Advice needed on what is causing rear pedal pressure lose.

[MidLife]

When bled correctly, you CAN completely eliminate the small drop

The pedal had a huge drop when I bought the 1300 18 years ago (dealer serviced). I did a bleed following the same instructions dduelin did, written by Leo 24 x 7 who was the go-to whisperer back then, and still had a drop, although smaller, still a drop. Then Leo invited me to Dallas to redo it under his supervision, still dropping after that.

Then it has always been dealer serviced ...and of course dropping.

Lately, this local old timer has been servicing the brakes and they've never been as tight before. Great feel. Tried to find out what he was doing that others weren't (including Leo and me) but he doesn't like to talk, all I could get out of him is "you ride it, I fix it!". But still dropping!

So it looks like the only way for me to experience rear brakes without a drop might be a trip to AZ!

 

[core-aggie]

So it looks like the only way for me to experience rear brakes without a drop might be a trip to AZ!

That's waaaay better than self speculation of how the system works

 

[dduelin]

Once I fixed the pedal drop on my bikes they had none as long as I owned them. People sometimes say it’s normal but it isn’t, at least on the two ST1300’s I owned. With both brakes applied and coming to a stop you should be able to trail off the lever and transition to pedal only with no change in rear pedal feel.

 

[MidLife]

I feel like you can forget about the front brakes when they are good and focus on the rear/SMC. However, I have never seen it mentioned it is OK to start skipping the Front hand lever process (Nipple 1 and 2). Perhaps lap the 5 bleeders for the rear/SMC brake until they are good, then go 1 through 7 to ensure everything is solid.

Good common-sense advice.....but a bit too advanced of a concept to be mentioned here!

And thanks for posting the Brake Toy, Had missed it.

 

[jfheath]

The 'drop' at the pedal is more of a sensation than a change in brake pedal position. And it makes perfect sense to me that I would be able to feel this. In the same way that you feel the pulsing of the fluid through the brake lever when the ABS kicks in.

The front brake operates the SMC. The rear pedal also operates the SMC with one smaller piston in the front left caliper. The SMC application isn't a switch - it is an analogue increase in pressure as the stopping force of the front left brake increases. I always assumed that the SMC would not operate nearly as much when the pedal is appying the brakes as when the brake lever is applying the brakes. This video seems to confirm that notion.

SMC in Action - YouTube

So as the pessure on the front pads is reduced by releasing the brake lever, the fluid pressure from the SMC to the rear outer pistons will decrease (but will not be removed completely). The SMC piston will move back a bit. - but the rear pedal also feeds in to the inlet port of the SMC. It seems logical to me that a change in the pressure of the fluid inside the SMC would be felt by the rear pedal if your foot was gently applying the rear brakes.

 

[MidLife]

I feel like you can forget about the front brakes when they are good and focus on the rear/SMC. However, I have never seen it mentioned it is OK to start skipping the Front hand lever process (Nipple 1 and 2). Perhaps lap the 5 bleeders for the rear/SMC brake until they are good, then go 1 through 7 to ensure everything is solid.

Actually....it has been mentioned!

Have a look at page 8 of the "Air Hunt" doc you posted.

It says that if you need to lap around as you suggested, start at step 3.

Some people truly understand what they are doing.

So you are right up there, with the SMC Master himself (and not many more apparently.....)!

 

[MidLife]

The 'drop' at the pedal is more of a sensation than a change in brake pedal position.

But it quickly turns into a real drop as soon as there is a bit of air (or moisture and heat like here in TX) in the PCV stretch of the system.

So as the pressure on the front pads is reduced by releasing the brake lever, the fluid pressure from the SMC to the rear outer pistons will decrease (but will not be removed completely). The SMC piston will move back a bit. - but the rear pedal also feeds in to the inlet port of the SMC. It seems logical to me that a change in the pressure of the fluid inside the SMC would be felt by the rear pedal if your foot was gently applying the rear brakes.

Not just logical, but like some like to say here, it is physics. Whatever pressure to the rear outers was lost from releasing the front needs to be replaced through pedal action. A blip in a well bled system but a drop with the slightest air the system.

It seems logical to me that a change in the pressure of the fluid inside the SMC would be felt by the rear pedal if your foot was gently applying the rear brakes.

That's the key word here. If you start with full pedal pressure, then the front middle is firmly engaged by the time you release the front lever and there is not much reenergizing to do with the pedal (The vid is great help for understanding that, thanks!)

When Leo 24 x 7, the way back then ST guru, helped with the drop on my ST, he couldn't feel the drop at first, because he was a very energetic type of guy and he was stomping on the pedal. But once he caught on on the "gentler" part, he immediately felt it.

AZ Larry judges how well the brakes work by how far the rear wheel lifts off and dduelin is reported as riding like a bat out of hell, so their energetic style is probably what is hiding the dip from them. No surprise there!

 

[Igofar]

We will have to disagree with your last comments directed towards me…
Every bike that has come through with this so call dip issue, was corrected, and the owners of the bikes, reported that it was no longer there, it was not my opinion, or my riding style that was not seeing it etc.
While some folks have never felt what correctly bled brakes feel like.
I have corrected new bikes, old bikes, worn out bikes, and straight out of the crate bikes.
I stand by my comments that I can remove the dip, if it’s caused by air in the system.
Many folks purchased these bikes used, and have no idea what the system should feel like.
Many folks feeling a rusted, non moving SMC, think that what they are feeling is normal.
On a correctly functioning system, when the SMC is working correctly, it moves when you come to a stop, when pressure is released from the brakes, it’s normal that the SMC will return to its resting position etc.
This, is not pedal dip, and just about everyone who has replaced their SMC will then understand this.
If the pedal drops down and dips further than what is needed to reset the SMC, then you have air in your system.
And as Dave pointed out, when bled correctly, there is no drop in either pedal upon release etc.
Just because your shops or mechanics could never achieve this, does not mean it’s not possible.

 

[Igofar]

@MidLife, I’ll make you a deal, ride your bike over here, and I will flush your system correctly, if I can’t remove what you perceive as a dip in the pedal it’s FREE, if it goes away you can donate to my tip jar.

 

[Langley863]

Managed to get out today. After spending the morning bleeding with no gains decided the SMC was at fault piston was sticking and not returning quick enough. Swapped the spring as the old one was 1 coil extra. Reinstalled bled in sequence and manually pumped the smc.

So now front brake perfect. Rear brake perfect. Combined to stop, front release, rear dips tiny bit but holds the rear brake (main problem solved) now if I slam on just the front and then apply rear shortly after I get full travel once to make the smc solid again then solid. Currently zip tied n weighted on the brakes will get out tomorrow morning to do the final steps suggested by Igofar and will report back but I think air and incorrect smc spring was the fault.

 

[Igofar]

If I'm understanding you correctly "if I slam on just the front and then apply rear shortly after I get full travel once to make SMC solid again"
What your describing is obstruction in either the bore of the SMC, or the filter cartridge behind it at the return port hole etc.
While the steps I outlined for you will help remove all the air out of the system, my money is still on you have a damaged SMC that needs to be completely replaced.
Mix & Matching parts on a brake system could be a serious concern, and down right dangerous.
I urge you to replace the entire unit, inspect the rear caliper bracket, and flush and bleed the system correctly, as outlined.
We don't want to read about you getting tossed off your bike, when your Parts SMC failed.

 

[dduelin]

As a rider knows his horse and the two become one it becomes impossible to separate the two. As you are slowing to a stop with both brakes and you bleed off front brake during the last few mph you automatically increase pedal pressure to keep the same rate of deceleration. It's not something you should even have to think about.... but if the pedal drops a few mm or more when the last of the lever pressure is bled off you will notice that and that is not normal. After removing that air there is never a drop but yes, a change of foot pressure to effect the same rate of deceleration.

 

[jfheath]

We are going off topic - that wasn't my intention. Regardless of my hypothesising, I still reckon that Jono has either:

* Still got air in his system - because that is how the system behaves when there is still a bubble in there.
* Has an egg shaped smc bore where the primary seal is not sealing properly
* A broken spring in the smc

The symptoms are all very similar. Pumping up the brake pedal is required before the rear brakes work.
But since he has it more or less working, it sounds like there is still air in there. It doesn't take much air to cause this.

I have dropped @Langley863 a PM so that we can chat by phone if he wishes. But a new SMC would cure all of these, and isn't a bad investment anyway. Unknown history, the SMC is going to fail at some time. Get one while they are still available.

 

[jfheath]

Currently zip tied n weighted on the brakes will get out tomorrow morning to do the final steps suggested by Igofar and will report back but I think air and incorrect smc spring was the fault.

How are they feeling now ?

now if I slam on just the front and then apply rear shortly after I get full travel once to make the smc solid again then solid.

I'm with Larry on this one.

You might have seen the cutaway version of the SMC in one of the threads. This one:



This SMC was removed from a bike that had exactly the same symptoms that you have. He had fitted a new service kit, and it made only a slight difference. He had to pump the pedal before the rear would bite, every time. His SMC would rattle.

This turned out to be a damaged bore. I saw when I think this happened. Setting off from a stand that we had at a motorcycle show, he changed gear just before turning a corner and selected neutral. With no drive to pick the bike up, he toppled to his left and his front wheel fell into the kerb - and I suspect, knocking the SMC bore out of shape - 'putting an egg in it'.

The new SMC fixed it.

 

[Langley863]

So the weight didn't fix it did a little more testing and I'm convinced the smc piston was sticking so going to replace that and will test again.

I think it's the smc because I took it around the corner and used my front brake no rear and then going over speed bumps hear a loud clunk 1st time ive gone further than my front street to test. Turned out it was the rod end hitting the piston as it hadn't returned fully up. So the dead travel in the rear brake wasn't air but was pushing the piston in the smc back in place hence why it all worked correctly on centre stand.

Will say if it's definitely that once I've replaced n bled the system again hopefully in the next few days.

 

[jfheath]

Get some new crush washers for the Banjo Unions. The SMC requires 4, but you will probably need to remove the ones to the caliper too.
I just bought 100 copper washers from demontweeks Buy Goodridge Copper Crush Washers | Demon Tweeks (demon-tweeks.com)
Since you have to replace them every time you dismantle, its handy to have some in stock if you ever have to separate a joint.
Some will (rightly) argue that you should have alloy washers otherweise you are likely to get galvanic corrosion - caused by electrical reaction by different metals.
But I have had less problems with copper washers than I have with alloy washers which seem to corrode due to salt from the exposed outside edge. I have never seen that with copper washers.

Check - but I think the washers are 10mm Inside diameter.

You might also find this useful - When I last went through the process, I took photos and tried to identify the potential snags. Download the pdf - post #1.
Please heed the warning about disturbing the rubber boot, and how to put it back properly if you do want to add additional silicone grease behind it.

Article [13] - ST1300 Maintenance - Replacing the Secondary Master Cylinder (SMC) | ST1300 Articles | ST-Owners.com

A neat trick I think is to bleed with the crush washers just tight enough to stop fluid coming out when you are bleeding. It gives you the opportunity to burp the joints and still have plenty of 'crush' left for when you tighten them to their full torque setting - 34Nm I think, but as always - check before proceeding with info from some stranger on the internet. Did I check ? Yes, of course I did. But you don't know that. Check yourself !

Don't forget the most important bit - to take apart your old SMC including the nylon cartridge in the inlet port, and report what you find.

Where abouts in the UK are you ? Are you close by - see under my avatar.

 

[MidLife]

@MidLife, I’ll make you a deal, ride your bike over here, and I will flush your system correctly, if I can’t remove what you perceive as a dip in the pedal it’s FREE, if it goes away you can donate to my tip jar.

Nice try, Larry!

And I know exactly what happens next:

--- The whisperer takes the bike hostage with threats to post pictures to the hall of shame.

But what the hell, I start saving for the ransom right now!

And I'll tip the Jar even if there is still a little dip. I can live with a little dip. In fact, I am so used to it, I might miss it if it completely goes away!

 

[mjc506]

Re: "the dip"...
If I've understood correctly, the dip is noticed when pulling to a stop using both lever and pedal, then when releasing the lever the pedal drops slightly?
If so, as the front lever is released, the front pads will squeeze less hard on the rotors* and the SMC may then retract. The SMC piston retracting increases the internal volume of that system (that's what brake master cylinders do), so the rear pedal will drop to compensate (if the load on it remains fairly constant)

*If, however, the rider is putting a lot of force on the rear pedal, the pressure from that front left middle piston will be sufficient to hold the caliper and SMC forward (compressed), so the SMC will not retract and the pedal will not dip until pressure on it is reduced, at which point any movement due to 'dip' will not be noticable...

Of course, plenty of things to affect how much pressure is required from the rear pedal to hold the SMC forward once stopped... Air in the lines? That'd probably do it. Old brake fluid? Yup. Faulty master cylinder or caliper somewhere? Almost certainly... And different riders in different situations will apply different pressures to that pedal while coming to a stop, so some riders may notice it, and some may not.

 

[Igofar]

Someone was paying attention in class

 

[MidLife]

Re: "the dip"...

.... as the front lever is released, the front pads will squeeze less hard on the rotors* and the SMC may then retract.

As well illustrated by the SMC vid posted earlier.

The SMC piston retracting increases the internal volume of that system (that's what brake master cylinders do), so the rear pedal will drop to compensate (if the load on it remains fairly constant)

What jfheath said and seems logical.... to you, and him, and me......but not many more!

*If, however, the rider is putting a lot of force on the rear pedal, the pressure from that front left middle piston will be sufficient to hold the caliper and SMC forward (compressed), so the SMC will not retract and the pedal will not dip until pressure on it is reduced, at which point any movement due to 'dip' will not be noticable...

Tried to make the same point earlier that a more aggressive rider may not feel the dip. But same concept. Not many believers for that either.

Some would even go as far as calling it....

....... self speculation of how the system works