Topic: CB550 front brake troubles

[asbissonnette]

Hey everyone, in progress of rebuilding a 75' cb550. I am mostly finished except for two problems (one which I will post in another thread). The big problem I am having is with the front brakes. I have read forum after forum and every time I think I find a solution, the problem shows up in some other form.

Right now, I can't get the caliper piston to fully retract. It kind of sits on the rotor, preventing the front wheel from moving freely. Putting pressure on the lever does cause the caliper to squeeze and fully stop the front wheel.

I took the brakes completely apart, cleaned everything up and put new pads on. The hoses are not clogged. I replaced copper washers on the banjo bolts. I am 99.9% sure there are no leaks. The master cylinder seems to be ok as I can tell it is pumping fluid, I also had the guy at the local cycle recycle check it, he said the cylinder was good.

I'm sure the hoses can probably be bled a little better, but I have spent 2 weeks trying this and the point I am at now is the best I've gotten it. I bought a brake bleed kit from harbor freight that hooks up to an air compressor, it is supposed to create a vacuum in the jar at the bottom to suck fluid through the bleeder, but I could never get the vacuum strong enough, despite meticulously following directions. So I basically modified it a bit to where I filled up the bottom jar with fluid, made sure the rubber hose is always under the fluid level, then blow compressed air into the jar to force the fluid through the caliper bleeder and out the master cylinder. In theory it should work, but I don't think I've gotten all air out. Because the lever is still spongy, I've tried traditional way also, with less luck.


I have it bled to the point where squeezing the lever all the way down does fully compress the piston, but it doesn't fully retract.


I rebuilt the caliper, took out piston, cleaned it up. There was some very small pits of rust, very small, but overall it looked good. I lubed it up with some silicone grease and used a C clamp to press it all the back in. I didn't have the caliper connected to the system when pressing the piston back in, but still needed the clamp to get it back in. Should it have been this hard to get it back in?

I know the quick answer is to get the brakes bled better. But is there any other possible culprits to this problem, before I invest all that time again? I haven't replaced the rubber hoses: could their expanding/retracting be enough to cause this? Any chance the caliper needs replacing?

I am getting close to losing it over this problem, any help would be much appreciated. Thanks.



I can't get the brake piston to retract fully.

[RAFster122s]

The seal will retract the pad just a tiny bit, the caliper will have very little drag on the rotor. Any runout on the rotor will push the piston back a tiny bit. Odds are it is working normally if your stationary pad is adjusted properly.
You may be obsessing over normal behavior.

[mickwinf]

May be the master cyl needs rebuilding, also could be the small hole in the fluid resovoir it is a tiny hole but needs to be clear to work correctly

[asbissonnette]

Oh, that is interesting RAFster. So the brakes may not normally retract fully if you are just testing them stationary while the bike is on a stand as the rotor sort of pushes them back in?

[LesterPiglet]

May be the master cyl needs rebuilding, also could be the small hole in the fluid resovoir it is a tiny hole but needs to be clear to work correctly

Just in case it isn't clear, there are TWO holes  in the reservoir.

[flybox1]

first..remove the brake caliper arm and separate the halves from the arm.  pressure force the piston back in while releasing the brake lever. now you have some working room.
replace everything, and before you squeeze that brake lever...ensure your stationary pad is gapped to your rotor properly.

if this does not work, you'll need to service your caliper(new piston seal) and possibly the master cylinder.

[TwoTired]

Wow, not many restore a 75 Cb550 to totally stock anymore.  Enjoy the challenge.

The retraction mechanism is a combination of the spring and square section seal distortion, and shape restore.

During set up, the stationary pad is is adjusted for .006 in. clearance when the piston side is not yet deployed (or pushed back into its bore, .020 inch or so).  The spring on the adjuster screw will make the whole brake arm swivel to that point.  Speaking of which, you have ensured the caliper brake arm swivels freely?

I don't know who told you to put silicone grease on the piston seal.  But, that was incorrect.  Either brake fluid or much preferably brake assembly lube is the correct choice as the seal needs to "grab" the piston to perform the piston retraction.    If it is now contaminated with silicone, it must be replaced, as it is nearly impossible to remove silicone that has permeated rubber.

The seal groove floor and sides must be totally clean, so as not to distort the natural seal shape.  As the piston is pushed toward the rotor, the square section shape turns to parallelogram.  When the pressure is released the seal returns to square section and draws the piston back with it as it reforms a square section.  Silicone will hinder its "grab" on the piston, as will significant distortions placed upon the seal by corrosion pits and any debris left in the seal groove.

Another possible hindrance to proper retraction is pressure remaining in the hydraulic system.  The master has a very tiny relief hole (~ .010) in the cavity nearest the outlet line.  Not only does that have to be clear for fluid to return to the reservoir, the master's piston must retract far enough to fully clear that hole.

I have also encountered rubber brake lines that have internally deteriorated and functioned as a one way valve, allowing fluid to go toward the caliper piston, but not toward the master cylinder.  No doubt is was restrictive, before it became a total block.  It was found by cracking each fitting at each end of the line's connections, sequentially, until the caliper pressure was released.  Note that full retraction is only about .012-.015 total.  Just enough to remove friction, if not full contact.

Bleeding.
The SOHC4 brake system was delivered with a naturally self bleeding physical arrangement.  Bleeding is the elimination of all the air inside the system.  And that tiny hole in the brake master was located at the highest point in the fluid system.  All the components have a natural rise toward that point, and bubbles will naturally rise to the top and exit that tiny hole all by itself overnight with the first small activation of the lever.

Be aware that suction at the caliper nipple will be counter to the natural rise of air bubbles.  Drawing an air bubble near the master all the way down to the caliper bleed nipple will likely require a lot of fluid travel.  Further, the bleeder nipple threads will leak air with suction on the nipple head making it difficult to know that all the air was removed.

About the caliper:
After cleaning, the piston is dry fit without seal to ensure free movement within the caliper bore.  The same is said for new brake pads as they are often supplied with interference fit, which must be filed to ensure free movement.  Further, there is a nylon alignment washer between pad and piston to help keep the pad from cocking/binding in the caliper bore.

Not mentioned in the trouble report, is if the lever is hard enough to prevent its contact with the grip.  If so, it is for certain not bled properly.

Inserting the assembly lubed piston into the caliper with seal installed is not easy.  But, you should not need a C clamp to force it in, particularly of lots of clamp pressure is required.  Could you be installing the piston backwards?  The steel "cap" on the piston must face the brake pad.

[RFogelsong]

Great info from TwoTired there, interesting info about the deformation of the seal. Does that mean that an o-ring x-section seal in that groove would not work for this system? (always been curious)

I'd like to add a general use tip.  An easy way to tell which component (caliper or master cyl.) is the problem is to tap the flat(ish) area of the caliper body with a rubber mallet lightly (not much force needed at all, you can do this with your palm if you'd like) about 5-10 times.  Now check if the pad has play and has retracted by seeing if you can wobble the brake assembly at the arm in/out (you'll hear a faint noise as the pads hit the rotor when you wiggle it if it's backed off). 

If the pad's backed off then your caliper assembly (read seal) probably wasn't the issue.  If it's still not released, chances are your piston is grabbing in some way.  High e electric guitar strings are .009-.011 in most cases, great for clearing that tiny hole at the master.

[asbissonnette]

Yes thanks for all the info twotired. I'll need to reread everything you wrote when I am actually in front of the bike. I'm not quite understanding the square/parralellogram seal concept. Are you taking about the rubber seal that is about halfway down the piston bowl?

The silicone grease was recommended by the Clymer manual. It says to put a light amount whereever the piston slides against the caliper. I found some brake parts lube with silicone at autozone, it is thick, sticky and green. Even the package says it can be used on calipers. Before purchasing, I read that silicone is recommended for use on rubber and that something like White Lithium Grease should not be used because it makes rubber warp and expand. There was research that went into the purchase.

I am pretty sure that hole is clear in the cylinder. I witnessed tiny bubbles coming out and can see the fluid flowing out when I squeeze the lever. Yes, I switched to trying to force the air up, rather than down after trying it once.

The pressure is still pretty low on the lever, I realize this is a problem. I am able to squeeze it all the way to the grip.

[TwoTired]

Yes thanks for all the info twotired. I'll need to reread everything you wrote when I am actually in front of the bike. I'm not quite understanding the square/parralellogram seal concept. Are you taking about the rubber seal that is about halfway down the piston bowl?

Yes.  It is the only seal inside the caliper.

The silicone grease was recommended by the Clymer manual. It says to put a light amount whereever the piston slides against the caliper. I found some brake parts lube with silicone at autozone, it is thick, sticky and green. Even the package says it can be used on calipers. Before purchasing, I read that silicone is recommended for use on rubber and that something like White Lithium Grease should not be used because it makes rubber warp and expand. There was research that went into the purchase.

There is a big difference between a product that contains silicone and a product that is pure silicone.  Auto zone has no idea about the particulars of the SOHC4 caliper, and it's needs are different than what your average auto caliper requires.
Even the Honda shop manual specifies silicone grease.  But, it is pure silicone, such as Dow Corning's High Vacuum Grease.  It will not weep, creep, bleed, or otherwise change phase under temperature or water contamination, to foul friction pads.
To be clear, the grease is not used for its lubrication properties.  It is used solely for its water shedding ability and stability when heated.  It prevents water contact against any of the metals that would otherwise be exposed to air and elements to keep it from corroding.  You only need a very thin coat, no thicker than paint, just to be a water barrier.  Pure silicone will not combine with water, so it sheds water, and stays put where placed, only a mechanical action will dislodge it. 

It specifically is NOT used on the caliper seal proper for the caliper of this design.  I don't care what auto zone cashiers say about suitability.  They think all calipers are the same.  And this one in particular, is not.  Check out the MSDS for the grease you bought.  If it is not pure silicone, or has any petroleum in it, it is certainly the wrong stuff.
In short, you don't care if the grease still lubricates at 500F.  You need to be certain that no part of it becomes a fluid at 500F and soaks into the friction pads, and it must not dilute with water.

I tested Dow Corning's stuff in comparison to Sil-Glyde, that was recommended by the parts store.  Dow's grease only appeared to glisten a bit more at 500F.  No other changes could be discerned.  Sil-Glyde puddled at about 300F, ten smoked and turned to tar at about 350-400F.  By 500F it was a brown smudge in the aluminum pan it was placed upon.

I had to toss my brand new brake pads, and wash my caliper components and rotor in solvent five or six times, after following the parts man's advice about Sil-Glyde.
I've just used the Dow Corning High Vacuum Grease ever since, as it is known to work properly with this caliper design.

[SohRon]

I'm not quite understanding the square/parallelogram seal concept. Are you taking about the rubber seal that is about halfway down the piston bowl?

The following pics will give you a general idea of how this works. These are a bit exaggerated to show the process, but the brake seal works generally as follows:

The seal holds the piston in position as shown




Applying the brake forces the piston to extend, causing the seal to distort



When the lever is released, the seal returns to its original shape and position, dragging the piston with it. This is what retracts the pad




There's a little more to it than this (not mentioning the spring on the caliper holder, etc), but it's a thumbnail expo on how it works.

Also, this pic shows the holes that must be cleared in the MC. The hole on the right (@.019") is the high pressure fluid return port and it's very important it remain clear.

[bryanj]

Sohron thats the best pictures I have seen for years , Any chance of the moderators posting that explanation in the FAQ's?

Only thing i would add is using the finger over the bajo hole of a rebuilt master to start the master pumping--I have explained that before but will write it out again if a mod requires.

[TwoTired]

Great info from TwoTired there, interesting info about the deformation of the seal. Does that mean that an o-ring x-section seal in that groove would not work for this system? (always been curious)

I don't think so.

[asbissonnette]

Ok, thanks everyone. As you can probably tell, this is my first time rebuilding a bike and am learning lots! About the silicone grease I used, it was actually right next to the SylGlide. I'll try and get a picture of the packaging.

One thing I haven't mentioned is that there is a lot of free play in the lever, it needs to be pushed halfway in before I feel any pressure. Could this be signs of a bad master cylinder?

Another possible culprit I came up with, haven't tested yet. So when I filled back up with fluid and did the bleeding, the piston was pretty extended out. I didn't think about this at time, but wouldn't it make sense to fill fluid and bleed with piston retracted all the way? This way the natural hydraulic fluid pressure would cause the piston to come back in all the way. I have read any specific directions for this but a theory of mine. Would it make sense to compress piston with C clamp and bleed out the brakes that way? I could be totally wrong, but thought I'd ask.

[TwoTired]

One thing I haven't mentioned is that there is a lot of free play in the lever, it needs to be pushed halfway in before I feel any pressure. Could this be signs of a bad master cylinder?

Could be.  Or, it just may have a lot of air to compress.

Pretty hard to diagnose hydraulic system mechanical problems when there is something other than hydraulic fluid in the system.

[asbissonnette]

Just wanted to update this thread with more of a conclusion for other people that encounter this problem. Honestly, I still don't know for sure the exact culprit. About the lube I used, I was incorrect when I said it was silicone based. The actual lube I used was "Permatex Ultra Disc Brake Caliper Lube ". It is a synthetic lube and is apparently safe for rubber parts.

To recap, the main problem I was having was that the front brake caliper was not retracting. It is about as fixed as I think I am going to be able to get it. I bought a master cylinder rebuild kit and a new OEM piston seal for the caliper. I cleaned all the parts of the whole system out pretty thoroughly, which included scarping any corrosion and buildup from the caliper and making sure lines were unclogged. When I took apart the master cylinder, the piston had a lot of yellow gunky build up around it that I could see was floating around in the lines - I definitely got all of this out.

I put a very light coating of the brake caliper lube on any rubber seals, bolts, springs etc. when reassembling. Before cleaning, the piston pretty much had to be forced down with a C-Clamp into the caliper. After cleaning, it basically dropped into the caliper when the seal wasn't present and I was able to push it all the way into the caliper with minimal effort after installing the new seal.

Before rebuilding the master cylinder, there was some free play in the lever. The cylinder was able to move fluid in/out of the reservoir and other than being a little dirty, the cylinder parts appeared to be in semi-decent working order. However, I felt a noticeable difference in the lever after installing the rebuild kit.

For bleeding, I initially forced some brake fluid through the bleeder valve using some tubing into an airtight, brake fluid filled jar, with an air compressor attached to it. After the fluid reached the reservoir (still had no pressure), I found that tapping the lines and straightening them out vertically, working the line in various angles, allowed the air to rise to the top. This was the most effective method for me. I think I got all the air out.

After reassembling and bleeding the brakes, there is way more pressure than I had ever felt before on the lever. And the caliper does retract now. I wish it was able to retract a little more as the pads still rub the caliper slightly, but not like before. I gave a quick adjustment to the caliper adjustment bolt per the Clymer manual, but haven't touched it since. So this might solve the problem, might not.

I guess the lesson to be learned is everything needs to be cleaner than I initially thought. There wasn't one obvious thing I overlooked or that was wrong. They were just old, dirty and needed some updated parts.

I won't go too in-depth with the troubles I was having getting that stupid snap ring out of the Master Cylinder. I basically C-clamped a flat head screwdriver to push down the cylinder piston, allowing better access to the ring. Then used a couple awls to meticulously squeeze, bend and pry it out. The snap ring pliers, the only ones that Lowe's sells, were too short and flimsy to grab the snap ring. Autozone had a beefier pair of SR pliers for 2$ more, but they still wouldn't have been long enough.