Bleedin' brakes!

[oldrookie]

Think I found the problem.

You were correct about taking the lever off. Spotted a hole in the new rubber outer boot that I just installed.

I HOPE somebody sells just the rubber boot; I really don't want to spend another 50 bucks just to get the boot.

[scott]

Think I found the problem.

You were correct about taking the lever off. Spotted a hole in the new rubber outer boot that I just installed.

I HOPE somebody sells just the rubber boot; I really don't want to spend another 50 bucks just to get the boot.

Does that outer boot actually function as a seal or is it just there to keep dust/debris out?

[oldrookie]

I thought it was just a dust seal, but it would appear that it has larger function. Hole in it resulted in a slow leak with brake fluid dripping off the bottom of the MC.

It is either that...or I have an issue with how the rest of the system went together. I think I got it right, but will be rechecking when the new parts come in.

[Steve]

1. Did the squeeze, open, close release dance--no joy
Any thoughts as to what I ought to do next?

I see you pretty much have it resolved, but thought I would add something.
I have always seen the "dance" done with these steps: Open, squeeze, close, release.
Have a hose on the bleeder nipple and have it end in a bit of fluid in a jar (baby food jar works well).

With the top off the MC the second hole shoots fluid into the air quite nicely.

That should only be a momentary squirt, not a continuous stream. The piston should be sitting just clear of that hole. As soon as the piston starts moving, the hole should be covered. You will not build ANY pressure in the brake lines until that hole is covered.


.

[oldrookie]

Thanks, Steve.

Appreciate the help.

[TooManyToys]

Not sure all this is necessary, but I'm bored.

M/C designs have some innovations over the years, but a basic M/C is a basic M/C still today. Our bikes use single chamber M/Cs. Finding a picture or diagram of a single chamber M/C is getting tough. This is a good representation of one, close enough to our M/Cs for my discussion.



The piston assembly is the most complicated of the parts. At the back end we have a spring, which acts as a return spring as well as in some cases a method of applying light pressure to the pistons primary cups so the edges of the cups are against the M/C bore at all times, eliminating a delay in their sealing which would require longer travel. The design of the primary cup is such that it seals tighter with the more pressure that is being exerted. Good thing.

If we had a “closed” hydraulic system we would have to use more travel to compensate for brake pad wear which extends the brake pistons farther outward. The way we make up for this wear and caliper extra volume is by the use of a compensating port, a small hole just in front of the resting point of the primary seal. The port allows more brake fluid to enter between the M/C and calipers to compensate for the larger caliper volume.

When the piston is starting to pressurize the brake fluid by moving forward some pressurized fluid will be pushed out of the compensating port and back into the reservoir creating a small geyser until the primary cup goes past the port. Once the cup is past the port, full pressure can be applied to the calipers in this now closed system.

The piston assembly is quite long and has a secondary cup at the other end where a lever or pushrod resides. This is the neutral chamber and is necessary as once the primary seal/piston end goes past the compensating port, fluid would just escape the reservoir and dribble out if the piston was only long enough to be behind the primary cup. Some caliper designs like the one shown have two ports where the reservoir is attached to the M/C body. This is done to make sure the neutral area of the long piston has a pressure relief when the primary piston is fully back and the neutral chamber no longer has access to the compensating port. It should be noted that the design of the secondary or sealing cup is a low pressure design, never intended to hold back full operating hydraulic pressure that is achieved by the primary seal/cup.

At the back of this M/C example is a flange that holds a dust boot, often accordion in shape. This is only to prevent dirt and moisture from accumulating at the back of the M/C piston/bore area. Some of these are fully enclosed, while other designers have a small hole to allow some airflow. These boots are never designed to hold back brake fluid.

If you have a non-sealing primary cup, brake fluid will blow past the seal and enter into the neutral area. This results in the neutral chamber of the piston assembly having pressurized fluid, and the compensating port develops an outflow during the entire piston stroke. Sometimes fluid will also come out past the secondary seal, but that usually only happens if the small compensating port can’t handle the volume of fluid rushing past the primary seal.

You can also have a weak, partially compromised primary cup. This often will result in a continuous moving lever or pedal under low pressure applications, while a hard pull or press will result in a solid holding situation. Remember that the primary cup is designed to seal tighter with higher pressures.

If you don’t have a long geyser coming out of the compensating port during application, just the initial squirt, but have fluid dribbling out of the secondary seal or dust boot then the secondary seal is compromised. You still can have good brakes, but you will lose fluid continuously.

The rubber parts of brake systems on these bikes are designed around glycol fluids, so any contact with petroleum products should be avoided. Brake cleaning fluid is fine and so is alcohol. When assembling M/Cs and calipers you can use brake fluid or brake assembly fluid for lubrication, as you never want to put everything together dry. Assembled dry raises the chances of nicking one of the seals exponentially.

The biggest detriment in rebuilding M/Cs is corrosion in the bore due to moisture entrained, poorly maintained brake fluid. Seals can last a long time. The proper way of dealing with oxides in the bore is to hone the bore out. You are limited to how much larger the bore can be and still maintain a good cup seal. And you also have the issue of how deep the pitting is in the bore. A honed bore that still has pits will wear away at the cups and seals so it’s just a matter of time (a few strokes or a hundred strokes) until the system has lost integrity again.

The end point is that looking at the compensating port output and fluid out of the back of a M/C tells you a lot about the M/C condition and understanding what all the parts do can save you a lot of grief.

Edit - This is an example of the assembly lube I mentioned, it is not brake slide pin silicone grease which is too thick for the assembly use. This is one brand but I've gotten it at auto parts stores such as NAPA. One bottle will probably last you for life.

http://www.jegs.com/i/Wilwood/950/290-11087/10002/-1?parentProductId=1402309

[Steve]

Not sure all this is necessary, but I'm bored.

I like it when you are bored.

Excellent write-up.

.

[oldrookie]

Me too.

Good info.

[oldrookie]

Got the new MC parts this afternoon and tore into the thing.

I think, after careful review of the piston and cup set, that the seal seated directly under the washer was installed upside down.

It appears that the rubber washer was made to sit down in the rim of the piece under it in order to form a seal against the underside of the washer.

Of course, I could be wrong. Happens frequently when I am working on something mechanical. The ridge on the rubber bit drops into the cupped part under it and seems content to sit there.

When I had it installed the other way one side of the rubber washer slid down inside and I think that caused the slow leak. (I hope)

I also used Q-tips to push the outer rubber boot into place to avoid another unfortunate puncture.

Worked on trying to bleed the brakes again, but had less success than I might have hoped. Tied the brake lever back and left it to sit overnight.

[JimmyR]

I had similar problems when I overhauled my front brakes a few months ago.I had completely rebuilt them - blasted and powdercoated the calipers, all new seals and boots, new stainless lines and fittings, new MC. Just couldn't get any pressure.

Turns out that when I reassembled the calipers I had twisted the bolts that join the caliper halves together so that they would register properly. Those bolts have flattened sides that register with the caliper so that they don't spin. Thing is, once you put the boots on them you can't see how they are oriented. Once we set them snug against the caliper body my brakes tightened up well. Not as good as the brakes on my other bike but they still work well enough to stop me when needed!

[oldrookie]

I am becoming remarkably good at taking the master cylinder off, disassembling-reassembling and reinstalling the little bugger.

Now if only I could do it right for a change. Have the lever tied back now and am hoping that will help with the bleedimg process tomorrow.

I no longer live in fear of that snap ring, so progress is being made.

[TooManyToys]

I've worked for 30 years in research testing of vehicle brake systems. I don't understand how the belief of keeping the brake applied overnight does anything.

The little air molecules will not be beaten into submission of staying smaller molecules once the pressure is released.

[tkent02]

The little bubbles will eventually float up hill to the upper end of the brake lines, they end up right by the master cylinder. That way when you release the lever, they go through the return port into the reservoir, instead of staying in the lines.

Sometimes it takes several times, but the brakes will get a lot more firm.

It's not like a car where the lines are mostly horizontal.

Try it, it works.

[oldrookie]

It appears to have done so. Brake lever travels smoothly through about the first third of its travel and then firms up quickly.

Not as solid as the new bikes, but I may be close to done. I've thought so before and been wrong, but this time it feels more firm.

I don't see any leaks from the piston bore, so it could be that I've finally got the seals back in correctly.

[exdirtbiker]

I read the whole thing here, and maybe I missed it, but you did try a vacuun bleeder that runs off an air compressor???
not a hand pump version... you may need the contious "pull" of a vacuum bleeder to make the air bubbles dislodge from where ever they are hiding.
I rebuilt, then replaced a Honda V45 hydrolic clutch m/c due to your same symptoms, then replaced the slave, and still no workee. ( total almost $350) Then someone told me "the air bubbles stick in the banjo bolt, and will never leave without a vacuum bleeding."
I was riding the bike 5 minutes after I bought the vacuum bleeder. And then realized I probably wasted $320... :-(

pS... don't buy the $20 Harbor Freight one, buy the $35 "name brand" model.