Faulty master cylinder?

[Its been a long time]

Having tried and failed miserably today to bleed the front brake on my bike, I think the new master cylinder I have might be faulty. Using a vacuum bleeder, I have put more than a litre of fluid through the front brake system but there's zero pressure building in the master cylinder. Everything in the brake system is new. It just will not pressurise at all. The vacuum bleeder is working just fine.

Please take a look at the linked video. As I pull the brake lever, the fluid is being ejected from the cylinder. That's not normal? Thanks.

Link to video. -

https://photos.app.goo.gl/7wX9DJ7ZZ9rRkJLq9

 

[yellowdog]

There should be a tiny hole in the master cylinder as well as the main hole check to see if its clear and not blocked up.

 

[yellowdog]

View attachment 323440[/ATTACH]

 

[Its been a long time]

There should be a tiny hole in the master cylinder as well as the main hole check to see if its clear and not blocked up.

Thanks. Yes the second hole is there.

 

[Its been a long time]

Just heard back from the shop I bought the new master cylinder from. They say they have sold around 500 of them with zero returns. Of course I could be the first with a bad master cylinder?

 

[SEd27]

My aftermarket master cylinder would not bleed.

It turns out that one of the tiny holes yellowdog mentioned above was blocked from the factory.

I ended up carefully chasing with a fine strand of wire to open up. The unit has worked great since.

 

[Its been a long time]

Well, failed again miserably to vacuum bleed the front brake after reverse filling it again through the bleed nipple. After another two and a half hours of bleeding I give up. The fluid flows clear but the master cylinder doesn't pressurise. As a final test I plugged the fluid outlet of the master cylinder. It pumps fluid but it doesn't pressurise. It sends a stream of bubbles though the smaller of the two holes inside the fluid reservoir. I have requested a return. See what happens next. Picture of master cylinder with outlet port plugged -

 

[yellowdog]

Try letting it sit overnight with the brake lever taped closed ,it can take 24 hrs to bleed apparently.

 

[Its been a long time]

Try letting it sit overnight with the brake lever taped closed ,it can take 24 hrs to bleed apparently.

Thanks. Tried that a while ago. Tied it back for best part of a week. No difference unfortunately.

 

[Wingedwheel]

If it wasn’t new I would say take it apart as it sounds like something is plugged but since it’s new definately get a replacement.

 

[Its been a long time]

If it wasn’t new I would say take it apart as it sounds like something is plugged but since it’s new definately get a replacement.

Agreed. But I don't think it's plugged. I think the U shape piston seals are backwards. That's what would happen if they are.

Any way, hats off to Yambits. They have shipped a new one with a postage paid returns label for the faulty one. So I can't complain too much. Sh!t happens but Yambits have stepped up on this one

 

[Jim]

I think the U shape piston seals are backwards.

I'd give odds that's the problem. Glad Yambits is stepping up.

 

[Its been a long time]

I'd give odds that's the problem. Glad Yambits is stepping up.

Indeed. I already have the postage tracking number. New master cylinder should arrive probably Saturday morning. Terrific response from Yambits really.

 

[Grimly]

They say they have sold around 500 of them with zero returns.

As soon as a retailer says that to me, I immediately think of them as lying abstards.

 

[Its been a long time]

As soon as a retailer says that to me, I immediately think of them as lying abstards.

I would tend to agree. However, I do give Yambits the benefit of the doubt here. And I'll have a new replacement master cylinder here by Saturday morning.

 

[Its been a long time]

Great service from Yambits. I received the replacement master cylinder by 10.30 this morning. I'll follow up this when I have worked on the brake again.

 

[gggGary]

Did you rebuild the caliper?
With time the piston tend's to stick to the seal in the caliper body. The master cylinder then moves the piston but the seal just stretches and pulls the piston right back where it was before the lever was pressed, result you can never build up pressure to move the piston out to the rotor. Disassemble, remove seal, clean out all the glop behind it, that's part of every disk brake overhaul.

 

[Its been a long time]

Did you rebuild the caliper?
With time the piston tend s to stick to the seal in the caliper body. THe master cylinder then moves the piston but the seal just stretches and pulls the piston right back where it was before the lever was pressed result you can never build up pressure to move the piston out to the rotor. Disassemble, remove seal, clean out all the glop behind it, that's part of every disk brake overhaul.

Everything is new on the front caliper except for the body casting. All new internal parts and bleed nipple. New brake line, bolts and washers. It's domestic chores today, I'll report back on this soon.

Thanks again for the interest.