Article [13] ST1300 - Brake Maintenance - Getting all the air out of the brake system

[TerryS]

I'm a little confused, and am wondering if anyone has ever experienced this? Or, does anyone know what the cause of this happening might be?

Between all of the bleed nipples throughout the bike, and doing the whole process through twice, I flushed almost a whole quart bottle through the entire system. So, it's definitely clean, and I'd like to think free of air, but again this happening confuses me and I just want to check with all of you to get your input! What do you think could be causing this?

Thanks so much for your help,

Tanner

I can see why this might be perplexing. I wonder if the Proportional Control Valve is at the root of this. As I understand it, the PCV's function is to prevent a wheel-locking amount of rear brake being applied; as the front brake is applied harder you get weight transfer off the back wheel which makes it easier to lock. The PCV operates to reduce the brake line pressure to the rear calliper when high inlet pressure (e.g. from the SMC being pulled forward by hard front braking) is achieved, as per the diagram below. At a guess, what happened to you may have been excessive pressure being applied to the pedal, which may have pushed the PCV into low pressure mode which would largely stop any brake bleed occurring. I've not seen this myself as I work solo with a speed bleeder with vacuum bleeder attached, and that has always drawn a decent flow of fluid whenever the pedal is pressed.

The diagram is from the VFR800Fi service manual; different bike but generally similar linked brake setup (VFR SMC drives just a centre rear piston, the two outers are driven direct off the foot pedal).

Whenever the foot pedal is pressed, pressure is applied to the rear outer pistons through the SMC and without the SMC moving. The SMC is pumped out fully by the line pressure and so would feel solid while the pedal is pressed.

 

[Burttj03]

I can see why this might be perplexing. I wonder if the Proportional Control Valve is at the root of this. As I understand it, the PCV's function is to prevent a wheel-locking amount of rear brake being applied; as the front brake is applied harder you get weight transfer off the back wheel which makes it easier to lock. The PCV operates to reduce the brake line pressure to the rear calliper when high inlet pressure (e.g. from the SMC being pulled forward by hard front braking) is achieved, as per the diagram below. At a guess, what happened to you may have been excessive pressure being applied to the pedal, which may have pushed the PCV into low pressure mode which would largely stop any brake bleed occurring. I've not seen this myself as I work solo with a speed bleeder with vacuum bleeder attached, and that has always drawn a decent flow of fluid whenever the pedal is pressed.

The diagram is from the VFR800Fi service manual; different bike but generally similar linked brake setup (VFR SMC drives just a centre rear piston, the two outers are driven direct off the foot pedal).

Whenever the foot pedal is pressed, pressure is applied to the rear outer pistons through the SMC and without the SMC moving. The SMC is pumped out fully by the line pressure and so would feel solid while the pedal is pressed.

Thanks so much, Terry. It definitely seems like it could be exactly that--my dad may have been pressing too hard on the foot pedal. Tomorrow sometime, I think I'll do one more round of bleeding just to be safe, and this time I'll ask him to press the foot pedal with a little less force, and then I guess we'll see what happens! I'll keep you updated!

Thank you,

-Tanner

 

[Igofar]

Standing by the

 

[jfheath]

Larry has this spotted, but it sounds to me like you have a blockage in the line somewhere. If fluid comes out at the front left centre piston without any issue (pressing the brake pedal) and it comes out at the PCV bleed valve ok, then any blockage has to be in the line from the PCV bleed valve to the rear outer pistons. That would suggest that the PCV pressure valve isn't of immediate concern.

Although the description of the PCV above is correct (on the ST1300 it reduces pressure on the rear outer pistons), it will not operate when the bleed valve is open as it requires pressure in the fluid in order to overcome the spring that allows the spool to move.

Blockage comes from a build up of the crystal 'sludge' which forms when fluid isn't replaced and thoroughly flushed at regular intervals. Eventually it forms hard crystals. Some may be stuck in the bleed valve itself. But wherever it is, this may be a warning sign. You have stuff in your system that will not be restricted to just one location, and the entire system will likely need a thorough going over before you are free from it. This could be lurking behind the piston seals, in compensation ports and in parts of the SMC, delay valve and Proportional Control Valve.

 

[Burttj03]

Larry has this spotted, but it sounds to me like you have a blockage in the line somewhere. If fluid comes out at the front left centre piston without any issue (pressing the brake pedal) and it comes out at the PCV bleed valve ok, then any blockage has to be in the line from the PCV bleed valve to the rear outer pistons. That would suggest that the PCV pressure valve isn't of immediate concern.

Although the description of the PCV above is correct (on the ST1300 it reduces pressure on the rear outer pistons), it will not operate when the bleed valve is open as it requires pressure in the fluid in order to overcome the spring that allows the spool to move.

Blockage comes from a build up of the crystal 'sludge' which forms when fluid isn't replaced and thoroughly flushed at regular intervals. Eventually it forms hard crystals. Some may be stuck in the bleed valve itself. But wherever it is, this may be a warning sign. You have stuff in your system that will not be restricted to just one location, and the entire system will likely need a thorough going over before you are free from it. This could be lurking behind the piston seals, in compensation ports and in parts of the SMC, delay valve and Proportional Control Valve.

Hmmmm. Well, I'll say that it hasn't had old fluid sit in it for very long. I did an extremely thorough flush (or so I thought) just last September, and it was stored in a garage just for a few months of winter. At that point, I had spoke with @Igofar to double check my work. At that point, this was not an issue at all, and each bleeder valve opened, released fluid properly, and flowed clear/clean fluid free of air, and the brakes worked great. Also, the entire SMC assembly (the entire caliper bracket assembly containing the SMC assembly) was replaced with a brand new unit just early last year.

Thinking back to yesterday, I believe this "delayed" flow of fluid when cracking the bleeded valve happened once in the third step when we we were bleeding the pcv as well, but after many more bleed cycles it never happened again.

Also, I noticed that the fluid coming out of each port was relatively clean, except for the front right center piston fluid looked pretty milky, but eventually it too ran clear and free of air with enough bleed cycles.

So, does any of this additional information bring up any new concerns or questions?

@Igofar, I've got a pretty busy day today, but I'll try to give you a call later at some point!

Thank you all so much for your help--I really just want this to be safe and proper for my dad to ride! We have a trip planned this summer, and I'm trying to make sure everything is tip top before we go!

 

[jfheath]

So, does any of this additional information bring up any new concerns or questions?

No, but it removes some concerns. It is difficult to diagnose issues from a distance. The SMC has been replaced and you will have had to have flushed everything out then, and providing that rubber boot has not been disturbed, it will not have got any water in. (It is very difficult to get the rubber boot back into position properly, once it has been disturbed - link ). The SMC in the photo was a friends who fitted a brand new service kit just two years prior to these photos. I don't believe that the seal was made properly with the top hat rubber boot.

I give mine a good flush every 12-18 months. However, I did once get a blockage in the line from the SMC to the PCV bleed valve. I never saw it come out, but the pressure released suddenly after which the fluid moved freely. I don't think the PCV pressure valve would be the issue - fluid flows in and out of that easily. Its only when the pressure increases that it allows a piston/spool to be pushed in taking some of the pressure away from the line.

Next time it does it (if it does) it would be worthwhile closing up the valve and just checking - is the pressure applying the rear brakes. Does it release the rear brakes when you remove the pressure from the pedal ? I'd be tempted to look at that bleed valve. If you have teflon tape around the threads, it may have disintegrated and blocked the hole in one side at the bottom of the threads. When you remove it, unscrew it a bit by hand or with a small socket between finger and thumb and screw it back in a touch. Forward and backward - just to feel that the threads are ok and not binding. You don't want to cross thread it when you put it back in.

 

[Burttj03]

No, but it removes some concerns. It is difficult to diagnose issues from a distance. The SMC has been replaced and you will have had to have flushed everything out then, and providing that rubber boot has not been disturbed, it will not have got any water in. (It is very difficult to get the rubber boot back into position properly, once it has been disturbed - link ). The SMC in the photo was a friends who fitted a brand new service kit just two years prior to these photos. I don't believe that the seal was made properly with the top hat rubber boot.

I give mine a good flush every 12-18 months. However, I did once get a blockage in the line from the SMC to the PCV bleed valve. I never saw it come out, but the pressure released suddenly after which the fluid moved freely. I don't think the PCV pressure valve would be the issue - fluid flows in and out of that easily. Its only when the pressure increases that it allows a piston/spool to be pushed in taking some of the pressure away from the line.

Next time it does it (if it does) it would be worthwhile closing up the valve and just checking - is the pressure applying the rear brakes. Does it release the rear brakes when you remove the pressure from the pedal ? I'd be tempted to look at that bleed valve. If you have teflon tape around the threads, it may have disintegrated and blocked the hole in one side at the bottom of the threads. When you remove it, unscrew it a bit by hand or with a small socket between finger and thumb and screw it back in a touch. Forward and backward - just to feel that the threads are ok and not binding. You don't want to cross thread it when you put it back in.

So, today I went ahead and went through the whole process again.

Last time, I used the old method of pumping the appropriate lever/pedal for the current step a few pumps, then holding that pedal/lever down, then cracking the appropriate bleed nipple (and feeling the lever/pedal sink as the nipple opens), and then closing the bleed nipple before releasing the pedal/lever.

Well, today I decided to try something slightly different. I tried the method that the forum member with the YouTube channel name "GTs Garage" used in his video. He attached the 8mm wrench and a piece of clear tubing, held the tubing straight vertically, cracked open the appropriate bleeder valve for the current step, then repeatedly pumped the appropriate lever/pedal/SMC for the current step, then finished with closing the bleeder nipple and removing the tube/dumping the brake fluid from the tubing into a bottle.

I wanted to try this method to ensure that I was really puling a good amount of fluid through the system on each brake line/bleed nipple--especially the last nipple for the rear outer pistons that I had the strange issue with last time.

Well, I think because I went through the whole process extensively last time, when I bled each nipple today only clear and clean fluid free of air came out of each one. I ran about half of a reservoir of brake fluid through each bleed nipple this time (and about a full reservoir through each one last time). This time, when I cracked open that last rear outer piston bleed nipple, I went back and forth pumping the foot lever and then the SMC while the bleed nipple was open to make sure that I was moving fluid a good amount of fluid, and this time there was no obstruction of flow, and only clean and clear fluid free of air moving just as easy as the fluid was moved in any of the other bleed nipples/circuits. So, personally, I think I'm feeling good that the system is now free of any dirty fluid, and air.


I cleaned everything up, then tested that the brakes fully engage and fully disengage when using the pedals/levers. Then, I layed on the ground next to the bike, turned the rear wheel forwards with my foot as I used my hand to engage the SMC to make sure that it would lock the rear wheel--and more importantly, completely release the brakes on the rear wheel when I let go of the SMC. Thankfully, it worked just as it should! Also, if I give it a good spin, the rear wheel rotates about three times around on its own without any brakes applied--just to get an idea of any drag/etc. present.

I know it's tough to make a call without seeing it in person, but would you guys agree with my thoughts that this brake system is now good?

 

[jfheath]

It sounds good to me. It's always a worry when you cannot find a reason for a behaviour. But it isn't doing it now and whatever it was could have been flushed out.

You don't mention how firm the brake pedal and lever feel, but I assume that they are good to go. The SMC should move only a mm or two.
I'm a worrier and having been caught out once (too much rubber grease on slider pins), many years ago, I still check for drag every now and then when I am on the move.

3 revolutions ? That is impressive.

 

[Burttj03]

It sounds good to me. It's always a worry when you cannot find a reason for a behaviour. But it isn't doing it now and whatever it was could have been flushed out.

You don't mention how firm the brake pedal and lever feel, but I assume that they are good to go. The SMC should move only a mm or two.
I'm a worrier and having been caught out once (too much rubber grease on slider pins), many years ago, I still check for drag every now and then when I am on the move.

3 revolutions ? That is impressive.

It truly is a worry when you can't explain it! Haha, but I agree with you that I think it is all good now. Thank you for all of your help!

Both the rear and front pedal/lever feel properly firm; and the SMC, when I was engaging it, did only move about a mm or two.

And as far as the rear tire rotations, I may have exaggerated a tiny bit; I rechecked, and it rotates just a touch under 3 revolutions--about two and a half total, and that's when I really give it a good spin with both of my hands from behind the bike.

 

[Fred Primmer]

I have a question. When do you re-install the left front caliper that you removed to tilt the SMC to 15° for the bleed at the PCV? My ST1300 service manual (pdf downloaded from manualslib.com) on page 17-11 tells you to remove that caliper and tilt the SMC for the PCV bleed but never tells you install it again. This article indicates on page 7 that the SMC needs to be tilted for the right and left center piston bleed (steps 5 & 6).

 

[Igofar]

The SMC needs to be replaced to properly bleed #6 bleeder.
While there are some instructions and PDF files floating around suggesting that you try and bleed it while its still at 15 degrees, and no re-mounted, these are incorrect.
The SMC must be mounted to the forks to bleed the lower bleeder on the left front caliper.

 

[Fred Primmer]

Is it okay to reinstall the left caliper after you bleed the PCV (step 3) or do steps 4 and 5 require the SMC to be at 15°?

 

[Igofar]

Is it okay to reinstall the left caliper after you bleed the PCV (step 3) or do steps 4 and 5 require the SMC to be at 15°?

The SMC remains tilted until it’s time to bleed #6.

 

[Fred Primmer]

The SMC remains tilted until it’s time to bleed #6.

Thank-you so much Larry!

 

[jfheath]

The reason that the SMC is tilted is to ensure that any air in the system is able to easily travel up to the outlet port of the SMC.

The problem with leaving it tilted when bleeding #5 and #6 is that these are the front centre pistons. You will be pumping the rear brake pedal. Unless you take precautions, leaving the SMC tilted means that there is nothing much to stop the pistons from being pumped out by the action on the brake pedal !

Personally I leave the SMC tilted until I have finished bleeding everything.
But then - you will note on Page 3 that I use blocks of wood which are slipped into the calipers to prevent the pistons from moving out. They are lined with a thin sheet of aluminium to stop the pistons from biting into the grain of the wood, and they are held in place so that they don't drop out. That way, I don't have to replace the caliper when I want to bleed the front centre pistons. Air and fluid will still move upwards from the front left centre piston with the SMC tilted.

I will go round the cycle a few times before I am happy - and one final time after the calipers have been mounted properly and the pads have been replaced.

 

[Fred Primmer]

Thanks Igofar and jfheath for your help. I took the right side fairings off two days ago and went through the bleeding procedure to replace the brake fluid. I used a one-way check valve on the hose feeding into an elevated empty Gatorade bottle in turn at each bleeder valve. Pumped the related lever or pedal to move the fluid until it was nice and clear. The old fluid was definitely golden. I tilted the SMC and blocked the caliper for step 3 and reinstalled it for step 6. I put about 3/4 of a liter of new fluid through the system.

The lever and pedal seemed pretty good after that so I left it until the next day with a soft-jawed clamp holding the lever and a 10 lb dumbbell depressing the pedal. That did give them a little extra firmness. Happy with the result, I put the fairings, seat etc back on. Thunderstorms in the area yesterday so I didn't get out for a ride until this morning. It all felt great. Thank-you for your articles and answers. On the road again, thanks to this forum and it's knowledgeable and helpful members!

 

[newtonhubcap]

I know this thread is a bit old and there seems to be further discussion regarding air in the system issue. I work in a local motorcycle shop here in Wisconsin. Now and then we do hit a brake job that it is nearly impossible to get bleed all the air out of the brake system. When this happens and it did with me this year (1994 ST1100) we typically bleed the system the best we can and then will turn the handlebars so the master cylinder is at it highest point (level also) and compress the brake lever, using a velcro strap to hold it over night. At time you can see micro-bubbles rising and by the next morning your brakes are solid! This works even with rear brake pedals reservoirs as well.

 

[ddemair]

I haven't yet done a brake bleed myself. I had a shop do the last one, but based on fluid color, I'm not sure they did a complete bleed.

Question... if I don't have air in the system and I'm only doing a fluid change, is it still necessary to tilt the SMC?

 

[Kevcules]

I haven't yet done a brake bleed myself. I had a shop do the last one, but based on fluid color, I'm not sure they did a complete bleed.

Question... if I don't have air in the system and I'm only doing a fluid change, is it still necessary to tilt the SMC?

I think it's always a good idea to tilt the SMC during a bleed. It will help expel the old fluid, any air bubbles and it gives you a chance to "exercise" the piston.

 

[Igofar]

I’ll go one further, it’s not only a good idea, it is necessary to correctly bleed and flush the system.