Piston Travel

[Richmond]

I recently rebuilt my caliper with all new seals (piston went in easily), got a steel braided brake hose, and got a new generic master cylinder off of ebay with a 14mm bore. I bench bled the MC and it worked with no air bubbles.

I hooked everything up and put on a speed bleeder and used that. Air came out, worked great. Now with the bleeder valve open if i pull the lever, a good amount of fluid gushes up the tube i have connected to the bleeder. Very solid very consistent.

The piston however, does not push far enough for the pads to apply pressure to the rotors. If i hold the caliper against the rotor, the piston will push back when the lever is pulled but it is like the piston puck needs to be out further just in general. Any ideas on how to do this? Should it do it on its own after some time?

 

[Greasey Fingers]

Sounds weird, the piston should ajust itself and finds its own position when you pump the brakes after you bleed and top up the MC and have everything tightened off. You have the caliper mounted properly I assume? Maybe put up a pic or even better a video. Im sure its a simple issue and you will fix it easily If everything is mounted properly and bled and locked off properly not much can really go wrong, its a very simple system.

EDIT: The only other possibility I can think of is the piston is the correct diameter but the wrong depth.

 

[drewpy]

as greasey says, it should push out and stay there. Even if the piston was too short it would pop out anyway. give the brake a really good squeeze, maybe it can't get past the piston seal and its rolling (which it suppose to do)

is the splooge hole blocked in the MC? it could set up a vacuum and return the piston more than it should

 

[Richmond]

Okay, I'm kind of on a short riding trip (sketchy I know) but I will post a video when I get home.

I have an aftermarket MC and I haven't specifically looked for a splooge hole but is it possible that mine doesn't have that or something? The brake system doesn't have air in it and it totally pushes the piston out but it needs to like sit 1.5mm tighter for it to engage the rotor. When I bench bled the MC I only noticed bubbles to come out of one hole at first? I'll post a video when I get home.

 

[Richmond]

Okay, I'm kind of on a short riding trip (sketchy I know) but I will post a video when I get home.

I have an aftermarket MC and I haven't specifically looked for a splooge hole but is it possible that mine doesn't have that or something? The brake system doesn't have air in it and it totally pushes the piston out but it needs to like sit 1.5mm tighter for it to engage the rotor. When I bench bled the MC I only noticed bubbles to come out of one hole at first? I'll post a video when I get home.

 

[Richmond]

Here is a video of the problem:

Any help appreciated!!

 

[Richmond]

On my brothers kz 440 he totally squeezed the piston in and then within 3 pumps on the lever it was brake tight...

 

[16VGTIDave]

There is a problem with the master. It isn't allowing fluid to stay in the caliper. I'd be checking for a blocked passage, or possibly there is still air in the system. Try pressure bleeding it first - connect a clear line to the bleeder screw and route it upwards a bit before going into a container. Squeeze the lever and maintain pressure, then crack open the bleeder screw. After any pressure has been released from the system, close the bleeder screw before releasing the lever. Repeat until no air is seen in the clear tube, then do it a few more times. If that works, repeat the process in a couple of days.

 

[Richmond]

Yeah I did that with the bleeding. It would pump the fluid up the line, then a couple bubbles. It hasn't pushed bubbles in a week or so though and I've checked it on a couple occasions. I asked my dad about it and he said that maybe the stock bike had a residule pressure valve in the MC so that not all of the fluid would return... Anyone have any idea if this is the case?

 

[16VGTIDave]

That would be interesting. I've only ever encountered residual pressure valves on hydraulic drum brakes. Keeping residual pressure on a caliper would have the brakes dragging all the time. Time to take a closer look at the master...

 

[Richmond]

Hmmm.. Im just running out of reasons it wont engage!
My dads was similar but after about 60 miles he said it started to apply pressure.

 

[16VGTIDave]

That is almost correct. In your video, the master is hardly pushing fluid to the caliper, and the caliper piston is retracting when you release the lever. Either the master is sucking the fluid back, or there is still air in the system and the piston seal is doing what it is supposed to and retracting the piston.

Have you checked to make sure that there is free play at the lever? If the piston in the master isn't fully returning, it won't open the fluid transfer ports. The adjustment screw on the lever is NOT for adjusting the lever reach, it is for adjusting free play. There should be a noticeable, but small, amount of play in the lever before the screw contacts the piston in the master.

 

[Richmond]

Um well Dave still really confused here but I guess in a good way? haha So I rebuild the whole front brake system, ride 550 miles with it not working at all, Doesn't work when I get home, I rebuild the clutch yesterday and yesterday evening I read your comment and went in the garage to contemplate. I squeezed the lever and went to fell how much play there was.... And it decided that now it wanted to engage, and sooo the front brake works. I'm pretty sure it will tighter up more on its own but for the moment I'm not going to question my confusion

 

[Richmond]

And thank you for all of the input! So grateful!