Thursday, November 11, 2010

Bleeding Motorcycle Brakes or Clutch

I don't update this blog much anymore because I'm occupied with motorcycle maintenance, riding and reading about motorcycles. I tend to be a bit obsessive when I'm interested in something ;-)

Anyway here's a bit about motorcycle brakes and clutches that I learned from reading and some hands-on -

Hydraulic brake systems operate using a master cylinder and a slave cylinder. The master cylinder contains a piston which moves when you apply the brakes and applies pressure to the brake fluid in the brake lines. This pressure causes a piston in a slave cylinder on the brake calipers to move and compress the brake pads on the brake disc.

Hydraulic clutches work using the same basic principle using a master cylinder and a slave cylinder that extends to move a rod when you pull in the clutch which disengages the clutch. The components are spring loaded to return to the relaxed state when you release the clutch/brake.

HowStuffWorks "A Hydraulic System" shows the concept better than a lot of words.

There are a few key characteristics of the fluid that are of interest for someone doing motorcycle maintenance -

1. It should be incompressible ie. no air bubbles.
2. It shouldn't contain a lot of water since water boils at a lower temperature then brake fluid and tends to corrode metal
3. It shouldn't contain gunk - e.g. dirt that dropped into the master cylinder when you remove the cap to look at the fluid or small particles that flake off the brake lines/components as they age.

Since hydraulic brake systems are essentially sealed it's debatable whether you ever need to change the brake fluid. For example - a 2005 Harley Davidson Sportster has no recommended maintenance interval for changing brake fluid. In other words the manufacturer doesn't consider changing brake fluid to be routine maintenance - just leave it alone - like most of us do with the brake fluid in our car braking systems. For a 2000/2001 Kawasaki KZ-P/Concours the manufacturer recommends changing the brake/clutch fluid every two years.

Part of the reason for Harley being maintenance-free vs. Kawasaki every 2 years is the type of brake fluid used by Harley vs. Kawasaki. Harley uses a silicone based Dot 5 fluid which does not absorb water Kawasaki uses a glycol based Dot 3 or 4 fluid which will absorb water.

There are some other differences between Dot 5 and Dot 3 or 4.

Dot 5 has a higher boiling point than Dot 3 or 4, and will not strip off paint - but has a tendency to aerate and retain air bubbles which will make your brakes a little (or a lot) spongy depending on how much air is in the brake lines. There's a little bubble or two in my Sportster master cylinder right now and that brake system has never been opened/disturbed. Dot 3 or 4 brake fluid will strip paint but has less tendency towards aeration (ie. you can shake a bottle and it won't get all bubbly/foamy). Dot 5 costs two or three times what Dot 3 or 4 costs - not a real big deal 4 bucks vs. 12 bucks.

Dot 3 or 4 fluid starts out clear and turns black/brown as it absorbs moisture. You may wonder how moisture gets into a sealed system? The answer is that it isn't quite sealed. There's a vent near the top of the master cylinder to allow air to enter. The reason for this is that as the brake pads wear the fluid level in the master cylinder will go down since the pistons in the calipers have extended more. The vent allow the pressure to equalize when the system is at rest. It also allows air, which depending on the humidity contains some water, to enter the top of the system. Glycol based fluids absorb the water from the air.

Dot 5 fluid may start out purple and turn yellow - I don't know for sure. Currently my HD Sportster Dot 5 fluid is yellow. If I'm bored sometime I might bleed and replace it and then I'll know for sure if it changes color. The maintenance-free philosophy on the brake fluid reminds me that bike has some very nice characteristics if you like to just ride - change the oil and oil filter, primary oil, air filter spark plugs and brake pads once in awhile. All very easy. The downside it doesn't go as fast or stop as fast as a bike with more involved scheduled maintenance...but I digress - back to brakes.

If you decide to add brake fluid to the full mark on the master cylinder after your brake pads have worn a bit you will need to siphon it back out before you put new pads in - otherwise when you push the pistons back into the caliper you'll end up with brake fluid dripping/shooting our of the master cylinder (don't forget Dot 3 and 4 brake fluid is not good for paint or your eyeballs...Dot 5 probably shouldn't be used for eye drops either).

I've bled and replaced the brake fluid on a couple of Kaw's in the last weeks, as well as the clutch brake fluid on one of them - so I've gotten the bleeding procedure down pretty good. These motorcycles have dual caliper front brakes, a front master cylinder, single caliper rear brakes and a rear master cylinder (so 3 bleed points per bike). The Concours has a hydraulic clutch - so one more master cylinder and a slave cylinder with a bleed point.

Here's the basics - you don't need speed bleeders or a mity vac or some fancy shop tools to do this. You need a phillips screwdriver, 10 mm box wrench, piece of 3/8 inch clear plastic tubing, a bottle to drain the old fluid into, some rags or paper towels, a bottle of brake fluid and a sacrificial turkey baster, unless you don't mind a turkey that smells/tastes slightly like brake fluid - in which case you will use your everyday kitchen turkey baster.

Remove the cover from the master cylinder - if it's covered in dirt or the area around it is dirty clean it up before you remove the cover. The tolerances of the cylinder bore and piston are critical so you don't want dirt in there scratching things up.

Suck out the old gunky fluid with the turkey baster. Clean any sediment/gunk out of the master cylinder with a clean cloth/paper towel.

Fill the master cylinder with clean fluid. This way you are flushing new fluid through the system instead of the old stuff.

Put your 10 mm box wrench on bleed nipple for the clutch/brake.

Stick the 3/8 inch plastic tubing over the nipple making a nice tight seal

Stick the other end of the plastic tubing into your jar that has a little brake fluid in it so the bottom of the tube is submerged in fluid. You are doing this to prevent air from entering the system. I'm not so sure about this...but it's a tradition so I do it anyway.

Position the tubing so the bend is higher than the bleed nipple. This way there's a column of brake fluid that doesn't siphon into the jar and is free to return to the system (instead of air which is bad - in this case).

Here's the main sequence of what you are doing -

1. Pull and hold the brake lever.

2. Crack open the bleed nipple.

3. Pull the brake lever to it's limit and hold it there.

4. Close the bleed nipple.

5. Release the brake lever.

Repeat steps 1-5 until you see clean fluid coming out of the plastic tubing. Make sure you top off the brake fluid in the master cylinder while your doing this so you don't suck air in from the top side. Same for holding the brake until you close the bleed nipple - if you get out of sequence and release the brake before you close the nipple you'll suck in air from the bottom end.

It's not absolutely necessary since there's no spurting action like when you push the calipers back in after replacing pads - but you should probably put the master cylinder cover back on loosely while you are bleeding the brakes to help avoid spilling fluid on painted surfaces. You'll want some paper towels/rags/water to wipe up any spills that do happen.

It's easy to do this on a motorcycle. A car is harder with the four wheels plus you need an assistant. I'm guessing a motorcycle with ABS would be harder too.

A bottle of brake fluid is several dollars and the plastic tubing was 20 cents at Lowes so this is cheap maintenance. Clutch and brake master cylinders, slave cylinders, brake calipers etc. are not so cheap. Having clean fluid with no water or crud in it may help those components last a long time as well as ensuring your brakes are operating at their maximum potential.

One final note - if you end up getting spongy brakes it's because you didn't follow the simple procedure of keeping the brake master cylinder full, holding the brake, cracking the bleed nipple, closing the bleed nipple and then releasing the brake.

You can buy some speed bleeders (one-way valves) that replace the regular bleed nipple, special tools to suck fluid out the top or bottom - but I'd recommend just do it the simple way, it's cheap and it works. Shops use special tools when replacing brake fluid because it's faster - not because the special tools do a better job. Considering the fact you will be replacing this fluid every couple of years you'll have to decide for yourself if taking 15 minutes doing it the old fashioned way is worth your time or if you want to cut that down to 5 minutes using special purpose tools - which may end up taking a lot longer than 5 minutes because you are messing around with a vacuum pump, removing the bleed nipples etc.

Have fun - happy riding and may all your stops be safe and enjoyable.