Yet another SMC question/theory

Last weekend I finally decided to bleed the brakes since u had about 35k on them. I figured I'd do the pads and then bleed the clutch since I had most of the plastic off ( which is a whole nother story getting back on when the previous owner has dropped the bike a few times and bent most of the mounting brackets). Anyway, when I got done with everything I had horrible brake drag in the rear. I know I bled all of the air out and I know I did it in the correct sequence. I also took apart the SMC to make sure it was moving freely in the bore, and it was.

In the process of trying to find what was dragging, I rotated the left caliper up to apply the SMC, which applied the outer piston in the rear. Now my question is, when the SMC rotates up, it forces the fluid to the rear caliper but as it falls back down, does it get replenished from the rear master cylinder, or does the fluid get forced from the outer piston back through the SMC? Because right now, the SMC does not refill as it falls back to the neutral position, unless I press the rear caliper pistons in with a pry bar. Does this sound right?

Now my second question is, the only thing forcing the fluid back to the rear caliper is that SMC piston, if you took that out and then bled the brakes as normal, theoretically when you applied the front brakes, you wouldn't have a linked brake set up, right?

Brian

Brian,
This sounds to me that you still have air in your system.
This might of happened when you took the SMC apart.
Did you bleed the PCV valve, it may take a few more bottles to burp all the air out of the system. I also tapped on the SMC when bleeding it and also changed the angle of it also.
Once the SMC piston is retracted, there is a small hole in the SMC allow the fluid to flow back to the rear reservoir.
I'm hoping you didn't change the travel or stroke of the SMC, as that small bypass hole may be blocked.
Also the piston might be hanging up in the bore if you changed the orientation of where it has been travelling before you removed it.
I heard that some of the SMC bores are oval.

I bled everything with a vacuum bleeder amd then went through everything one more time once I was complete. I had the manual and the notes I wrote from the write ups on this site with me the whole time so I did tilt the SMC so I could get all of the air out. I will do it one more time this weekend and really make sure everything in the PCV is bled as well. In fact, I'll probably take the SMC back apart just to double check it but I do know it was and is moving smoothly in the SMC bore.

Brian

I changed my pads at 16k miles, ( they were a bit low, and I had a week long ride planned), and after changing them, I bled the brakes. I was using a Motion Pro bleeder, and in some cases, a vacuum unit, and I had to bleed the whole rear system 3 times to get all the air out. I rode it and checked for rear drag ( I used a Fluke 179 to measure the temp pf the discs), and then re-bled it when I found the temp was too high in the rear. After the third time, the front and rear discs were about even , and there was no drag when on the centerstand. It's not an easy bike to bleed!

diablo1101 said:

I bled everything with a vacuum bleeder amd then went through everything one more time once I was complete. I had the manual and the notes I wrote from the write ups on this site with me the whole time so I did tilt the SMC so I could get all of the air out. I will do it one more time this weekend and really make sure everything in the PCV is bled as well. In fact, I'll probably take the SMC back apart just to double check it but I do know it was and is moving smoothly in the SMC bore.

Click to expand...

Personally I'd do the bleeding with a motion pro check valve type bleeder and throw the vacuum bleeder back in your tool box. I've tried vacuum bleeders in the past and have never had good luck with them. Doing it the old fashioned way with either an assistant or the motion pro worked 100% of the time. I always got good, air free bleeds. When you're through, zip tie your front brake & hang a weight on your rear brake and let them sit overnight like that. This has always made for very firm brakes in my experience.


When you get to the PC Valve in the bleeding sequence, continue bleeding even after the fluid become clear because the air often takes much longer to exit the PCV than the other bleed points.

I've had good luck with a Mityvac kit. It's all I used single handed for the last 10 years. Never a problem flushing and bleeding my bikes and cars. I did replace the last piece of hose to the bleeder with a size that fits the bleeder without the adapters supplied in the kit.

I'm going to redo everything this weekend, even though I dread pulling the right side covers off to get to the PVC. This is the first time I've had problems with using an air bleeder but I do have speed bleeder that fit the threads on the calipers I'll try Saturday. It sucks but it beats the alternative of burning a set of pads up in 200 miles.

I'm going to redo everything this weekend, even though I dread pulling the right side covers off to get to the PVC. This is the first time I've had problems with using an air bleeder but I do have speed bleeder that fit the threads on the calipers I'll try Saturday. It sucks but it beats the alternative of burning a set of pads up in 200 miles.

Mick said:

When you're through, zip tie your front brake & hang a weight on your rear brake and let them sit overnight like that. This has always made for very firm brakes in my experience.

Click to expand...

Also, I highly recommend the Motion Pro (or similar) check-valve bleeder. Personally, I've had poor luck with the screw in Speed Bleeders.

Thanks for taking the time to explain the order for me. This has got to be the most confusing brake system I've ever worked on, and I've seen some goofy set ups before.

One of the things I was able to do last night was force the rear pistons in after taking a tooth brush to them. After reinstalling the pads it seems to be much better but I'm still going to go through the whole system and bleed it again. I'll keep you updated on what I find but I won't be surprised to find air in the system after reading everything that has been posted.

Brian

diablo1101 said:

Thanks for taking the time to explain the order for me. This has got to be the most confusing brake system I've ever worked on, and I've seen some goofy set ups before.

Click to expand...

Just in case you need the info, here are a few links that will help in understanding the brake system:

https://www.st-owners.com/forums/attachment.php?attachmentid=37122&d=1223995327

http://world.honda.com/motorcycle-technology/brake/p5.html#01


You seem aware of the correct bleeding sequence but here's a cheat sheet from my personal archive that used to be on the site (link no good now) that has a different twist (in red). It mentions something I never did which is to to activate the SMC piston while bleeding the PCV. jfheath mentioned this in his post above. Like I said, never did it but it might help in your case if your problem is stubborn air in the system:
************************************************************************************************************
From: https://www.st-owners.com/forums/showthread.php?53678-ST1300-Brake-Bleeding-Sequence-Cheat-Sheet

1.
2. Re: Brake Bleeding Sequence Cheat Sheet

When I changed the fluid in my brakes several weeks ago, I modified the procedure slightly to the following (in red):

Brake Lever

1. Left Outer (upper) bleed valve.
2. Right Outer (upper) bleed valve.

Brake Pedal

1. Remove and tilt left front caliper 15 degrees (wood shims inserted between the pads will keep the pads from extending while~if pedal is pumped)
2. Proportioning Control bleed valve. While the SMC (front left caliper) is removed and tilted, manually pump the SMC piston one stroke inward and hold, and have a buddy pump the rear brake pedal to continue bleeding from the proportioning control valve bleeder, which simultaneously re-extends the SMC to the fully out position. Repeat the sequence as necessary.
3. Rear Brake Center bleed valve.
4. Right Front Center bleed valve.
5. Left Front Center bleed valve.
6. Rear Outer (forward) bleed valve.
**************************************************************************************************************

Excellent info there. I'll definitely try it this way since brake fluid is relatively cheap, I'll see if this a difference.

Brian

It was only by activating the SMC that I got all my air out, both at PCV, and at the rear. It seemed to take hours, but it DID work in the end. After reading this thread, I checked my brakes after a longer ride yesterday, and all 3 discs seemed to be the same temp.

In a rare morning off, I decided to go ahead and bleed the brakes again. Here's what happened during this bleed.

I went through the entire system in order and pulled a very tiny amount of air out, it could have come from the bleeder screw threads, it was that small. But, during the bleeding of the rear caliper and circuits, the SMC snap ring popped off and blew the piston out. I reset it back in but after 6 or 7 pumps it popped out again. I know it's seated all the way as I double checked that the ring was in the groove all the way around. This got me thinking, can you activate the rear brake with the SMC disconnected from the mounting without harm?

The second thing I noticed is the return spring on the SMC will compress if I push the piston down but the piston stays in the down position and doesn't rise back up from the spring pressure, is this right? When I press the rear brake pedal, the piston will rise up like it should.

Any insight as to what anyone thinks is going on would be appreciated here.

Brian

Brian PM me a contact number and I will call you and assist you in this matter. In the mean time DO NOT attempt to ride this bike.
Igofar

Please entertain my dumb question: Why isn't the brake bleeding procedure to place the left front caliper in the 15 degree position, the first step in the process?
It seems to me,and I am open to educating, that bleeding the non-SMC related part of the caliper would not be affected by 15 degrees, and the tilted caliper is only to make sure the SMC part is bled.
Your earnest consideration, please.

Because you're bleeding the front brakes first. Placing the caliper at the 15 degree angle would probably trap air in the front caliper circuits. That's my theory at least.

Pm sent. And don't worry, I'm not going to ride the stupid thing. I think I hear an FJR calling me in the background though.

PM sent...call me tomorrow, can't use either hand to type...just had surgery on both hands and wrists and have a bunch of stitches in them.

This is just a guess. Could it be that the snap ring that pops out is flat on one side and slightly rounded on the other side? GM used to use snap rings like this in their A/C compressors and if you put them in backwards they would pop out of the groove under pressure.