St1300 rear brakes

[Memorechy]

Hello guys,
I recently took appart all brake calippers and master cylindrers including secondary master and bleed with a vacuum pump

Front brake lever works great

Rear brakes goes soft after parking. Need to pump it.
Also i noticed that if apply Front brakes first the rear and if i release Front brakes i can feel the brake pedal go down.

Any ideas?

 

[Gus1300]

Search brakes in the ST1300 Articles section and read up on the system to ensure you're bleeding it correctly. There is a lot of good information on the site that should help you.

 

[dduelin]

Hello guys,
I recently took appart all brake calippers and master cylindrers including secondary master and bleed with a vacuum pump

Front brake lever works great

Rear brakes goes soft after parking. Need to pump it.
Also i noticed that if apply Front brakes first the rear and if i release Front brakes i can feel the brake pedal go down.

Any ideas?

That kind of pedal drop after releasing the front brake is almost guaranteed to be a big bubble of air in the Proportional Control Valve circuit. Like Gus1300 posted I would start with a really good flush and bleed of all circuits in the proper sequence.

 

[ST11Nut]

This thread may be helpful

Article [13] - ST1300 - Brake Bleed Procedure [VIDEO] | ST1300 Articles | ST-Owners.com

 

[Igofar]

The Factory Service Manual indicates that the SMC should Never be taken apart. The measurements of the bore/piston are very small, and this could block the return port causing dangerous brake issues etc.
With that said, did you also remove the two 8mm bolts on the back of the SMC line attachment bracket? There is a small filter cartridge in there, that is also NOT supposed to be removed etc.
And you may be able to see if its clogged up with crap, causing many of your issues etc.
I would recommend NOT using a vacuum pump while bleeding the brakes on these bikes, and instead, use a simple one way check valve (motion pro hydraulic brake bleeder) $13 dollars on Amazon instead.
The pedal drop, as pointed out, is due to trapped air in your PCV area.
You must follow the correct procedure, sequence, and method to remove the air from here.
Reach John Heaths article on avoiding the pitfalls for the correct advice.

 

[jfheath]

The pedal drop occurs because the front lever operates the SMC which applies the rear caliper outer pistons. With the air bubble in there, they won't be very effective - but you can't tell. The SMC effectively blocks off the route from the rear pedal to the rear outer pistons, so it feels firm. When the SMC is released when the front brake lever is released, the rear pedal then gets the job of squeezing that air bubble, so the pedal dips.

Beware though - once all of the air is out of that line, the behaviour of your SMC may change: If previously it had been compressing air, then the usual SMC checks might have demonstrated that the rear brake releases ok, when in fact it was just allowing the air bubble to expand. Always check that the rear wheel locks, and releases when the SMC is applied and released by hand. Movement at the SMC plunger is typically about 1-2mm. Closer to 1mm than 2.

My article that Larry refers to talks about the advantages of using a vacuum pump to pull fluid through an empty system. Don't confuse that with bleeding an already full system. For that you need the non-return bleed tube. I cannot get a good tight feel at the pedal/lever without putting the vacuum pump away and doing 3 or 4 circuits with the non-return bleed valves. I have 3 to save removing one to use in a different location. And use the pump, pump, pump technique and then open the bleed valve, closing it before the fluid stops coming through. The built up pressure makes the air bubbles smaller and they more readily detach themselves from the joints and tube sides, as well as eliminating the 'suck back' when releasing the lever - although the non return valve should help with that.

 

[TerryS]

With my VFR800 (similar linked brakes) I struggled to firm up the back brake feel until I enlisted my wife to assist with the PCV bleed procedure. I repeated the following in sequence and it worked out great.
1. Press pedal to pressurise SMC
2. Press SMC and hold in
3. Crack PCV bleed, then close
4 Press pedal to push SMC back to full extension
now back to 2

 

[Barnabasb90]

I am having issues with my rear caliper in that it is seizing on after applying brake pedal. I have replaced the seals on all three calipers with no leaks and good pressure. One anomally seems to be that the brake pedal operates operates the two outer pistons. This shouldn't happen should it?

 

[Igofar]

Your SMC is what is causing the two outer pistons to move, not the foot lever.
Your foot lever moves the rear center piston, however, ANY lever also operates the front left caliper, which operates the SMC, etc.
Inspect/Test your SMC unit first before going any further.
Also inspect your rear caliper mounting bracket for wear/damage/dry sliding pins etc.
Lastly, tightening the rear axle BEFORE you tighten the stopper plug can also cause issues.
You have to tighten the 14mm stopper plug BEFORE the axle is torqued etc.

 

[Barnabasb90]

Thanks for that but at present I have both wheels in the air so no movement on the SMC so does that mean I have some form of back feeed through the SMC even when not activated?

 

[Igofar]

It may mean that your SMC is frozen in its bore with rust and corrosion, blocking the return port, causing pressure on things etc.
It also could be the guide dowel dry/dirty/damaged and the rear caliper is not floating smoothing, binding things up.
What you think may be pressure issues could also be alignment issues.
Standing by the white courtesy phone if needed.

 

[Barnabasb90]

The SMC is moving freely as I have stripped cleaned and reassembled. I will have a good look at the caliper binding issue as that is a problem as its not releasing. Not sure what the courtesy phone is about. I live in UK. I do have Whats App though.

 

[jfheath]

Just to add a bit of detail to the above.

When the bike is in the garage, pressing the rear brake pedal will put pressure to all three rear pistons. It will push out the centre piston through the brake line that goes direct to the centre piston from the rear pedal master cylinder - behind the alloy step near your right heel. It also feeds the line to the SMC. The fluid pushes past the primary seal in the SMC (because like a bicycle pump, it seals in one direction only) and that applies pressure to the rear outer pistons.

Now - the reason that the rear brake goes solid when you press the brake pedal is that the fluid that you pumped in has got no means of escape when you let go of the brake pedal. The primary seal prevents it from flowing in that direction away from the pistons. This behaviour is the same for the master cylinders at the brake pedal and at the brake lever.

So how is the pressure normally relieved after applying the brakes? Well - there is a very tiny relief port - called a compensation port - It is almost invisible, and it lies just in front of the lip of the primary seal. As soon as the piston in the master cyclinder is moved, it seals off that compensation port, preventing the fluid that is under pressure from escaping. Which is what you want if you are applying the brakes. When you let go of the brakes, the piston moves back past the compensation port. Any excess pressure is released. There is always a bigger volume of fluid to move back than there was when you started squeezing the brakes - because the pressure causes it to warm up, and some of the heat from the brake pads is also passed into the fluid. The fluid expands, and it needs to get out through the compensation port - back to the reservoir.

Here is a cutaway of the SMC showing the obvious inlet port, and the very tiny compensation port, circled in red. The outlet to the rear outer pistons is the big hole at the right hand end..



And this is what it looks like with the piston, seals and spring in place. The red marks on the drawing indicate the centre of the inlet port on the left, and the tiny compensation port on the right.
The piston only has to start moving and it blaocks any fluid fromt he right hand side of the primary seal from esacping. The primary seal is the black rubber seal to the left of the spring.



So your SMC may well be able to move freely, but if it cannot move to the left to the position where the compensation port is revealed, then it isn't moving far enough.

I could not tell you by operating the SMC whether or not there is enough movement to clear that compensation port. But there is a very obvious tell tale sign:

If your rear brakes are free to allow the rear wheel to spin, and you then press hard on the brake pedal to apply all three pistons in the rear caliper, the rear wheel should lock solid.
If you then release the brake pedal and the rear wheel still will not turn, then the obvious explanation is that one of the compensation ports is still blocked. And my money would be on the one in the SMC.

You can also tell whether the SMC is working correctly by applying it by hand at the fron fork leg. It should only need to be moved between 1 and 2mm to lock the back brake, and as soon as you let it go, the rear wheel should be free to turn again.

If this is the case, the best solution is to get hold of a brand new SMC. And then learn how to service it every year, so that the build up of crud in the brake fluid that is responsible for this behaviour can be prevented.

 

[Igofar]

Well said John, as always, your explanations are outstanding!
Thanks for stepping in to assist, I’ve had my hands full the last couple weeks, and have not been able to reply as much as I’d like.
Thank you.

 

[Barnabasb90]

Thanks a clear explanation. Will investiage further. I have a spare SMC and in fact refitted the best of two after a good clean but I did lubricate the piston rubbers with some silicine grease. When actuated the SMC does operate the the centre piston and relaxes but when I operate the brake pedal firmly all three pistons actuate and then locks up. If I bleed off the rear most bleed nipple the wheel frees immediately. If I understand correctly the problem could be the SMC or the rear master.

 

[Igofar]

Sounds like you are correct in your thinking.
Using a refitted (best of two) may be your issue.
These units are not to be opened up, or taken apart, per the factory service manual etc.
The yoke is a very close measurement, and its easily possible to adjust it incorrectly, which will disable the unit buy blocking the port with the rubber skirt.
On the backside of the SMC (2-8mm head bolts) there is a small screen cartridge that filters stuff. This also has the same very small port, and often is clogged up.
The hole is so small that the only thing that fill fit through it is an acupuncture needle. And it is impossible to do so without damage to the o-ring, and/or filter cartridge.
The bore, piston, and/or skirts are not the issue.
Your filter behind the unit is probably clogged up.
This part is not available from Honda.
This is why replacing the entire unit with a NEW unit is advisable, and usually the only option that fixes things.
The other point I'll make is, when your rear caliper is locked up, and you say bleeding off the pressure by opening the bleeder corrects it, you may be missing the point of alignment issues.
While it may appear to be a pressure issue, because the bleeder seems to fix it, by relieving the pressure via the bleeder, will also allow the rear brake caliper bracket to shift/settle/re-align itself, which may be your issue.
Inspect the rear caliper mounting bracket in three areas.
1. look inside the egg shaped hole where the stopper plug sits, do you see flutes, or spiral markings? If so, this is DAMAGE.
2. look at the lower ledge where the front foot of the brake pads sit, is it loose, or worn away allowing the pads to shift around, binding things?
3. look at the sliding dowels that connect the caliper to the bracket, are they dry/dirty/bent/damaged? Often folks will try and remove these dowels thinking they have to, to pull the caliper apart, and cross thread them back in, or over tighten them, swaging the into the soft aluminum, and misaligning the pin/caliper etc.
I will try and attach a couple pictures of these comments, what you are looking at is DAMAGE and should be replaced.
I am providing pictures of the cartridge, and needle just to give you an idea of the size your dealing with. I strongly urge you NOT to try and take the SMC apart, and advice you to replace it with a new OEM unit if that is called for.

 

[Barnabasb90]

Thanks all very useful. I can only find part numbers for individual components of the SMC not the complete unit. Total cost is about £384 around $500 and then that means utilising existing ancillaries e.g. Slide bolts etc and assembling myself. The larger slide bolt on the rear caliper is ok and screws and unscrews well. I will have a look at the other slider components when I get the caliper off. I will have a look at the caliper bracket as well although I note the stop bolt hole is oval top to bottom although again this does not impede the stop bolt and there is no free play around that when torqued up. There are no scores or marks caused by chattering. The pistons are moving freely. Not sure which is the correct presentation of the Stainless pad retainer. Should it be flat or have cut outs. I do have doubts about that allowing free movement of the pads. The bike is an 2002 work horse having covered 86500 miles. Really appreciate your help as starting to make sense.

 

[Andrew Shadow]

I can only find part numbers for individual components of the SMC not the complete unit.

It is called a bracket sub assembly by Honda.
2002 ST1300, England Model:
BRKT SUB ASSY L F, Honda Part Number 06454MCSG03

Confirm the P/N for your model ST1300 before ordering.

 

[jfheath]

When actuated the SMC does operate the the centre piston and relaxes but when I operate the brake pedal firmly all three pistons actuate and then locks up.

Hmm - are you sure about that ? The SMC - the bracket that pivots on the front left fork leg - that should operate only the two outer pistons on the rear caliper.

The brake pedal will actuate all three pistons when the bike is not moving in the garage - along two different brake lines.
It can be difficult to tell which ones are stuck. Slide a feeler gauge or a slip of paper between the disc and brake pad. That will tell you which pad is piston is not returning.

If I bleed off the rear most bleed nipple the wheel frees immediately. If I understand correctly the problem could be the SMC or the rear master.

The rear most bleed nipple relieves the pressure on the two outer pistons. I'm trying to think whether it will also relieve pressure on the centre piston in a properly working system.
The answer is - I don't know. It may do. There is a none return valve in the SMC, but if there is pressure in the centre pistons line and you relieve the outer piston line, the fluid will be able to flow the correct way past that valve - assuming that the pressure isn't building up in the outer piston line. I still don't know.

It could be the rear pedal master cylinder compensation port that is suspect. The fluid from the rear outer pistons has to go through the compensation port in the SMC and also through the compensation port in the rear master cylinder - so if the rear master compensation port is blocked, then all three pistons on the rear will lock.
You can test for this. Try pushing in the front left centre piston by hand (make sure the piston is clean first). If that pushes in, then the rear master cylinder compensation port is fine.

Do the paper test to see which pistons are not returning.

Oh - and before you do any of that - just make sure that there is space at the top of your rear reservoir. If that is brim full, the fluid has nowhere to go, and if for example you have just put in new pads, or different pads, that is a very real possibility !

Keep us informed.

 

[Barnabasb90]

It is called a bracket sub assembly by Honda.
2002 ST1300, England Model:
BRKT SUB ASSY L F, Honda Part Number 06454MCSG03

Confirm the P/N for your model ST1300 before ordering.

Thanks for that gteat explanation. I have spoken with David Silver Norwich UK and they confirm the part number is correct for the complete assembly. I have ordered the SMC plus the stainless seating for the rear bads and a the smaller of the two guides in case its bent. Can you say is it possible to remove the rear caliper with the wheel in place by feeing the bracket at the wheel and removing caliper centre bolt.