Kiwi Rider
Registered
At work today we had a guy bring his 2014 Harley D. Night Train in for a service, W.oF check (safety check) and he said "can you sort the front brakes out, very soft and no stopping power".
The calipers are 6 piston billet alloy with stainless steel braided hoses.
I opened up the M/Cyl reservoir and took a look, orange coloured fluid and very nasty looking stuff!
The lever came all the way into the handle bar, and very soft feel.
Now, if you look closely at the cailper you may notice the position of the bleed nipple . . .
at the BOTTOM!!! I mean, WTF?? To bleed them I had to unbolt them from the fork legs and sit them on a stool with the bleed screw pointing upwards.
Yes, there was a fair amount of air bubbles in the lines, but the real problem was the pistons extending and retreating back around 2mm every time I squeezed the brake lever, this is the reason for the excessive lever travel, and lack of stopping power!
The cause is the piston seals being stuck to the piston.
Pull the lever, the pistons move out dragging the seal instead of passing through the seal and then dragging the piston back every time hydraulic pressure is released.
I decided to strip the calipers and have a look at what horrors may lie inside of them . . . given that the fluid was rusty water contaminated . . . . . . . just look at what I found.
So with a thoroughly good clean, and pistons all cleaned and examined, seal recesses cleaned out and seals checked and lubed, I reassembled them and refitted them to the brake hoses.
I then flushed heaps of fluid thru them and bled the air out.
Bolted them up the fork legs and pulled the lever . . YES!!! result!
The lever has a firm feel and only comes in about 30% of the way to the bar.
By the way, the pads are sintered and only about 20% worn, in good condition.
So, I hope anyone reading this thread has learned something here,
moral of the story . . Flush and bleed your brakes on a regular basis, once a year is good.
Here's a description of the piston seal "stiction" problem. . .
The brake system has a ratcheting action. Every time you apply the brakes, the pistons press the brake pad against the rotor creating friction which slows the bike down and also results in loss of material on the brake pad itself. This wearing process occurs slowly or else we would be changing pads daily instead of after thousands of miles. When the lever is released, the pressure is released and a set of springs and a little bit of vibration and bumping from the rotor cause the brake pads to push the pistons backward slightly to remove the friction from the rotor. As the pads get thinner from repeated brake application, the pistons slowly move out of the caliper over time so that the brake lever engages more or less at the same distance from the hand grip. This is the so-called ratcheting action of the hydraulic brake system. As this occurs, the level of hydraulic fluid in the brake reservoir will be dropping so maintenance top-ups are required.
Over time, dirt and brake dust coat the pistons in the brake caliper. Some of this dirt may work itself into the piston seal when the pistons retract a bit after brake application. Sometimes, instead of the piston sliding forward through the seal when the brakes are applied, the piston gets stuck to the seal and rolls the seal forward when the brake lever is pulled. When the brake lever is released, the seal restores its normal shape causing the piston to be pulled backward. The ratcheting action is lost. As the brakes wear, the piston gets further and further from the friction point. The result is that the brake lever activation point moves closer and closer to the handgrip.
The speed at which the piston retracts back into the caliper due to elastic action of the seal is quite slow so 2 quick pumps on the brake lever generally result in the pistons making contact with the pads, and the lever works at the correct distance from the handgrip. The reason pumping works is because the first pump moves the piston outward as much as the hydraulic fluid pushed into the system from the hand lever allows, and the second pump forces additional hydraulic fluid in behind the piston which moves it out to the pad and generates enough friction to stop the motorcycle.
Thanks for taking the time to read this, comments welcome
The calipers are 6 piston billet alloy with stainless steel braided hoses.
I opened up the M/Cyl reservoir and took a look, orange coloured fluid and very nasty looking stuff!
The lever came all the way into the handle bar, and very soft feel.
Now, if you look closely at the cailper you may notice the position of the bleed nipple . . .
at the BOTTOM!!! I mean, WTF?? To bleed them I had to unbolt them from the fork legs and sit them on a stool with the bleed screw pointing upwards.
Yes, there was a fair amount of air bubbles in the lines, but the real problem was the pistons extending and retreating back around 2mm every time I squeezed the brake lever, this is the reason for the excessive lever travel, and lack of stopping power!
The cause is the piston seals being stuck to the piston.
Pull the lever, the pistons move out dragging the seal instead of passing through the seal and then dragging the piston back every time hydraulic pressure is released.
I decided to strip the calipers and have a look at what horrors may lie inside of them . . . given that the fluid was rusty water contaminated . . . . . . . just look at what I found.
So with a thoroughly good clean, and pistons all cleaned and examined, seal recesses cleaned out and seals checked and lubed, I reassembled them and refitted them to the brake hoses.
I then flushed heaps of fluid thru them and bled the air out.
Bolted them up the fork legs and pulled the lever . . YES!!! result!
The lever has a firm feel and only comes in about 30% of the way to the bar.
By the way, the pads are sintered and only about 20% worn, in good condition.
So, I hope anyone reading this thread has learned something here,
moral of the story . . Flush and bleed your brakes on a regular basis, once a year is good.
Here's a description of the piston seal "stiction" problem. . .
The brake system has a ratcheting action. Every time you apply the brakes, the pistons press the brake pad against the rotor creating friction which slows the bike down and also results in loss of material on the brake pad itself. This wearing process occurs slowly or else we would be changing pads daily instead of after thousands of miles. When the lever is released, the pressure is released and a set of springs and a little bit of vibration and bumping from the rotor cause the brake pads to push the pistons backward slightly to remove the friction from the rotor. As the pads get thinner from repeated brake application, the pistons slowly move out of the caliper over time so that the brake lever engages more or less at the same distance from the hand grip. This is the so-called ratcheting action of the hydraulic brake system. As this occurs, the level of hydraulic fluid in the brake reservoir will be dropping so maintenance top-ups are required.
Over time, dirt and brake dust coat the pistons in the brake caliper. Some of this dirt may work itself into the piston seal when the pistons retract a bit after brake application. Sometimes, instead of the piston sliding forward through the seal when the brakes are applied, the piston gets stuck to the seal and rolls the seal forward when the brake lever is pulled. When the brake lever is released, the seal restores its normal shape causing the piston to be pulled backward. The ratcheting action is lost. As the brakes wear, the piston gets further and further from the friction point. The result is that the brake lever activation point moves closer and closer to the handgrip.
The speed at which the piston retracts back into the caliper due to elastic action of the seal is quite slow so 2 quick pumps on the brake lever generally result in the pistons making contact with the pads, and the lever works at the correct distance from the handgrip. The reason pumping works is because the first pump moves the piston outward as much as the hydraulic fluid pushed into the system from the hand lever allows, and the second pump forces additional hydraulic fluid in behind the piston which moves it out to the pad and generates enough friction to stop the motorcycle.
Thanks for taking the time to read this, comments welcome
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