Topic: Dual Front Brakes Won't Lock Up

[zeech]

I rebuilt the dual front brakes for my 650, new o-rings, pistons, steel braided lines, upgraded Nissin MC. I've bled and bled and bled and all the air is out of the system (I bled the far caliper first then the closer one) but I still can't get the brakes to lock, it will want to just keep rolling. There are also no leaks from any of the banjo bolts. The only thing not new is the brake pads, but they seemed to have more than enough life on them. The rotors are 5mm.

I've also tried tapping the lines etc. taping the brake level back etc. and nothing seems to do it. Perhaps I need new pads? Any thoughts would be great, thanks.

[Duke McDukiedook]

Would probably be a good place to start. You sure the MC is still good?

[zeech]

What would be a good place to start, pads? The Nissin MC is brand spanking new.

[Don R]

Check the pads to see if they are perfectly square on the rotor. sand them to deglaze, scotchbrite the rotors. I have a K0  like that. I haven't taken time to fix it since I haven't been riding it.
 If the new MC has a bigger piston it will take more force to make it work. If the lever isn't spongy the air is out.

[zeech]

Thanks Don I'll give that a try. The Nissin has a 14mm piston, and being a modern MC it has a smooth feel all the way through, even though it is applying maximum pressure.

[Oldtech]

The bore of the Nissin MC is probably too small. The 14mm is for a single disc. I have the same setup on two vintage Hondas but I use the 16mm (5/8 inch) bore and it brakes really well. In theory the smaller bore should give higher pressure in the lines but it doesn't move the same volume of fluid.

[zeech]

Interesting Oldtech, I was told this would be fine but maybe you're right. Do you happen to know what the stock dual brake MC size is? Is the banjo bolt still 10mm on the larger Nissin you have?

[bjbuchanan]

Thanks Don I'll give that a try. The Nissin has a 14mm piston, and being a modern MC it has a smooth feel all the way through, even though it is applying maximum pressure.

There is no such things as that 'smooth feel' all the way through. Even two finger brakes have some type of progressive force. If someone sold you on that or has tried to convince you of this, that is the first guy to not trust to know what direction to steer you in.  You only have that smoothness from power assist and you certainly don't have that
The MC isn't sized right or you have air in the lines or MC, somewhere.

Did you use fresh crush washers? Sometimes you can't really see a leak but air can make its way through that re-used washer.

[zeech]

Sorry bjbuchanan that must have been misleading, it has the smooth feel but definitely firms up near the grip. The fact that it's too small is probably the bigger issue. Yes I used fresh ones all tied the lever back all night and checked for leaks and there are none.

[scottly]

The bore of the Nissin MC is probably too small. The 14mm is for a single disc. I have the same setup on two vintage Hondas but I use the 16mm (5/8 inch) bore and it brakes really well.

Do you have 650 calipers? I've used a 14mm MC with both dual 38mm 750 calipers and 42.8mm late 750 calipers with good results.
Zeech, does the brake lever pull easily all the way against the handle bar? A brand new, dry MC can be VERY difficult to bleed!

[zeech]

Hey scottly, yes I have 650 calipers 38mm. It pulls easily almost all of the way there then stiffens up neat the end when you can almost pull no more before it wants ti hit the handle, should it stiffen up well before then? Any tips for really making sure the MC is bled? Can the lines be bled without the MC being fully bled?

[scottly]

Have you actually tried the brake, or are you going only by the feel at the lever?

[zeech]

I roll the bike down a small hill and it just keeps rolling, though braking is being applied, it does not stop the bike as it should.

I can see the calipers applying pressure to the pads but it's obviously not stopping. Perhaps all I need is a good deglaze and roughen on the rotors. Will try tonight and see if that does anything...

[scottly]

Not good. Don't bother with the deglaze. You should feel resistance in the lever within about 1/2 of it's travel. I'm guessing you still have air trapped in the system, most likely at the master?

[zeech]

I doubt it's in the lines as I have bled and bled and they are clean. I'll disconnect the MC and manually bleed it and make sure I guess.

[flybox1]

Keep bleeding.

[MiGhost]

Did you replace those old rubber brake lines with new braided lines?
The old rubber lines will bulge under pressure effectively reducing braking application.

[zeech]

flybox1 I've bled more than enough — MiGhost if you read my post you'll see I have steel braided lines.

I took the MC off and bench bled it to make sure, it all seemed fine. Then I took the calipers off to inspect them and found that the piston looked like it was getting hung up somehow on the arm that attaches to the fork— the arm was stuck in place not able to move freely which wasn't letting the piston move out further than it needed to. I pulled it apart, cleaned everything and put it back together and the arm was able to move freely again.

It seems like braking is now better, need to get my brand new leaky fork seals fixed now so I can really test it out....