Front brake pressure disappears

[Bill]

After I changed the front tire, I squeezed the brake several times, as usual.

Everything was fine until I started riding and as soon as I was moving, I lost the pressure in my front brake lines.

If I pump the brake a few times, everything returns to normal but as soon as I move, the pressure disappears.

If I'm parked, the pressure stays normal.

I've never experienced something like this.

Anyone know what I need to fix?

 

[garyahouse]

Bill: you created your own spam attack on the website. Ya gotta give it a little time to respond before you re-submit your post.

Concerning your brakes, I assume you've tried bleeding your brakes? Sounds like air in the line to me, though based on your description, I can't figure out how. You didn't happen to squeeze the brakes when the wheel was OFF, did you?

Gary

darksider #44

 

[MEM]

Holy crap.

How many front brakes do you HAVE ? :dribble:

 

[SkooterG]

Whoa! Easy on the trigger finger there Tex!

When was the last time you bled the system? My vote is air in the system and/or old fluid that is bad. No matter what, your first step is to perform a bleed.

 

[Bill]

Whoa! Easy on the trigger finger there Tex!
When was the last time you bled the system? My vote is air in the system and/or old fluid that is bad. No matter what, your first step is to perform a bleed.

The brake fluid is a bit over 1,000 miles and 5 weeks old and the brakes were working perfectly until I removed the calipers and reinstalled them today.

 

[SkooterG]

Whoa! Easy on the trigger finger there Tex!
When was the last time you bled the system? My vote is air in the system and/or old fluid that is bad. No matter what, your first step is to perform a bleed.

The brake fluid is a bit over 1,000 miles and 5 weeks old and the brakes were working perfectly until I removed the calipers and reinstalled them today.

Then my best SWAG would be that something went wrong with your prior bleed. A bubble was in the system that was moved when you removed the calipers, or something was jarred lose when you removed the calipers. I would check everything closely, and re-bleed. Then go from there.

 

[Bill]

In reading other forums, there are only two opinions on this type of problem:

1) There was an air bubble trapped somewhere close to a caliper that started moving when the calipers were removed and reinstalled. I thought of that originally but also wondered why the brakes kept their pressure when stopped and only lost it and became very soft when the bike moved.

2) The second opinion is that one of the rotors is warped or bent. The idea is that when I'm moving, the rotor is pushing the brake pad pistons back into the caliper and requiring me to reestablish the pressure in the lines but if this is the case, it wouldn't be any different than the pads being pushed back normally after being used for braking.

The rotors and pads only have 2,000 miles on them and when I remove the calipers and rotate the wheels, I see only the slightest hint of side to side motion and even lining the rotors up with a straight edge doesn't really show any bending or warping.

Since I was changing the tire today, I suppose that one of the rotors could have lightly bumped against something but I never knew it and I'd surprised if OEM rotors were so fragile that they bent only because the front tire was removed from the bike.

 

[Fred W]

Because it happened after you had the front wheel off, I am voting it is a warped rotor, induced during the tire change.

If the rotor is warped it will push the pads back, and the pistons back into the calipers. Then when you squeeze the brakes there is nothing there until you get them back down to the thickness of the rotors.

Easy to check for. Just jack the front up and spin the front wheel. Hold something stationary near the rotor and see how much the distance undulated as you rotate the wheel. You really shouldn't see any.

Hopefully it is something that you can fix easy.

 

[Bill]

Fred,

I just removed the rotors and followed your suggestion. When I place a straight edge next to the right side rotor, there is no perceptible movement but when I do it on the left side, I can see a small amount of side to side movement.

DAMN!!!

I can't remember hitting it against anything and I've changed my own tires for years and have never experienced this.

Thanks

 

[garyahouse]

Seems to me that if warped rotors had something to do with this, they would have had to be REALLY warped, and you would have felt it every time you hit the brakes. And you're not going to bend your rotors by accident. It takes some serious force to bend a piece of steel that thick. My money's still on air in the line.

Gary

 

[Mihalis fjr]

The front deflection of the rotor is 0.10 mm maximum as per service manual writes..These float rotors have a very very small movement left and right and for that reason you can see some times a very small amount of side to side movement.I don't believe that this small amount of movement can cause this trouble in your brakes!For a reason,some air bubble is trapped in or was in the system and cause the trouble now!

Do a good bleeding and everything will come back to normal again i believe..!

If the rotor is toooo warped,you can feel the movement in your brake lever!Do you feel it?

 

[Fred W]

Seems to me that if warped rotors had something to do with this, they would have had to be REALLY warped, and you would have felt it every time you hit the brakes. And you're not going to bend your rotors by accident. It takes some serious force to bend a piece of steel that thick. My money's still on air in the line.

The front deflection of the rotor is 0.10 mm maximum as per service manual writes..These float rotors have a very very small movement left and right and for that reason you can see some times a very small amount of side to side movement.I don't believe that this small amount of movement can cause this trouble in your brakes!For a reason,some air bubble is trapped in or was in the system and cause the trouble now!Do a good bleeding and everything will come back to normal again i believe..!

If the rotor is toooo warped,you can feel the movement in your brake lever!Do you feel it?

You are both missing the most important clue given in the first post, highlighted below

After I changed the front tire, I squeezed the brake several times, as usual.Everything was fine until I started riding and as soon as I was moving, I lost the pressure in my front brake lines.

If I pump the brake a few times, everything returns to normal but as soon as I move, the pressure disappears.

If I'm parked, the pressure stays normal.

Air in the lines doesn't care if if the wheels are moving. If he had air in his lines it would go soft again after just sitting still and waiting for a few minutes. That isn't the case here. The brake lever only goes soft again after moving the bike.

And I'm not sure that you would feel pulsation in the brake lever if the rotor thickness is still uniform but the rotor carrier (inner part of the rotor) has been bent. I don't think the rotor brake surface itself is warped in this case, just that the carrier has been bent so the rotor isn't parallel to the wheel any more. Plus he has already noticed a visual irregularity, which is most likely more than 0.1mm spec if he can see it.

I don't know how the OP changes his tires, with tire irons or a tire machine, but I can see this happening if one were to bear down on the rotor inadvertently during the tire removal or replacement.

FWIW, before I broke out the credit card, I'd try to identify the high and low sides of the rotor and see if I could flex it back to straight. What do you have to lose?

 

[Mihalis fjr]

I read the first post more carefully..I missed the ''If I'm parked, the pressure stays normal''.

Agree with Fred!

 

[Bill]

Seems to me that if warped rotors had something to do with this, they would have had to be REALLY warped, and you would have felt it every time you hit the brakes. And you're not going to bend your rotors by accident. It takes some serious force to bend a piece of steel that thick. My money's still on air in the line.

The front deflection of the rotor is 0.10 mm maximum as per service manual writes..These float rotors have a very very small movement left and right and for that reason you can see some times a very small amount of side to side movement.I don't believe that this small amount of movement can cause this trouble in your brakes!For a reason,some air bubble is trapped in or was in the system and cause the trouble now!Do a good bleeding and everything will come back to normal again i believe..!

If the rotor is toooo warped,you can feel the movement in your brake lever!Do you feel it?

You are both missing the most important clue given in the first post, highlighted below

After I changed the front tire, I squeezed the brake several times, as usual.Everything was fine until I started riding and as soon as I was moving, I lost the pressure in my front brake lines.

If I pump the brake a few times, everything returns to normal but as soon as I move, the pressure disappears.

If I'm parked, the pressure stays normal.

Air in the lines doesn't care if if the wheels are moving. If he had air in his lines it would go soft again after just sitting still and waiting for a few minutes. That isn't the case here. The brake lever only goes soft again after moving the bike.

And I'm not sure that you would feel pulsation in the brake lever if the rotor thickness is still uniform but the rotor carrier (inner part of the rotor) has been bent. I don't think the rotor brake surface itself is warped in this case, just that the carrier has been bent so the rotor isn't parallel to the wheel any more. Plus he has already noticed a visual irregularity, which is most likely more than 0.1mm spec if he can see it.

I don't know how the OP changes his tires, with tire irons or a tire machine, but I can see this happening if one were to bear down on the rotor inadvertently during the tire removal or replacement.

FWIW, before I broke out the credit card, I'd try to identify the high and low sides of the rotor and see if I could flex it back to straight. What do you have to lose?

 

[Fred W]

Whoa... Deja Vu all over again!

 

[dcarver]

This is like a mystery movie! What dunnit?

 

[Fred W]

This is like a mystery movie! What dunnit?

Easy! It was Mr. Bill, in the Garage, with a tire iron!

 

[Chuck35]

I suppose its safe to assume all the pads and clips are in the correct positions and free to move as designed.

 

[Bill]

I would approximate the side-to-side movement of the left rotor to be approximately .2mm. I do not feel pulsing in the brake.

I change my tires with a Cycle Hill changer and MoJo lever. Yesterday, I was having a lot of trouble balancing the front tire and put it back on the Cycle Hill several times in order to turn the tire on the wheel. I don't remember hitting the rotor against anything but the left side rotor does have some visual side-to-side movement while the right side rotor does not.

I'm just going to replace the rotor and I'm going to open the brake reservoir, tie the lever to the grip and see if an air bubble will come out of the lines because I know that the bike will not build pressure with air in the lines but I also know that if there was air in the lines, the lever will be soft whether I'm moving or not.