say so.

Just out of curiosity, Which seal do you think needs to be replaced ? #1 or #2 in your photo?

I'm surprised that you got the rubber boot off without undoing the stem of the bracket. It makes me wonder if someone has put normal grease behind the boot and the seal has swelled - making it ineffective ?

The length of that bracket component is absolutely critical to the safe operation of the SMC. I would not undo that bracket without first measuring its length with a vernier scale caliper. (Digital calipers do not have the necessary accuracy). Check the length on reassembly. Even the smallest bit of grit can prevent the screw thread from tightening to exactly the same position.

There is a nylon filter cartridge behind the inlet port mount on the outside of the SMC. It contains a one way valve, so you need to know which way it fits before you remove it. Inside there is second compensation port, almost impossible to see. It needs to be clear as well as the compensation port on the metal at the bottom of the inlet port.

It may be a trick of the light, but the metal ribs on the left end of the piston seem to have been polished. Check out whether or not this is the case. If so, the bore itself has been polished. Then ask why. The primary seal #1, and the secondary seal #2 should keep the body of the piston away from the sides of the SMC bore. Has the SMC been knocked and the bore damaged ?

My concern here is one of safety. The SMC is not just another master cylinder. It is a pressure relief valve for the fluid in the rear caliper to escape when it has been pressurised - either by braking or by temperature change . The primary seal sits immediately behind the compensation port in the bore. The first tiny movement of the SMC piston seals the compensation port so that full pressure is applied to the rear brake outer pistons. If the piston cannot return to clear the compensation port - eg due to incorrect length, damaged bore, damaged spring or corrosion due to an ill fitting rubber boot - then there is no pressure relief. Without much warning, the rear wheel will lock on solid.

The problem is that you cannot know if the SMC was working properly or not before you removed it.

I assume that is was brake fluid leaking out ? If fluid was getting out, then air was getting in - so a piston that was failing to return properly may not have been noticed: any unrelieved pressure would be absorbed by compressing the air in the system. Or by pushing it out past the secondary seal to emerge through the drain hole.

All theoretical mumblings, but if the problem was mine, I would (and have) simply replaced the entire SMC rather than have the constant niggle of impending doom.

Please check everything very carefully.

 

[STRider]

I would not undo that bracket without first measuring its length with a vernier scale caliper. (Digital calipers do not have the necessary accuracy).
<snip!>
Please check everything very carefully.

John, who hurt you? I agree with everything you wrote, but this?

Someone get this man a genuine Mitutoyo 500-196-30 Advanced Onsite Sensor (AOS) Absolute Scale Digital Caliper, 0 to 6"/0 to 150mm Measuring Range, 0.0005"/0.01mm Resolution, LCD

Then again, John Harrison constructed the most accurate timepieces in history (up to that point; the 1700s) without digital calipers, so there's that.

 

[jfheath]

Maybe I should have used the word generally ?

A while back I looked for a higher resolution model without success. Many claimed only about 0.1-0.2mm in the specification, despite displaying values to 2dp. I clearly didn't know where to look. Of course 0.01mm accuracy is a +/- so theoretical discrepancy between two separate readings could be 0.02. But that is way better than I thought existed with digital caipers.

Thank you, I stand corrected.

I think the point that I should have made was that just because a digital caliper shows 2 decimal places, it doesn't mean that is how accurate it is.

I don't often need to use one, but when I do, the vernier scale will get me to slightly better than 0.01mm.
At full price, your accurate model is only $30 shy of a new SMC ! I'd buy the SMC instead.

SMCs are £245 now in the UK.

We had a series on TV over here about John Harrison's attempt to win the prize for a timepiece for hekping to calculate Longitude. Brilliant story.

 

[STRider]

I get your skepticism... Particularly when it's based on real world experience. And clearly avoid the $10 digital calipers from Harbor Freight or whatever the UK equivalent is.

That series was "Longitude" I bet, featuring the recently passed Michael Gambon as John Harrison. That series introduced me to the work of Sir Gambon long before he became Dumbledore. I loved it. Watched it within the last year for about the fourth time. Great story. Horrible how he was treated by the Royal Astronomical Society however.

 

[jfheath]

Yes that was the one. Harrison didn't seem to get the recognition that he deserved - until 'Only fools and horses' brought him some brief glory! I don't suppose many will have spotted that link though.

 

[Sadlsor]

Then again, John Harrison constructed the most accurate timepieces in history (up to that point; the 1700s) without digital calipers, so there's that.

This name caught my attention, as the J.H. I know is a good friend, lives across town from me, is a former Kawasaki factory race tuner, a 3-time Iron Butt Rally finisher, a perennial IBA volunteer, and a heckuva swell guy.
So I was intrigued for a moment, when John was mentioned in the context of accurate timepieces... and then I saw the timeframe.
Yes, children, context is everything.