Caliper pistons: a funny story and a question

[IG.]

So, on Saturday, the day before the Sunday ride, I decided it was time to clean my calipers for the first time ever, since my front wheel was out. Removed one of them from the fork, removed the pads. Pumping the brake lever to extend the pistons. Only one of the pistons is moving out. So, I am thinking well it's gonna stop at some point, right? Wrong - one of the pistons just falls out and the brake fluid is leaking. And you know what this means. Cursing myself, I realize what happened. I realize that I need to have at least the pads installed to make sure no piston pops out.

So, I said to myself, don't panic. All you need to do is put the piston back and bleed the system. Otherwise, the bore looks nice and clean, and the part of the piston which goes inside also looks nice and clean. I notice that the dust seal is partially out. So, I carefully stick it back. Then had enough sense to cover the piston in brake fluid and carefully push it inside. After some time, I succeed and the piston is in. Luckily, I had a new bottle of brake fluid. I use Mighty Vac to bleed the fluid, check the lever - mushy - still got air in the system. So, I switch to the basic method of press and hold the lever, and playing with the bleed screw - my wife helps me with this. I have to tell you that this basic method does get rid of the air trapped inside the caliper. The lever became really hard when squeezing. Great! Now, all I need to know there is no leak at the piston. I try a few more times to squeeze and push the piston out and then push it back. It goes back easy and smooth and there is no leak - whew! Quck cleaning with water and soap, and both calipers are done. A quick test ride confirms I am OK. What a way to spend most of your Saturday!

Now, to the question. Besides the piston which popped out, most others are pretty hard to push back by hand. Also, when I sqeeze the lever most of the pistons are hard to come out. Observing the pads wear, I notice that the further away the piston is from the incoming brake line, the less wear the pad has - which is consistent with my observation of how hard it is to push the pistons back. I tried to move them all out and push them back a few times, and thought it will improve the ease of their movement, but this wasn't the case.

So, what makes the pistons hard to move?

Another thought I had, that maybe I just need to push the pistons out and lubricate them with brake fluid prior to pushing them in, doing it a few times, and this should make the pistons come out/be pushed in easier. Any thoughts on that? Keep in mind, I pushed out all pistons about 1/2" and aside from their very top portion, they all look clean and shiny.

 

[Busa1166]

I didn't think you were supposed to apply the brakes without the rotor to stop it They are supposed to be hard to push back in, taking the top off cylinder should ease up the pressure but watch out for over flow.

 

[IG.]

Hard. But not that hard.

Nothing wrong with applying the brakes - just have something to stop the pistons.

When I squeeze the lever, the fluid escapes from the reservoir and goes into the caliper, so when I push the pistons back, the same amount of brake fluid goes back into the reservoir. But, you are right, if add the fluid prior to pushing the pistons back, the reservoir might overflow.

Pushing the pistons that far out (which never happens in normal braking), creates a vacuum in the reservoir which in turn pulls the pistons back in a little.

Now that I think about it, as the pads wear out and get thinner, the pistons are being pushed further out, and the vacuum inside the reservoir gets stronger and stronger. When the brake lever is released, that extra vacuum pulls the pistons slightly 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 to get rid of the vacuum.

I think I'm gonna post this in the main forum.

 

[Dehning]

In a perfect world, fluid dynamics should make all the pistons come out at the same time. Putting any kind of pressure against the one closest to the line should cause the others to pop out but I realize that this isn't a perfect world and I have read lots of complaints about the 6 piston setup.

 

[Dehning]

Hard. But not that hard.

Nothing wrong with applying the brakes - just have something to stop the pistons.

When I squeeze the lever, the fluid escapes from the reservoir and goes into the caliper, so when I push the pistons back, the same amount of brake fluid goes back into the reservoir. But, you are right, if add the fluid prior to pushing the pistons back, the reservoir might overflow.

Pushing the pistons that far out (which never happens in normal braking), creates a vacuum in the reservoir which in turn pulls the pistons back in a little.

Now that I think about it, as the pads wear out and get thinner, the pistons are being pushed further out, and the vacuum inside the reservoir gets stronger and stronger. When the brake lever is released, that extra vacuum pulls the pistons slightly 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 to get rid of the vacuum.

I think I'm gonna post this in the main forum.

I don't know that I agree with you. After bleeding my Hayabusa, CBR1100 & VFR800 all in the last month I'd have to say that there must be negligible vacuum caused by releasing the lever certainly not enough to pull the pistons back to the same place every time. What is the point of self-adjusting hydraulic brakes if they don't self-adjust to brake pad wear ? The piston system at the master is more of a one-way valve to cause hydraulic pressure when moved in one direction, and release air in the line back into the airspace in the reservoir when moved in the other.

I am happy to be corrected here, but that has been my experience.

 

[IG.]

In a perfect world, fluid dynamics should make all the pistons come out at the same time. Putting any kind of pressure against the one closest to the line should cause the others to pop out but I realize that this isn't a perfect world and I have read lots of complaints about the 6 piston setup.

I know what you are saying. I was amazed at the discovery that the further the piston was from the incoming brake line, the less pressure such piston would apply to their respective part of the pad, which is clearly confirmed by the pad wear on both calipers. Not only the outer pad (closer to the incoming brake line) is used up more than the inner pad, but each pad has also slanted wear pattern in such a way that the closer the part of the pad is to the incoming brake line, the more it's worn (i.e. the closer the piston is, the more force it applies to that part of the pad).

 

[Dehning]

I know what you are saying. I was amazed at the discovery that the further the piston was from the incoming brake line, the less pressure such piston would apply to their respective part of the pad, which is clearly confirmed by the pad wear on both calipers. Not only the outer pad (closer to the incoming brake line) is used up more than the inner pad, but each pad has also slanted wear pattern in such a way that the closer the part of the pad is to the incoming brake line, the more it's worn (i.e. the closer the piston is, the more force it applies to that part of the pad).

I remember a technical post on this board a while ago that explained why a pad always wears more quickly on the edge that first hits the rotor, this is why many pads have one or more slots in the pad surface to as to even out the force involved. I'm looking, but not finding the thread.

 

[IG.]

I remember a technical post on this board a while ago that explained why a pad always wears more quickly on the edge that first hits the rotor, this is why many pads have one or more slots in the pad surface to as to even out the force involved. I'm looking, but not finding the thread.

This is interesting. Please post if you find something explaining it this way.

Regarding the vacuum, it's not the release of the lever which creates the vacuum. As the brake pads wear out, some of the fluid from the reservoir goes into the calipers because the pistons have to extend more. Assuming that the vent hole in the membrane is clogged and does not let the air to enter, this creates vacuum in the reservoir. When the lever is not pressed, the fluid/air are free to travel from/to the reservoir with no restriction. So, in such conditions (some vacuum in the reservoir), after you release the lever, the vacuum sucks small portion of the fluid back into reservoir which forces the pistons back in. Even a very small backing in of the pistons will be felt the next time the brakes are applied - it will feel as the lever needs to travel more to apply braking force.

How about any insight about hard to move pistons?

 

[Dehning]

This is interesting. Please post if you find something explaining it this way.

Regarding the vacuum, it's not the release of the lever which creates the vacuum. As the brake pads wear out, some of the fluid from the reservoir goes into the calipers because the pistons have to extend more. Assuming that the vent hole in the membrane is clogged and does not let the air to enter, this creates vacuum in the reservoir. When the lever is not pressed, the fluid/air are free to travel from/to the reservoir with no restriction. So, in such conditions (some vacuum in the reservoir), after you release the lever, the vacuum sucks small portion of the fluid back into reservoir which forces the pistons back in. Even a very small backing in of the pistons will be felt the next time the brakes are applied - it will feel as the lever needs to travel more to apply braking force.

How about any insight about hard to move pistons?

I know it's a P.I.T.A. but have you thought about swapping pistons and/or seals around to see if the problem moves ?

 

[djsin]

heres what you do...take it all the way apart..pistons seals everything separate the 2 halves...buy a new piston and seal kit for 75$ and no more problems..or atleast get the 12 new seals..they are hanging up and at some point the caliper wont release all the way and begin to wear your brakes out fast...ask me how i know.. had them get stuck so bad i couldnt roll the bike under my own foot power..

 

[djsin]

over time the cleaners we use to make our babies shine as well as dirt and grime begine to make the rubber swell..then the pistons start to hang..when everything wrks right u can move them with your finger and minimal force in both directions....

 

[Dehning]

That's $28.91 per side for seals or $63.55 per side for pistons and seals from Service Honda. :down:

 

[IG.]

The piston which fell off, and I put it back in while lubing with the fluid, that piston moves great. I can push it in with my fingers easily. That's why I am considering pushing them out (without popping them out), lubing and pushing back.

 

[Ludicrous Speed]

The reason they do not move in and out as easily is that they are dirty. Look at my caliper rebuild thread, pinned up top.

Also, all that moves the pistons back in to the bores is the pull that is on the pistons by the seals. And the runout of the rotor bumping the pads pushing them back in. THere is not a vacuum in there anywhere pulling them in