Rear brake problems on S2R800. Problem Solved.

[Monstyr]

I've had no issues with my S2R rear brake, however, I went through a couple of rear masters on my old 916 which had a similar setup, the problem was corrosion of the pushrod, take the master off and remove the rubber boot then clean out the gunge which tends to collect in there and clean everything up, corrosion means the rod doesn't slide through the seal smoothly and will stick, if it sticks on you'll have obvious problems, if it sticks off then the continued pressure of using the brake may blow the seal. If the pushrod gets too crusty it also tear the seal leading to leaks and air in the system.
This might not be a factor on your bike but worth checking anyway.

[corey]

It sounds like your brake is bled properly.  The problem is like stated above - the push rod needs to be adjusted shorter.  When the brake pedal is released, there should be some free play between the rod and the slave piston.  The piston needs to back out far enough to uncover the internal bleed back port for the fluid to return.  I don't think you'll see a lot of fluid return, not like when you push the pads back.  Does the lever move easily?

Push rod has more than enough free play unfortunately. I have it backed out entirely.
The lever is rock hard, unless i push fluid back into the reservoir by spreading the pads.
It justt seems like fluid does NOT want to come back through the bleed hole. There is something preventing it from happening.
Does anyone have any internal knowledge of these Master Cylinders? Is there a seal or something that could go bad?
Could it be something OTHER than the master cylinder? I can't really think of anything else it could be, because fluid flows free and easy through everything while the bleed screw is cracked open. Fluid just won't go backwards back through the master into the rezzy. Even with the cap off.
Could this be a caliper issue? Is there something that makes the caliper spread back open that forces the fluid back through the system? or is that just a natural thing that happens with hydraulics?

I didn't use the wrong fluid did i? DOT4 correct?

Where is the bleed hole located on the M/C? Is it something i can get to and clean/clear out manually?

Should i just buy a new M/C? Are there aftermarket options that are less prone to failure?

[Howie]

You may have to disassemble and check.  Yes, you used the proper fluid, but what about previous people?  If someone before you introduced petroleum product into the system the seal is swelled.  The vent port can also be restricted.  Rebuild kits are close to impossible to find.  New OEM replacements are inexpensive. 
http://www.yoyodyneti.com/Category.aspx?CategoryID=3007
PS11B is your master.

The caliper can be a problem also, but not likely.  If extra force is needed to retract the piston that can cause the problem.  In normal use twisting of the piston seal is what returns the piston.  If in doubt, let someone with a more educated feel check. 

[corey]

Thanks for the reply howie.
I bought the bike with only 2600 miles on it, and I'm pretty sure the rear fluid that was in there was the original.

I did some searching around on some other ducati forums, and found this statement:

"So lets try this. IF the compensating port is blocked, blocked because either the piston rod is screwed too far out of the clevis, OR if the screw that contacts the brake light switch is screwed out (towards the switch), then fluid WILL NOT return to the reservoir."

If my brakelight switch screw were too far out, and the rod returned to the resting position it could achieve within the area allowed by the brakelight screw, the vent port would remain obstructed by the rod. If the vent port were to remain obstructed, would pressure applied to brake when pressing the pedal down STAY pressurized? Thus creating some free-play in my pedal that i THINK i have, but actually may not?

Edit: After thinking this through, i have even tried REMOVING the rod entirely after engaging the brake. The pressure still does not release. The pressure builds and builds, but will not release/return fluid.

Is it possible that the M/C just will NOT allow fluid to return?
It's like a vacuum is forming inside the system...

[Bill in OKC]

Is it possible that the M/C just will NOT allow fluid to return?

something is keeping the piston from returning to its resting position.  It looks like time to take the master apart and check it out.  Dirt, corrosion, broken internal spring,   if there is nothing external pushing on the piston then it must be internal.

[corey]

some more information.
if the bleed screw is cracked, the pedal works as normal. Full push, full release.
the push-rod (bill is that what you are calling piston?) is at the rest position, with plenty of play.

on this caliper, do both pistons move? or only one.
i remember only being able to move one brake pad while prying the pads apart.
is it possible that one of my pistons on the caliper could be stuck? causing the fluid to remain "mostly" in the system, but allowing just enough retraction to disengage the brake (i.e., the OTHER piston retracting a bit)?

i'm not sure how this could have occured though, as the caliper has never squeezed onto anything aside from it's specific rotor...

any guides out there on disassembling the master?

[Langanobob]

It almost sounds to me like you got used to riding around with some air in the system.  Now your brake is bled and working right, but you're not used to the positive action.   But this is just a guess from long distance.

If the bleed port wasn't bleeding, I think the brake would pump up with each stroke and the tire would stay locked when the brake pedal is released.   From your description it doesn't sound like the wheel is locking up.   If everything is working correctly the pistons  do not move very much at all, there is not much fluid going to the caliper and there might not be a jet of visible return fluid in the reservoir when the pedal is released.

The seals in the caliper are supposed to flex a bit when the brake is applied, allowing the pistons to move then act like a spring to pull the pistons back very slightly.   

Howie, correct me if I'm wrong but if the bleed port isn't open, he wouldn't be able to force fluid back to the reservoir by moving the brake pads with a screwdriver?

[EEL]

I just did a rear brake bleed on my S2R 800 yesterday. From what I've heard so far corey, your caliper does not seem like the source of the problem. Your problem is in the rear master cylinder if there is one..Have you gone for a ride and tried the brake or is this all "bench" testing..

[Langanobob]

As a reference point for what it might be worth I just went out and looked at my M620.  I don't know what the spec is, but moving the pedal by hand I get about 1/4" of motion to take the slack out of the actuator rod.  Then about another 1/4" of actual M/C piston motion (measured at the boot toe end of the brake pedal)  before the brake is fully engaged.  There is no visible activity in the reservoir when the pedal is released.

[corey]

EEL:
I have ridden the bike.
It rides fine, there is no lockup of the wheel, and the caliper is not engauged unless i push the pedal.

Langanobob:
Your measurement experience sounds about exactly the same as mine.
I wonder if you are correct about me getting used to running with air in the line.

Can anyone else add any feedback as to how far their push-rod moves from zero-engagement to full-engagement?

My pedal just seems very stiff, but i guess it could possibly be the way it is SUPPOSED to be.

Previously, my pushrod would travel roughly 1" to 1.25" when using the rear brake pedal.

[Howie]

I think Langanobob may be onto something, you may be trying to fix something that ain't broke.  Go for a nice ride, use the back brake and see what happens.  Be cautious though, the back brake is ineffective enough at speed that you might not feel the drag, but you can build enough heat to cook your back brake.  I know someone who cooked his back brake twice on a 748. 

<snip>
The seals in the caliper are supposed to flex a bit when the brake is applied, allowing the pistons to move then act like a spring to pull the pistons back very slightly. 
Nice explanation
Howie, correct me if I'm wrong but if the bleed port isn't open, he wouldn't be able to force fluid back to the reservoir by moving the brake pads with a screwdriver?
If the port is completely restricted one would have so put enough force on the piston to get the seal to bypass, possible with the combination of leverage and hydraulic advantage.  If one did accomplish this he would most likely notice brake fluid escaping through the rear of the master.  Partially restricted, maybe.  .Therefore you are correct.  One way to catch a master that is not releasing is to bleed the brake(s) affected after it binds.  The brake instantly releases, bingo!

The spec I gave you is for the push rod, not the pedal.  You will have much more travel at the lever.

[corey]

Sounds like I am beating a dead horse... or I guess a live horse now?
like i said, the brake is not engaged unless the pedal is depressed. the actuator push-rod has about 1/4" free play (to be safe, it's a little more than spec'd 1.2mm), and then about another 1/4" to 3/8" of travel before i am out of pedal. A very firm braking action indeed.

By everyone's description, it doesn't sound as if my bleed port is obstructed, since I am EASILY able to force the fluid backwards by spreading the pads inside the caliper. It doesn't require much effort at all. I can do it by squeezing with my hand if a I really get into it.

My only worry is that if there is an obstruction, when the bike gets warm, the fluid will expand and lock up my back brake. I guess i will have to be sure there is not too much fluid inside the reservoir. Is this a legitimate concern?

I will ride it a few times and see what i get, but as stated, it sounds like i actually fixed it, and it's now working like it should, and I'm just used to a little bit of a mushy pedal (like a car) and not the stiff nature of the rear brake on the bike.

I'll keep you guys posted. I'm going to have my dealer check it out when it goes in for the 6k service in early June.

I appreciate all the thoughts and help, this could be a great informative thread for the future!

Cor

[woppini]

Not sure if I can be of help, but your symptoms sound like 2 things that come to mind. Brake hose going to caliper. If they fail internally they can act like a one way valve. The caliper may not release, and or hydraulic supply maybe cut off. I didnt read all the pages, but i am assuming your removing the rear brake caliper to bleed it. The bleeder screw must be bled as it faces upwards. You would never know if the hose was at fault since your twisting the whole thing as you bleed it. It may only act up when installed since the hose falls into its natural bend. Was any fluid other than brake fluid used? If thats the case, every rubber seal and that comes in contact with the fluid is toast. That includes caliper, master cylinder and brake line. One last thing, you MUST use D0T 4 fluid. DOT 3 does not have the temperature rating that bikes need. Once things heat up while riding, I have heard that the DOT3 can actually apply the brake because of fluid expansion and boiling point.

[corey]

Not sure if I can be of help, but your symptoms sound like 2 things that come to mind. Brake hose going to caliper. If they fail internally they can act like a one way valve. The caliper may not release, and or hydraulic supply maybe cut off. I didnt read all the pages, but i am assuming your removing the rear brake caliper to bleed it. The bleeder screw must be bled as it faces upwards. You would never know if the hose was at fault since your twisting the whole thing as you bleed it. It may only act up when installed since the hose falls into its natural bend. Was any fluid other than brake fluid used? If thats the case, every rubber seal and that comes in contact with the fluid is toast. That includes caliper, master cylinder and brake line. One last thing, you MUST use D0T 4 fluid. DOT 3 does not have the temperature rating that bikes need. Once things heat up while riding, I have heard that the DOT3 can actually apply the brake because of fluid expansion and boiling point.

Thanks for the reply woppini!
To answer your questions...
I'm pretty sure the line is fine, as I am able to bleed plenty of fluid (an entire canister in fact) in both the mounted AND unmounted, upside-down bleeding position. I have also checked to see if fluid would go back through on it's own by leaving the bleed screw cracked and releasing the pedal to see what would happen. plenty of suction back the other way.
As far as brake fluids, the stuff that was in there looked original, and i used DOT4, high-temp specific stuff.
As stated, I'm pretty sure i got everything right, it just "feels" different to me than what i'm used to, which is a spongy pedal i guess...

[woppini]

the master cylinder is mounted at an angle. If there are air pockets in there, they will always stay toward the highest end. Have you tried removing the master cyl, and keeping it level? Crack the bleeder lose and let it gravity bleed or take a measurement of how far the push rod moves in with the master cyl installed. When you remove the cylinder to hold it level, push on the rod the same amount you just measured. Crack open the bleeder each time you push in the rod, and close it each time you release the rod. The reason why I mentioned taking a measurement is because you can push the master cyl rod too far in which could cut the seals.