Topic: SOFT front brake - i'm outta ideas...

[Franky]

Hi guys

I have read a gazzillon theads about the subject, but I can't seem to nail this one.

I recently decided to do some servicing on my CB550 frontbrake as it wasn't retracting as it should and dragging quite a bit. I had descent grip and stopping power.
I, of course, snapped the bleed screw and had to drill it out, do new threads and put in a slightly larger one. It seals fine with no leaks.

Now I simply won't get a firm grip or actually any grip at all. The best I can do, is about a centimeter with some resistance but it's still no prob to squeeze it down to the handlebar.

Here's what I've done so far:

- Took caliper apart, some gunk, cleaned it up (especially the groove behind the seal). Piston had a few minor pits but looked fine. Put in new Honda seal.
- Took MC apart, not much gunk, cleaned it up. Piston, the seals, etc. didn't look bad but I rebuilt it anyway with Honda kit.
- Tried bleeding the system the manual way for literally hours, but could not get any grip. No visible bubbles in the fluid coming from the bleed nipple. Used same techniques as shown here: Motorcycle Brake Bleeding Procedure from Sportbiketrackgear.com
- Tried using a vacuum system. Ran about 400 ml of dot5 fluid through.
- Did some knocking with the back of a screwdriver on MC and caliper to loosen any trapped bubbles.

Other info:

- The small hole in the MC is clear and I get squirts flying high in the air without the lid on.
- The new MC piston has a different 'o-ring' as you can see in the image. The rest of the parts that we're in there and the Honda rebulid kit were identical. Tried using both pistons - no grip.
- Checked for leaks at varius banjo bolts and for cracks, bulges on the hoses - all seem fine. Braided steel on top, regular rubber on lower part.
- The caliper piston moves nice and freely, retracts beautifully.
- The stopping power is there, it works, but obviously very weak.

Hope you can point me in the right direction 

[05c50]

The rubber cup on the mc piston marked as "new" in the picture, appears to be smaller then the old one. Maybe it just looks that way, but could it not be sealing correctly? I would try to separate the mc from the rest of the system (plug the hole where the line attaches) and see if the mc holds pressure.

...............Paul

[Kong]

That flat faced seal that goes in front of the piston, are you sure its not in backwards?  I've seen it happen more than once.  The flat face contacts the piston and the cupped face faces the return spring.

[Rio_CB750]

I know that for my 750, I had to do a "bench bleed" after my rebuild, no manner of normal bleeding would get the brakes right.

If photobucket worked, I could show you the pics, but basically, put a first length of brake hose on and dip the end into your reservoir and then pump. You'll see bubbles coming out, when that is clear, start putting it together down to the caliper making sure that you don't drip that crap on anything!  Maybe that will do it, good luck brother!

[Magpie]

Try tieing the brake lever back to the handle bar and leave it over night. If there's any air in the fluid it will make it's way up to the master and escape through the small hole as it's now open because the lever is back. It's worked for me several times.
Cliff.

[Kevin400F]

Slight mod to Magpie's comment......

Tying the lever back (assuming you were able to build pressure in the system) does help air migrate up to the master cylinder to be "burped" out of the small hole, but the hole is not open until the lever is released fully.  (When the lever is tied back, the seal cup inside the master has moved past the small hole.) 

Building pressure and tying the lever back squeezes any trapped air bubbles down to a fraction of their original size, which lets the bubbles sneak through fittings more easily.  After it sits overnight, turn the bars so the right side handlebar is pointing slightly upwards, slowly release the lever, and with the MC cap off, lightly cycle the brake lever a few millimeters, while watching for any air to be burped out of the small hole.

All this being said, the fact that one of the cups in the MC rebuild kit didn't match the original is a bad sign.  If the other seal cup (closest to the brake hose connection) is also "short" like the one in the picture, you will have excessive lever travel before you build pressure, and no amount of bleeding will change things. 

[Kong]

If the brake system has been put back together properly and none of the parts are bad then you won't have to go through any of those rituals.  There is a problem somewhere, it needs to be found and fixed.

Open up the bleeder at the caliper.  Put a length of clear plastic hose over it, one that fits tight.  Put the other end of the hose in a quart jar that has about an inch of brake fluid in its bottom.  Make sure the end of the tube stays submerged all the time.  Now fill your master cylinder and start pumping.  Do not let the master cylinder drop to less than 1/3 full, replenish the fluid as you pump it out.  It should immediately start pumping fluid through that tube.  Don't close the line when you let off the brake handel because if the pressure drops it can't suck any air into the system because the end of the tube is submerged.  Anyway keep on pumping until you've pumped about a half a quart of brand new brake fluid through the system.  Then close the bleeder and take the hose off.  There will be no air in the system after you have done that.  If it still doesn't work and there is no fluid leaking out of the system anywhere you may safely assume that your master cylinder is for crap and you might want to find another one.  Remember, a rebuild kit won't do squat if the bore of the master cylinder was pitted more than you were able to hone out - you did hone the master cylinder before you put new seals in it, didn't you?

[tlbranth]

I have a Goldwing which makes my 750 look pretty simple. On the Goldwing forum, brake bleeding is a popular topic because is virtually impossible to do on a Goldwing. BUT, the consensus there is that air likes to hang out in banjo bolts so successful bleeding includes bleeding the bleeding banjo bolts.
When I rebuilt my 750, I couldn't get any fluid at all through the MC. Once I finally got something moving though, the thing bled easily with standard bleeding technique.

[haill]

did you crack open the banjo bolt at the master cylinder after pumping the brake lever with the lever held to the bar?

[camelman]

I usually tap on the various pieces with the handle end of a screwdriver.  It helps dislodge bubbles.  I've also resorted to unbolting things and moving them around, such as the master cylinder and the stop switch.

Camelman

[HondaMan]

- The small hole in the MC is clear and I get squirts flying high in the air without the lid on.

That indicates air in the line somewhere.
When bleeding, here's a tip that helps: turn the handlebars to the side where the master cylinder is level first. Also, make sure the brake lines go ONLY DOWN from the master to the caliper. Sometimes a PO has installed a longer-than-stock upper brake hose and it snakes around, trapping bubbles...make sure the whole path between the caliper and master is UP. Then pull in the lever about 1" (as measured at the ball end) with a piece of wire, install a 3' long piece of hose on the caliper bleed bolt and crack it open, fill the master, and go have a cup of [tea, coffee, Jim Beam] and come back. Close the caliper bleed and check it (watch out for that squirt, brake fluid eats paint!) for grip. You may find that short strokes of about 1" on the lever now will let the tiny bubbles finish out.

[Franky]

Thanks a lot for the many replys. I really appreciate it!

@05c50, Paul: You're right. The new one looks smaller in the picture, but they are identical except for the positioning of the rubber in the middle. The idea of separating the MC makes sense. I just hope it ain't busted...

@Kong: I'm sure it does! I had both the Haynes and Shop Manual out when reassembling, so even a moron like myself would be hard pressed to #$%* it up :-) About the honing of the MC cylinder: I did clean it out. Not much gunk and no scratches or the like. I am worried that the MC could be crap though...

@haill: I did this to every banjobolt down the line!

@Hondaman: The upper part of the line is in fact quite long and winding - i'll look into your advice (and probably have the Jim Bean to calm down)...

[MADCB400F]

Mornin, looking at your PIC of the plungers the new shows you have the rubber seal still rolled inside out and not seated correctly in comparison to the old. Fix the seal before you do anythin else.

[Franky]

@MADCB400F

I'm sure I put the new rubber on the right way. As I said, the rubber itself and the positioning of it in the  Honda rebuild kit is slightly different to the existing one. The bump or groove on the MC piston that holds the rubber is different too. So, I guess my MC is not original or Honda made the slight alteration because it works better/easier to produce...?
In any case, the stopping power was there before and I tried both the old and the new MC piston afterwards with the same negative result.

Today I have time to mess with it again - hoping some of your suggestions will work. Thanks again guys. 

[fishhead]

Measure the diameter of the new and old pistons. Pictures may be deceiving, bit they do look like different sizes.

   I don't know everything, but the Honda master cylinders (US models) that I have seen with the plastic reservoir pictured were for the 75-77 GL 1000 Goldwing (they usually have a larger reservoir for the front, but the rear reservoir is smaller and fits on the front and look like what you have). The 75-77 GL master cylinders are 11/16 inch in diameter.

[Franky]

Just a quick update to let you know, that the MC is not busted.

I have a CB360 with good brakes so I tried swapping MC's. Result: the CB360's brake was still nice and firm, whereas the CB550 was as soft as ever - even with the 'new' CB360 MC installed.

[socalenduro]

when i replaced my lines i could pump and pump and pump....and it never firmed up... literally an entire bottles worth of fluid....
after taping every centimeter of the line to make sure there were no bubble i gave up and went to bed. came back the next day and re bled the system. i guess overnight all the bubbles made their way up...
have you let the system sit with fluid in it?

[Roach]

when i replaced my lines i could pump and pump and pump....and it never firmed up... literally an entire bottles worth of fluid....
after taping every centimeter of the line to make sure there were no bubble i gave up and went to bed. came back the next day and re bled the system. i guess overnight all the bubbles made their way up...
have you let the system sit with fluid in it?

+1 on that same thing happened to me with my 550 had to waited over night because i got frustrated cam back next day and started pumping again and it all worked out.

[ryani]

Another trick I have used on my dirt bike front brake is to bleed it and then if it's a bit spongy wire or tape lever back towards the handlebar and let it sit overnight.  It opens up the passage in the M/C to let the air bubbles escape.

[HondaMan]

Just a quick update to let you know, that the MC is not busted.

I have a CB360 with good brakes so I tried swapping MC's. Result: the CB360's brake was still nice and firm, whereas the CB550 was as soft as ever - even with the 'new' CB360 MC installed.

Then, it may be that the O-ring on the piston in the front caliper is not sealing like it should, and there may be a sticky brake puck there as well. If you just replaced the pads, make sure the paint is sanded off the edges of those pads (or the rust, if old) so they will move freely in and out.

[socalenduro]

it most likely air or a fast leak if you are not getting any build up in the lever...
if your pads were stuck the lever would still feel firm the front wheel just wouldnt roll.
and for it to not build up any resistance in the lever fluid would REALLY have to be leaking

[Franky]

PROGRESS!

1.
I did the sitting overnight with lever tied down to the handlebar trick and that helped quite a bit. Both instantly and when I'd bleed the thing I'd get tiny bubbles thru the bleed nipple. Did it a couple of times so far.

2.
Something must be bent in the fork or the caliper assembly. Even before Rotortiller suggested it (thanks a lot though  I saw that the brake pads wouldn't meet the rotor at a right angle. I added a washer on the lower part of the caliper-arm where one bolt attaches it to the front fork. That helped a lot! A much larger area of contact means much better braking.

3.
'Gaither' PM'ed me with his experience with DOT5 fluid. Slow lever action when bleeding - regular fast pumping will build tiny bubbles in the fluid

All in all I now have OK braking, but there's still some way to go I think. Maybe I'll try to get a hold of a giant hypo and try that out---

[FunJimmy]

When I purchased my first CB550, the PO had stripper the bleed nipple so badly that I knew it wouldn’t come out without snapping off in the caliper, but the brakes needed bleeding so I removed the caliper and pumped the lever until the piston was just about to fall out. Then I emptied the master cylinder reservoir and refilled it with fresh fluid before I pushed the caliper piston all the way back in again. You wouldn’t believe how fast the air was purged from the lines. I repeated this procedure a couple more times before remounting the caliper and the system worked perfectly.

It’s very similar to forcing the brake fluid through the bleeder up to the master cylinder, but you don’t need a hypo and there is no fluid spills to contend with.

[Roach]

When I purchased my first CB550, the PO had stripper the bleed nipple so badly that I knew it wouldn’t come out without snapping off in the caliper, but the brakes needed bleeding so I removed the caliper and pumped the lever until the piston was just about to fall out. Then I emptied the master cylinder reservoir and refilled it with fresh fluid before I pushed the caliper piston all the way back in again. You wouldn’t believe how fast the air was purged from the lines. I repeated this procedure a couple more times before remounting the caliper and the system worked perfectly.

It’s very similar to forcing the brake fluid through the bleeder up to the master cylinder, but you don’t need a hypo and there is no fluid spills to contend with.

wow that's really good info I will have to give that ago.  makes allot of sense.

[flybox1]

Hot air rises! 
i had a similar experience, and here's what i did. 
i tactically acquired my wifes hairdryer and w/ brake lever tied to handlebar and bars turned to the left,  i gently heated every component of my system.  paying closer attention to any transition, especially the brake light switch and banjo intersections.  i did this every 30 minutes for about 2 hours while i worked on other stuff on my bike. i was pretty surprised at the bubbles still coming out of the MC when i finally pumped it.