Front disc brake

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pierodn

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Hi.
I replaced the caliper with new seals, the frontmaster cylinder has a new kit, the brake pads only are old but seem good.
With the bike stop on the stand the brake lever is well hard to thight and so that wouldn't seem to be air in the hose.
But when i ride and brake, the lever thightnes but at soon comes back up like a spring and it is not possible to keep the braking lever pulled because it tends to go back.
So is impossible braking because the brake attacks and off continuously, as if it were an ABS with the same noice!
The disc is in good condition.
Sorry but i cannot able to explain better.
Thank you.
Piero
 
pierodn said:
Hi.
I replaced the caliper with new seals, the frontmaster cylinder has a new kit, the brake pads only are old but seem good.
With the bike stop on the stand the brake lever is well hard to thight and so that wouldn't seem to be air in the hose.
But when i ride and brake, the lever thightnes but at soon comes back up like a spring and it is not possible to keep the braking lever pulled because it tends to go back.
So is impossible braking because the brake attacks and off continuously, as if it were an ABS with the same noice!
The disc is in good condition.
Sorry but i cannot able to explain better.
Thank you.
Piero
Click to expand...

If I understand correctly, your brake lever pushes back against your hand as if the internal pressure forced it back. If this is so, then there's something in the caliper that's boiling as the brake gets hot. 'Not meaning to insult, but did you fill the system with new fluid from a closed container?
If using DOT 3, it can absorb moisture directly from the air. Even though you can't see the moisture, it will then violently boil as the caliper gets hot, thus causing the system pressure to rise. Granted, the hole in the master cylinder reservoir will allow some to escape, but, if the volume of steam is sufficient, the bellows will contain the pressure, causing the pressure to rise.
If you went with DOT 5, the same can happen if any solvents were to remain in the caliper. Of course, if you used any kind of volatile solvents with either type of brake fluid, this could be the cause. Worst case would be a brake that locks up, and won't release until the brake cools down. Very dangerous!
It sounds like what you're feeling is when something in the system boils, the piston is pushed back far enough to uncover the hole in the master cylinder which allows the piston to move part way back into the bore thus dumping some pressure, then the piston moves partway back into the bore from the force of your hand against the lever, and this cycle repeats, feeling like a vibration in the lever. 'Sorry for the run-on sentence!
I would start with a complete flush of the system with new dry brake fluid. If the problem persists, you may need to remove and dis-assemble the caliper to rid the system of any residual moisture and/or solvent.

Nathan
 
Hi Nathan,
I think i will desmount and clean the front master cylinder, caliper and hoses and replace them wirh a new oil dot ????.
What else i can do!!!!
I dont like my Norton "ABS" front brake!!!!!!!
Thank you.
Ciao
Piero
 
pierodn said:
So is impossible braking because the brake attacks and off continuously, as if it were an ABS with the same noice!
The disc is in good condition.
Click to expand...

From that description it sounds more like the disc is warped or is running out of true (or the hub is)? Or one of the wheel bearings is breaking up?
 
L.A.B. said:
pierodn said:
So is impossible braking because the brake attacks and off continuously, as if it were an ABS with the same noice!
The disc is in good condition.
Click to expand...

From that description it sounds more like the disc is warped or is running out of true (or the hub is)? Or one of the wheel bearings is breaking up?
Click to expand...


I agree with you L.A.B. i would look closely at the disc and mounting on the wheel. When i have over heated brakes and boiled the fluid it make the brakes feel spongy as the fluid expandes and bubbles form (like when water boils). As the brakes cool these tiny bubbles are absorbed or fined there way out of M/cylinder vent. But this does harm fliud and it should be changed.
 
Air or solvents in the system vaporizing from heat will cause brake fade, that is no matter how hard ya squeeze nil force transmitted to pads as the bubbles just squeeze down instead, but no kick back to lever. Look below the master cylinder as advised. if the plunger spacer between lever and m/c piston is a tad too long it can cause light lever squeeze to brake too hard > lock up and not let up. Wobbly rotor pushing against pads is known for pulsing back into lever or peddle. Its possible I've heard an internal swollen or collapsed hose can feel like this.
 
Hello Piero,
Dot 3/4, both are compatible, Dot 5 will not mix, and it will be almost impossible to clean the 5 out of the system. I know I tried and it took almost 2 months to get it clean again with Dot 4(higher temp handling), though the 5 will handle the heat of racing brakes.
Peter
 
toppy said:
L.A.B. said:
pierodn said:
So is impossible braking because the brake attacks and off continuously, as if it were an ABS with the same noice!
The disc is in good condition.
Click to expand...

From that description it sounds more like the disc is warped or is running out of true (or the hub is)? Or one of the wheel bearings is breaking up?
Click to expand...


I agree with you L.A.B. i would look closely at the disc and mounting on the wheel. When i have over heated brakes and boiled the fluid it make the brakes feel spongy as the fluid expandes and bubbles form (like when water boils). As the brakes cool these tiny bubbles are absorbed or fined there way out of M/cylinder vent. But this does harm fliud and it should be changed.
Click to expand...


My experience too....

Fluid problems, including simple air in the fluid, equal soft and spongy lever feel.....worst case coming back to bar....

Disc run out, lack of clearance disc to caliper or disc warping causes the hydraulics to feed the movement back to the lever...or may just cause excessive lever movement...

These oddball symptoms occur more often with floating discs which can either actually warp or get stiff on the buttons and not float properly.....I have had it on two GSXRs of different ages and caliper piston count. Having pads with too much friction material on them for your specific set up can cause issues too.....there can be huge variations between brands, or they can have variations in the metal backing size that limits their free movement...check the pads move freely back into calipers by hand pressure alone. Many calipers will give these symptoms when there is a build up of corrosion inside the piston seal.

Worn wheel bearings will cause the same symtoms due to letting the disc run out etc.

So, check the wheel bearings first, because its quick, then check clearance, free pad movement, run out and warping.

But change your fluid anyway because, one you don't seem 100% sure it is good so you don't want to have that in the back of your mind, and two...by now it has probably suffered excess heat.
 
Sometimes you can learn a lot by just pushing the bike quietly around the garage.
Pull the front brake lever; does the brake come on then release cleanly. Any remaining friction?
Any shudder/shake in the wheel bearings or fork bearings?

Step on the rear brake pedal and release while you are checking. Bad rear brakes can make a scraping sound.

My vote on your pulsing disc brake problem is a loose on the hub or warped disc.
 
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