Air in SMC Query

MidLife said:
That may help explain why at times, when getting close to needing a bleed, a pedal that feels somewhat spongy at rest still turns out to be a bit firmer while in motion?
Click to expand...

It is certainly a possibility, but not a 100% diagnosis. For example, a distorted (oval) SMC bore may produce the same result.

But think of this little scenario. You have an air bubble in the brake line. Imagine that it is located between the SMC and the rear caliper. That line goes up from the SMC to the proportional control valve, past the bleed valve behind the mid fairing near your right knee (near the ignition coil) and then down to the rear caliper outer pistons.

When you are stationary, the SMC is not activated and pressing the rear brake pedal causes fluid to pass the SMC and through that brake line - applying the two outer pistons of the rear caliper. (This is why you get fluid out of the rear outer bleed valve when bleeding and you press the brake pedal).

With me so far ? Ok - well there is an air bubble in that line, so when you press the brake pedal, the pedal feels soft as it squeezes the air bubble. Pressing it a few times puts more fluid into the line and it gets a bit firmer. Wait a while and the fluid returns via the compensation port and it is as soft as it was before.

When you are moving and apply the brakes, the SMC is activated and that applies pressure to the same line - squeezing the air bubble and applying pressure to the rear pads. Not as much as it should because of the air bubble - but you cannot feel this sponginess.

With the SMC applied, the pressure from pressing the rear brake pedal cannot push any more fluid past the SMC - the SMC has already done that - so the brake pedal now feels firm.

This to me, is a good indication of where the air bubble is in a spongey rear brake. And it is difficult to get rid of, because there is an 'n' shaped bend in the brake line behind the headlights - and a couple of unions around the Proportional Control Valve. Tapping, flexing and a vacuum really help to shift this and bleed it out at the valve near your right knee.

You can verify this with help from a friend. Get them to squeeze the caliper toward the fork, to apply the SMC. Press the brake pedal. Does it feel firm ?
Keep your foot on the brake pedal, and release the SMC. The brake pedal may dip as the SMC releases its pressure on the bubble and the brake pedal takes up the slack.

This is just me hypothesising - but its based on my own experience, and I have convinced myself from observations. Just waiting for someone else to disprove it !!
 
Andrew Shadow said:
Out of curiosity after the brakes were bled did he overfill the master cylinders? I have no idea how much brake fluid expands with heat. If there is no other outside influence but increased heat causing this binding it seems to indicate that there is still air in the system that is expanding with heat, moisture in the system that is expanding with heat or the system is overfilled and when/if the brake fluid expands due to the heat it has no where to go but to force the pistons out a little.
Click to expand...

Thanks Andrew. I have checked both reservoirs and they are not overfilled. I believe you may be right about there still being air in the system. The symptoms are definitely connected with heat. The lever and pedal are both firm and the brakes sharp, but I cannot think of anything else that may be causing this weird behaviour.
 
Clutching at straws time.

If there is a bubble in the line, then expansion due to heat is likely to compress the bubble !. But that is just an aside - the effect any expansion of fluid that would cause the brakes to bind SHOULD be catered for by the release of pressure through the compensaton port(s).

What is that 's' doing on the end ? Well - there is one compensation port in the SMC. It is blocked off when the SMC is applied and it should be uncovered whan the SMC is released. In faulty SMC's this may not be happening due to corrosion / sludge in the bore or due to a distorted bore or damaged piston.

But Fawlty now has a new SMC fitted.

The fluid fluid that escapes through the compensation port in the SMC has to go somewhere. The only other place that it can escape is through the compensation port in the Rear Brake Pedal Master Cylinder. Again, this is shut off when the brake pedal is depressed, and cleared when the pedal is released. Unless there is a problems with the master cylinder piston movement.

Question - Can fluid return when you press in the rear centre piston ?(clean the piston first).

Also - is all of this a red herring - is there some other reason for brake drag ?

One of my favourite 'gotchas' - cos it caught me out the day before I was due to embark on a 2000 mile trip to Europe - and had my rear brake smoking on the test run - was the slider pins. I couldn't find anything wrong so I took the brakes apart again and re-did what I had serviced from scratch, double checking everything. The only fault that I could find was that I had put rather too much rubber /silicon grease in the boots of the caliper slider pins. This reduced the air space and caused an airlock. When I took out the pads and checked the movement of the calipers by grabbing hold of the caliper and moving it in towards the wheel and out again - and established that it moved freely - I was rather gobsmacked when I turned back from picking up the pads, to see the caliper moving back in towards the wheel, all by itself !

I separated the caliper from the bracket, cleaned out the holes and the shiny guide pins and put a smear of grease on the surface. Re-assembled and the caliper stayed put. It's the only thing that I could find that wasn't quite right - and the problem went away.

Apply the rear brakes and the piston pushes out the pads and also draws the caliper across. Release the brakes and the caliper should stay put. If air pressure pushes it across, one pad will be pressing lightly against the disc rotor.
 
Thanks again John. A lot to take in for a novice like me! Sounds like the rear brake caliper needs a good clean and lube, although this was done when new seals were fitted a couple of months ago. The rear brake cylinder was also rebuilt at the same time. Another bleed is on the cards as well. Deep joy!

I am afraid this is beyond my abilities at the moment, so it's back to the mechanic. I will keep you all posted.
 
Remember that I am only guessing. Educated guessing - but there are so many possible alternatives, its difficult to say exactly what the real issue is. Murcia is far too far for me to come and help.
 
jfheath said:
Remember that I am only guessing. Educated guessing - but there are so many possible alternatives, its difficult to say exactly what the real issue is. Murcia is far too far for me to come and help.
Click to expand...
Many thanks again John. I will keep slogging away until we sort it (poco a poco) as they say in Spanish (little by little) If you are ever in my neck of the woods, give me a shout and I'll buy you a beer or three!
 
I forgot to mention a trick that is worth doing. Get three strips of paper or something thin and slide them up between the pistons and the pads (if you can). Hopefully they will stay put. Then when the brakes start binding (without riding it) you can find out which piston is causing the problem. Don't forget to remove the strips before riding !
 
Thanks John. I haven't been able to ride this week, but have been monitoring the rear wheel movement on the centre stand and so far it is normal. Last Sunday night I thought I would try zip tying the lever and weighting the pedal overnight. Since doing this the rear wheel has had free movement. The real test is out on the road, which I am hoping to do this weekend. Will post back the results.
 
OK. Went for a ride yesterday. Straight onto the motorway, rode about 10 miles at 80mph. Got off motorway and rode a few miles on ordinary road. I had not touched either brake during this ride.(virtually no traffic, used engine braking and did not need to stop). Rear brake started binding. I coasted to a stop, without touching either brake. I pressed the brake pedal lightly and the rear wheel freed up. Carried on riding another 40 miles without any problem. Once again I believe that heat played a part in this. There must have been pressure in the system, which was cleared when the brake pedal was pressed.

Just to recap. New SMC fitted, rear master cylinder rebuilt, rear caliper serviced, cleaned and new seals fitted, system bled according to workshop manual procedure.

Very strange. It seems that unless I use the rear brake, pressure builds up and the brake starts to bind.

I am going to try the same run again, using the brakes normally and will report back.

Once again, this morning, bike on centre stand, rear wheel spinning freely. Just checked again 4 hours later and rear wheel binding, pressed brake pedal and freed up. Current temp 36c.
 
Last edited:
Basil, as no one's chimed in yet, they must be sleeping in on the left side of the pond, I'll give my two'penneth.
Yes, from reading about this previously it could be worth giving everything a fresh bleed, but thinking outside the box, I don't do it well, the fronts aren't dragging slightly and applying the SMC?
I'll keep thinking, but it don't arf hurt!
Upt'North.
 
Upt' North said:
the fronts aren't dragging slightly and applying the SMC?
Click to expand...
He has written that the rear wheel spins freely when checked on the centre stand. After the bike has sat soaking several hours in hot ambient temperatures and without having been ridden the rear wheel is then found to be binding when checked again. This would eliminate any influence from the SMC as far as this test goes.
 
Fawlty said:
Very strange.
Click to expand...

Sure is.

Two things I would need to know if this was happening to mine:

1) Which piston(s) are activating the rear brake - the outer pistons or the inner. With brakes binding - test with thin material - very thing feeler gauge, thin paper etc. You may be able to insert strips do it when it is free and see which one are clamped 4 hours later (No, not when riding !)

2) Does the SMC move when you apply the rear brake pedal to free up the rear wheel.
 
These would be my guesses....
1. Your mechanic does not have a clue what he is doing.
2. You probably still have air in the system.
3. I bet your going to find either a bent guide pin, or too much grease in the boot causing suction.
I believe your throwing yourself off, as well as all those trying to help you, by telling everyone e that heat is causing this.
Rather that build bigger fences, gates, or traps, it may be time to open the gate and let the rabbit out.
If one of the rear guide pins is bent, or you have too much grease in there, TIME, not heat may be causing the pistons to jam up, heat could be allowing the grease to warm up enough for the pistons to move even though the pin is bent, or pressure may be allowing the caliper to tilt then work straight again, the go crooked again etc.
Throwing money and parts at it at this point may be wasted.
Get a different mechanic, remove the rear wheel,inspect the pins for damage (bent), cleAn out excessive grease, and make sure the rear wheel is put on in the correct order (stopper bolt gets tightened before axle nut).
Then report back to us.
Good luck
 
Last edited:
Rwilson1061 was having the same issues, and tried the same fixes, until he realised the prior owner and or mechanic BENT his rear caliper bracket!
Maybe he will be kind enough to post pictures.
 
Upt' North said:
Basil, as no one's chimed in yet, they must be sleeping in on the left side of the pond, I'll give my two'penneth.
Yes, from reading about this previously it could be worth giving everything a fresh bleed, but thinking outside the box, I don't do it well, the fronts aren't dragging slightly and applying the SMC?
I'll keep thinking, but it don't arf hurt!
Upt'North.
Click to expand...

Thanks for this. I have checked the front and it is not binding.
 
jfheath said:
Sure is.

Two things I would need to know if this was happening to mine:

1) Which piston(s) are activating the rear brake - the outer pistons or the inner. With brakes binding - test with thin material - very thing feeler gauge, thin paper etc. You may be able to insert strips do it when it is free and see which one are clamped 4 hours later (No, not when riding !)

2) Does the SMC move when you apply the rear brake pedal to free up the rear wheel.
Click to expand...

Thanks. I will try these and report back.
 
Igofar said:
These would be my guesses....
1. Your mechanic does not have a clue what he is doing.
2. You probably still have air in the system.
3. I bet your going to find either a bent guide pin, or too much grease in the boot causing suction.
I believe your throwing yourself off, as well as all those trying to help you, by telling everyone e that heat is causing this.
Rather that build bigger fences, gates, or traps, it may be time to open the gate and let the rabbit out.
If one of the rear guide pins is bent, or you have too much grease in there, TIME, not heat may be causing the pistons to jam up, heat could be allowing the grease to warm up enough for the pistons to move even though the pin is bent, or pressure may be allowing the caliper to tilt then work straight again, the go crooked again etc.
Throwing money and parts at it at this point may be wasted.
Get a different mechanic, remove the rear wheel,inspect the pins for damage (bent), cleAn out excessive grease, and make sure the rear wheel is put on in the correct order (stopper bolt gets tightened before axle nut).
Then report back to us.
Good luck
Click to expand...

Some good ideas and suggestions. Many thanks. I am a complete novice when it comes to brakes. Heat seems to be the connected to the cause, whatever it is. You could be right about throwing myself and others off. That is not my intention, I am just reporting what happens and looking for a solution.

The problem I have here is finding a mechanic who knows these bikes. They are not common in this area. I could take it to a Honda dealer, but I am not convinced that even they would be competent.

I will do my best to investigate and action your suggestions and report back.

One other question. Could an internally collapsed brake hose cause these symptoms, or can we rule that out?
 
MidLife said:
Could we also have the outers and inner all binding together, in case the issue is with compensation to the rear reservoir?
Click to expand...

Yes. I believe that all of the pressure that is released via the SMC compensation port also has to be released via the master cylinder compensation port to return fluid to the reservoir. If that is blocked, the pressure can't be released.

But - if that rear master cylinder port was blocked, then the front two centre pistons would bind - noticeably the left one (The right is affected by the delay valve) - and Fawlty says that the front brakes are not binding when the rear wheel binds. However - it is a good check to make - that fluid is actually returning to the reservoir.
 
You must log in or register to reply here.
Back
Top Bottom