General rear brake question.

[dmoon64]

After SantaFeRider's rear brake meltdown I decided to look at my rear brake caliper. Just because I was curious I felt the rear rotor after my 30 mile highway commute. The rotor and caliper were very hot. I mean real hot. Last night I took the caliper off and did the caliper cleaning and brake fluid bleed. I also went ahead and replaced the pads.

 

In other threads I read where you can pump the piston out enough to see a groove in the piston. I never saw the groove, but was able to get the piston pretty far out. As I pressed on the brake lever the piston would move out then retract when I let up off the brake pedal. I cleaned it with simple green then put it all back together. Took a ride and all seemed fine.

 

Now for the stupid question. Only one piston comes out of the caliper. The other one seems fixed. Is that normal and how does the brake actually work. I understand the fluid presses on the piston and forces the piston and brake pad against the rotor, but how does the other brake pad come into play? Does the caliper or rotor deflect enough to cause the pinching action between the brake pads?

 

Thanks for any answers.

 

Darryl

 

2003 V11 Lemans

[Guest ratchethack]

Now for the stupid question. Only one piston comes out of the caliper. The other one seems fixed. Is that normal and how does the brake actually work. I understand the fluid presses on the piston and forces the piston and brake pad against the rotor, but how does the other brake pad come into play? Does the caliper or rotor deflect enough to cause the pinching action between the brake pads?

Darryl, my compliments for taking a heads-up approach and using the proper procedure following a symptom that IMHO is very common and often goes entirely unnoticed until damage and/or roadside grief ensues. Your care here will no doubt serve you well down the road.

 

No stupid Q's here. The Brembo P32B caliper (as mentioned recently in the aforementioned thread) works by both pistons acting in opposite directions on the rotor at equal pressure (Pascal's law). In proper operation, assuming the pistons are free to move in their bores, they act with equal force, "clamping" the rotor between the pads. Since the pressure on each piston is the same, the location of the rotor between them will cause the pistons to find the exact point of extension of each where pressure on the rotor is exactly the same on both sides.

 

I think the "groove" you're referring to is a description of the circular gap between the piston and caliper.

 

I've encountered wot you describe many times, including on this particular caliper. When pressurizing the pistons to expose the seals for cleaining, one piston typically comes out, leaving the other retracted. This DOES NOT mean anything is wrong, it just means that one piston has a tiny bit more "opposition" to overcome than the other. You don't want to use too much more pressure in the case of cleaning it in an attempt to make it come out without putting a clamp on the one that HAS come out, to prevent it from coming out far enough to pop its seal and dust cover. You've got substantial trouble at that point that's easily enough avoided! When you hold one back, the other will move. It typically only takes a very tiny amount of additional brake pressure to make the other one come out, unless you've got a serious problem and this would be very rare by my experience. Relative to the internal caliper pressures under braking, the usual differences between the pistons' resistance to move (even when caked with substantial crud) are more or less negligible and instantly overcome on the road.

 

EDIT: Now that I think about it a little more, I remember that I've used a well-padded Channelock pliers to squeeze the extended piston back in, which causes the retracted one to come out -- as long as the relief port on the master cylinder is held closed, that is. Holding down the brake lever will do it. Takes very little pressure on the pliers.

Does the caliper or rotor deflect enough to cause the pinching action between the brake pads?

Sorry, not sure I follow you here?

[docc]

d'Moon,

 

Good on ya for cleaning up your brake. How many miles on it? I've not ever gotten over 10,000 on rear pads and the least was 6,000; and that's without using the rear brake much!

 

As Ratch says, both pistons should move. It's probably pretty common for the inboard piston to get splattered by the tire spray more. Try clamping the movable piston and applying pedal pressure to the other to free it up. In a perfect world, they will move identically as there is no movement in the caliper mount or deflection of the rotor.

[dlaing]

Took a ride and all seemed fine.

I guess you fixed it without a thorough cleaning of the piston that was sticking more.

But I would check it pretty frequently to make sure it is not overheating, again.

If you really want to be cautious, you should clean it again, this time cleaning the other piston.

It may be worth the effort now to give the brake pads a more balanced wear, and ensure a cooler running rotor.

[dmoon64]

I was under the impression the second piston was fixed so it looks like I will need to free up that stuck piston. When you guys mention clamps is there a specific brake clamp or just anything that can get in the space to hold the moving piston?

[Guest ratchethack]

I was under the impression the second piston was fixed so it looks like I will need to free up that stuck piston. When you guys mention clamps is there a specific brake clamp or just anything that can get in the space to hold the moving piston?

Darryl, per my post above, I doubt if it's really "stuck". If you've done a thorough cleaning and brushed down into the circular groove between the piston and caliper, I'd expect you're OK, but by all means, if the peace of mind of knowing for sure is of interest, have at it again.

 

WRT how to hold the "loose" piston from comong out any further, anything that works will do. Per my edited post above, I've used a well-padded channelock pliers to push it back in, which will cause the other one to come out. Don't forget to hold the relief port closed while you're doing this by pushing down on the brake lever. As I also recall now, I have a tool with a wooden handle that happens to be the right dimension to fit the caliper very well. If it's handy at the time, I'll stick it in there, pump the brake lightly, first one piston comes out, hits the handle, stops, the other one comes out, and Robert's y'er Mum's favorite brother.

[dmoon64]

Bob's your Uncle!

 

[dmoon64]

Breaking, cleaning, and bike storing habits.

 

Breaking. I use both fronts and rears. I had been using the bike as a commuter. 35 miles one way, mostly 70-80 mph on I-80. Pretty straight run in Iowa. Some weekend fun rides but still not huge amounts of miles. I carpool now so I very rarely ride.

 

Cleaning. Not a very important to me. I may spray down with the Honda clean, but I usually wash it once or twice a year.

 

Storing. Goes under the house into a heated basement. So no real winterizing. If it gets over 35 I may go for a spin. Last year I rode until late January. Iowa had a warm winter until February, then all hell broke loose.

 

I didn't notice a smell but I had the brake groan. After I cleaned that up I happened to touch the rear rotor after getting to work. It was very hot, and that was about the time I read of SantaFeRiders melting rear brake isuue. So that started me on this path.

 

I still haven't got the one piston free, but I will give it one more try tonight before taking it to a dealer or trading it for a KLR650!

 

 

My 03 LeMans has two single pistons inboard and outboard. They squeeze the pads against the rotor.

[GuzziMoto]

Hope you don't overreact, sell it and get a KLR. Disc brakes are amazingly simple devices that should be fixable short of serious catatrophic failure. Hang in there.

[Dan M]

The latest rear calipers on the big Guzzi V's have a different piston arrangement, with two smaller ones on the outside, and a fixed pad on the other. This is similar to an automotive design. The single piston calipers on my 4 wheel disc Jeep have the same basic setup, and the fixed pads wear much less than their counterparts.

 

 

When did they switch to a sliding caliper? I've never seen it on a V11.

 

 

Oh yea Darryl, If brakes are an issue with you, don't buy a KLR. I remember squeezing as hard as I could (both the brake lever and the seat vinyl!) and that bike just doesn't stop.

[BrianG]

The latest rear calipers on the big Guzzi V's have a different piston arrangement, with two smaller ones on the outside, and a fixed pad on the other. This is similar to an automotive design. The single piston calipers on my 4 wheel disc Jeep have the same basic setup, and the fixed pads wear much less than their counterparts.

 

This is called a "floating caliper" design, and the caliper floats on it's mounting bolts. By virtue of the same Pascal's law, a properly floating caliper should provide equal pressure to both pads. These floating calipers foul on their mounting bolts far more frequently than caliper pistons foul in their bores, hence a both-sided piston arrangement is better functioning, albeit more expensive.

[GuzziMoto]

By newer, I believe he was refering to the Breva's and Griso's. Not newer V11"s.

[dmoon64]

Ah screw it. Can get both pistons to move but they don't move the same. I'll ride this weekend and see if I still get the high temps on the rotor. If I do anyone want to buy a LeMans. $6500 and it yours...

[Guest ratchethack]

Darryl, I'm sympathetic with you to the point of discovery that rebuild kits are evidently not available for the P32B caliper.

 

HOWEVER:

 

As I've tried to make clear, it would be more the exception than the rule to have both pistons move the same until the pads make contact with the rotor. Once they do, the pressure on both will be identical (yes, even when they move unequally until they contact the rotor), unless you've got something very very unusual going on. Probabilities of this are slim to none.

 

When properly cleaned, the probabilities of one brake pad dragging due to insufficient retraction of the piston and not the other are also slim to none. I don't know what caused your rotor to heat up, but it sounds like you haven't found the problem.

 

The likelihood that you actually have no problem at all other than something like a bad adjustment or a bad habit of resting your foot on the brake lever is MUCH higher than a "stuck" piston. Worst case scenario, a simple refurb of the piston and caliper to de-burr or remove a particle between the caliper cylinder and piston would likely suffice -- using the existing seals and dust cover, etc.

 

If you don't have FULL confidence in your brakes (both front and rear), IMHO you shouldn't ride the bike. FWIW, any OTHER bike with opposing piston calipers that you do brake service on may be expected to exhibit the identical unequal movement of pistons you've seen here. Will you also be dumping your next bike when you discover this?

 

If your tolerance level is that low for something apparently this trivial and you can't find a way to gain full confidence in your brake, It's probably best you move on, since you're likely to continue to be a very unhappy Guzzista over many more trivial matters to come -- and also likely, far less trivial matters.

[dlaing]

Ah screw it. Can get both pistons to move but they don't move the same. I'll ride this weekend and see if I still get the high temps on the rotor. If I do anyone want to buy a LeMans. $6500 and it yours...

Whatever happened to "Bob's your uncle"?

They don't have to move exactly the same.

The one that was sticking may need to get some use as it may have been stuck for quite a while. You did clean it, right?

If the rotors get too hot to touch after a five minute ride with no rear brake use, remove caliber and pop out the piston that sticks more. Clean it and the seal very well, lube them with brake fluid, put it back to together and Bob's your uncle....

If not, I'll bet a new caliper from http://www.guzzitech.com is probably under $100.