I think I figured out the extra brake lever travel

[IG.]

A while back I was complaining about this condition which I consistently notice as the brake pads get thinner - the lever needs to travel more before you feel it starts applying the pressure. I think that's what's happening.

As the pads wear out and get thinner, the pistons are being pushed further out taking along more brake fluid from the reservoir, and the vacuum inside the reservoir gets stronger and stronger. When the brake lever is released, that extra vacuum pulls the pistons slightly back in, which in turn creates an extra space between a piston and its respective pad. So, next time the lever is squeezed, there is a certain distance it needs to travel until the pistons get close to the pads again - which of course is waste of time and effort.

To combat this condition, all that needs to be done is to open the reservoir for a moment to get rid of the vacuum. Additionally, during the transition from warm weather to cold, the vacuum in the reservoir gets stronger - because the air temperature inside drops. This only makes it worse.

A good visual indication of the fact that there was vacuum in the reservoir once you open the cover would be the rubber membrane pulled down instead of being flat.

 

[Tufbusa]

There is no vaccum in the reservoir. It's vented to allow the rubber boot to expand and contract with heat as well as fluid added or expelled when applying and releasing the brake.

You are however correct in the fact that as your pads wear it takes more lever travel to apply pressure to the caliper.

 

[IG.]

I am confused now. The reservoir cover seems to seat tight over the rubber membrane, and I didn't notice any openings letting the air through the cover... unless the screw holes themselves are designed to let air in, so in this case if something seals the holes, the vacuum can still build up. Or maybe the gasket itself has small air passages, and again if those get something in there, then the vacuum will build up?

Hmm.. if it's not vacuum related, then why more lever travel is required as the pads wear off?


Oh, yeah check out my funny story and a question in Maintenance section.

 

[Twobrothersbusa]

The inner membrane will have a pin hole in the top of the membrane (usually in the center) to allow venting. For Gen IIs the brake reservoir top has a small vent notched into the cap on the right side. I don't know where it is on the cap for Gen Is, but it is vented.

Here is some info comparing Dot3/4 to Dot 5. It is a good read.
D.O.T. 5 Brake Fluid

If you are getting spongy brakes you probably have some air sneaking around in your brake lines. This can come from overheating the fluid or excessive moisture in the system from old fluid. Read of the article, it will help.

 

[kromdom]

then why more lever travel is required as the pads wear off?

As pad wears off, piston needs to travel more due to less pad material to push into. Consequently, more fluid needed to push pistons which in turn demands greater lever pressure.
P.S. Above is at least how I understand it

 

[IG.]

As pad wears off, piston needs to travel more due to less pad material to push into. Consequently, more fluid needed to push pistons which in turn demands greater lever pressure.
P.S. Above is at least how I understand it

Disagree with this. After the brake lever is released, the pistons are staying the same microscopic distance from the pad regardless of the pad wear. Since the bore is constant, it takes the same amount of fluid to push the pistons over the same distance.

In your explanation, there seems to be an assumption that the pistons go back to a predetermined position regardless of the pad wear. Then, you statement would be true, but then you would need more than one stroke of the lever just to move the pistons enough to apply any pressure.

 

[kromdom]

Hey, I never claimed I was right. Hopefully someone with the smarts can break it down to your satisfaction. Good enough for me that I squeeze the lever and the bike stops.

 

[IG.]

The inner membrane will have a pin hole in the top of the membrane (usually in the center) to allow venting. For Gen IIs the brake reservoir top has a small vent notched into the cap on the right side. I don't know where it is on the cap for Gen Is, but it is vented.

Here is some info comparing Dot3/4 to Dot 5. It is a good read.
D.O.T. 5 Brake Fluid

If you are getting spongy brakes you probably have some air sneaking around in your brake lines. This can come from overheating the fluid or excessive moisture in the system from old fluid. Read of the article, it will help.

Thanks for the good read. Wow!

Now as to the small hole in the center of the membrane. So, if this hole is clogged with something, then there will be vacuum build up. Matter of fact, depending on whether the hole is clogged on the botton or on top, it can act as a one way valve, thus either promoting internal pressure build up or a vacuum build up. And if it's clogged, not letting air in both direction, then it can build up pressure on a hot day and agressive braking, and will build up vacuum as the pads wear and/or on a cold day.

The feel of the lever is not spongy at all. No complaints about that. As I slowly squeeze the lever, the resistance comes up suddenly and stays hard.

 

[Tufbusa]

Thanks for the good read. Wow!

Now as to the small hole in the center of the membrane. So, if this hole is clogged with something, then there will be vacuum build up. Matter of fact, depending on whether the hole is clogged on the botton or on top, it can act as a one way valve, thus either promoting internal pressure build up or a vacuum build up. And if it's clogged, not letting air in both direction, then it can build up pressure on a hot day and agressive braking, and will build up vacuum as the pads wear and/or on a cold day.

The feel of the lever is not spongy at all. No complaints about that. As I slowly squeeze the lever, the resistance comes up suddenly and stays hard.

There is no hole in the membrane (Rubber boot) If by chance you plug the vent in the cover your brakes will lock up and dump you on your head. Just ask Brendan P how that works!

 

[Twobrothersbusa]

Thread jacking in process....How does glycol based brake fluid degrade based on moisture content over time if the system is not vented to air? Just asking, because that is my understanding. Dang google is failing me all over on this one.

There is no hole in the membrane (Rubber boot) If by chance you plug the vent in the cover your brakes will lock up and dump you on your head. Just ask Brendan P how that works!

Resume normal thread content here:

 

[kromdom]

Thread jacking in process....How does glycol based brake fluid degrade based on moisture content over time if the system is not vented to air? Just asking, because that is my understanding. Dang google is failing me all over on this one.

not sure if this will help: MOTORCYCLE BRAKE INFORMATION

 

[Twobrothersbusa]

Discuss the fluids ability to absorb water over time, not how the moisture enters the system. Thanks, still looking.

 

[Tufbusa]

Thread jacking in process....How does glycol based brake fluid degrade based on moisture content over time if the system is not vented to air? Just asking, because that is my understanding. Dang google is failing me all over on this one.



Resume normal thread content here:

Thats an easy one.

You have air trapped under the boot. Look in your window and you'll see the fluid level mid window with air above.

Moisture (Condensation)

 

[VFRcanada]

There is no pin hole in the rubber membrane. The brake system is a sealed system. As the brake pads wear, the pistons remain farther out from the calipers. The brake fluid fills the space the pistons leave in the calipers and the fluid level goes down in the master cylinder. The rubber membrane expands to fill the void. When you install new pads you need to push the rubber membrane back to its original shape.

 

[Twobrothersbusa]

So the moisture in the system is from when the brake fluid is changed. The vent in the reservoir cap allows the membrane to move based on system properties?

Thats an easy one.

You have air trapped under the boot. Look in your window and you'll see the fluid level mid window with air above.

Moisture (Condensation)

 

[Tufbusa]

So the moisture in the system is from when the brake fluid is changed. The vent in the reservoir cap allows the membrane to move based on system properties?

I think you have it pretty much figured out Charlie.

When you add fluid or change the fluid it's best to get as much of the air between the boot and fluid out, as possible.

I don't know enough bout oils to tell you exactly what happens to degrade the fluid over time but I do know it deteriates.

 

[Tufbusa]

Charlie, are you coming down to Barber for the 23-24th? I see there are still a few spots left. Would be a gas to have you there!

 

[cheferman65]

This is why I bleed my lines every 2 oil changes. It takes all of 20 minutes if that.
If you are gonna ride fast, ya better be able to scrub off the heat just as quick.
It's a habit I got into when i first moved to Fla. 15 years ago. With very humid and long hot summers it just made sense and my racing amigos confirmed it.

 

[IG.]

OK, thread hijackers. I took out the reservoir cap and finally found the vents - one on each side of the cap - looks like someone took a sharp file and made a 1 mm notch on each side of the cap. So, the vent issue solved. This is cool - on one hand the air enters the system to compensate for fluid level change and temperature change, on the other hand, the membrane separates the fluid from the air to protect it from the moisture.

Now I realize that if the gunk gets into both of those tiny notches and solidifies plugging them good, this may cause all sorts of trouble. Something to be aware of and to check once in a while. If the vents get plugged by the gunk on the outside only, this might work like a one way valve - let's the air out, but doesn't let air in - this would result in creating a vacuum inside the reservoir. Of course, plugging the vents on the inside or solid, would make it possible to create a vacuum or excessive pressure depending on conditions and riding scenarios.

The unanswered question still remains, why the lever needs to travel more as the pads wear? My current pads are half way done, but the lever is good on this particular set of pads which are the same as many previous ones EBC HH (can't notice any excessive travel). I am guessing this might something to do with the fact that I just cleaned the pistons, and exercised them by forcing them out and in a few times.

So, my next guess would be that if the pistons bind excessively, then when the lever is released the pistons pool back a bit. So, the next time the brakes are applied, the lever needs to travel more to compensate for that pull back. I still cannot explain why the piston's pull back would be worse when the pads are worn. I could only guess that as the piston moves out more, the less frequently used part of the piston binds worse.

Anyone knows? Can't get a solid answer which makes sense.

 

[djsin]

change the piston seals and leave the Einstein part to Einstein problem will be solved