By: Ron Milam
Last month as you may recall we began a discussion on brakes. Since that article was published, several people have mentioned brake problems that they are having. I think that next to clutches, brakes are the system that cause riders the most headaches.I believe we left off on the subject of sticking pistons.
If your pads are in good shape and your pistons are sticking, you may need to clean the seals. What I have found is that debris can build up in the seal grooves of the calipers and cause the seal to grip the piston too tightly. The first time I took a caliper apart I was scared to death the seals would get damaged and have to be replaced. Replacing them would be easy. Waiting a week for the parts was the worry. Happily, I found it to be no big deal. In fact I think everyone should disassemble their calipers yearly and clean them and change fluid. You will find the caliper to be made in 2 pieces, with a small o-ring sealing the 2 halves. The pistons will likely be very difficult to remove. The answer here is to blow them out with compressed air. Did I say CAREFULLY blow them out with compressed air? This is another one of those areas when I must say that this is a fairly dangerous thing to do if caution is not used. It is entirely possible to inadvertantly launch a projectile at a velocity that could kill,maim or really get you in a lot of trouble. Just be sure to point the piston away from yourself or anyone else and into a rag or other soft item to avoid damage to the aluminum piston.
You can use a small screwdriver or a packing puller to remove the seals. If you are careful, the seals can be reused. If you have had the forsight to order new seals, replacement would probably be better. After removing the seals you can clean out the grooves in the caliper housings. You should next reinstall the seals. My next step is somewhat unusual and really hard to do unless you have Houdini hands. I fill both
caliper halves with fluid, put the pistons barely in and lay the o-ring in one half. Then, at a high rate of speed, bring the two halves together without making a huge mess and leting air into the system.This will require great manual dexterity to keep the ports covered with your fingers and to prevent the o-ring from falling out of position. You can then bolt the parts up tight. The reason you left the pistons hanging out is that you can now push them in some to push out any air that may have been trapped inside.
I have two theorems concerning hydraulic brake systems. Theorem 1- The less air you trap in a system, the less you will have to bleed out later. This means it is a good idea to fill all of your hoses and other parts. As you connect the hose to the cylinder, push the pistons in some more to expell any air in the port and fill the end of the hose. You’re going to trap some air. Your goal should be to keep it at a minimum. The next theorem ( lets call it Theorem 2 ) states that bubbles go up easier than they go down. If you don’t believe this just try pushing some bubbles to the bottom of a swimming pool. This sounds like the kind of thing only nut would try. Well then, why do most people bleed their brakes from the top down? The best way to bleed brakes ( by the way this whole column is simply my humble opinion ) is from the lowest point, which is normally the calipers to the highest. I have always used a large medical syringe to push fluid into the caliper. You should first remove the pads (if they are still in) to avoid contaminating them. If you get fluid on your pads they will probably never work right again. You should push the pistons all the way in and insert some object to hold them there. The less internal volume there is , the less place there will be for those pesky evil bubbles to hide. On some bikes the bleed fitting screws down onto a steel ball sitting over the passage way. The function of this ball is to drive you nuts if you don’t know it is there. You can take it out to bleed the system and put it back in afterwards. Since this open port is lower than the master cylinder, it will be under a positive pressure, allowing you to insert the ball and reinstall the bleed fitting without allowing any air to enter. If you have a large syringe , say 5 ounces, one shot should be sufficient to completely evacuate the system. You obviously have to put something under the mastercylinder and have its top off, since 5 ounces of fluid will overflow.
There are a couple of tricks to using the syringe. First it is transparent. You should take advantage of that fact by making sure all air is out. Don’t get bubbles in while you are filling it. If you shake or bump the fluid container, you may entrain air. Any bubbles you put in the syringe must be expelled.You should also use transparent tubing to connect the bleed fitting to the syringe. Again, this allows you to check for bubbles.
Next month, we will cover master cylinders and fluid and other interesting stuff like that. If anyone has good ideas or techniques they would like to share about brakes or anything else, please tell me about it and I will get it in next month.
Ron