I am having a terrible time with the rear brake of my son's bike. It was binding so I removed and clean the pistons, one was stuck solid. I then set about bleeding the system, I removed the master cylinder pushrod and used a crosshead to obtain full strokes. My problem is that it never stops producing air bubbles, I must have put a litre of fluid through. I've tried forcing fluid in the bleed nipple with a syringe until it fills the reservoir. There is still hardly any brake, I then tried to catch any surging bubbles with conventional bleeding from the rear pumping the pedal and it continues to produce air bubbles. I then took the master cylinder apart, the seals look fine. I soaked them on hot soapy water to plump them up a bit and reassembled. Repeating the above bleeding procedure, still loads of air. I'm out of ideas now. Any help would be gratefully received.

Cheers

Clint

Hi

I had the same problem. The pipe goes up then down on the bike and its hard to remove all the air

It is easier to remove all the brake system and bleed on your bench  hold the piston higher than the caliper. 

Once bleed refit.

Can this be done without breaking the line again to refit it? Seems a lot of work but I'm more than happy to do it if it works. My introduction to working on trials bikes is proving painful.

 He has told you completely backwards. You want to lift the caliper higher than the master cylinder. That way gravity is on your side. Get at least 3 feet of clear hose to fit the bleed nipple and drape over the rear fender. A vacuum bleeder is good just to catch the fluid. Again keep gravity working for you, this gets the fluid above everything. Pump the m/c until it starts up the hose. Burp the banjo bolt at the m/c to release trapped air. Be patient. Are you sure you have fixed the pistons?

 Two weeks ago my God Son broke off the bleed nipple the night before he left for a 2 day event. I stole a nipple off his Dads 525 KTM and bled it from the M/C.

https://advrider.com/f/threads/what-did-you-do-with-your-trials-bike-today.644122/page-645#post-40822158

Thanks guys, I'm a little confused though. lineway, you say hold the caliper at the highest point, but your photo shows it at the lowest point.

Hi

I can tell you the caliper needs to be at the lowest point i know it seems strange but it works.

Normally I would bleed then hold the lever in over night allowing air to travel up.

Working on bikes can be both rewarding and frustrating at the same time

Thanks Rustybay, this one is certainly frustrating, but I'm determined to get it done. I'll take the whole rear brake system off today and hang it from the reservoir and bleed again. Do you just leave it to gravity or pump the master cylinder?

5 hours ago, clintrandall said:

Thanks guys, I'm a little confused though. lineway, you say hold the caliper at the highest point, but your photo shows it at the lowest point.

 You actually have to read what I wrote. The nipple was broke off at the caliper, so I installed one at the MC. So you have to still bleed uphill. You cannot bleed down hill. Both of you are crazy. Crazy people will spend days bleeding a simple system.

Thank you Mr Lineway, that's very kind of you to clear up that little misunderstanding, it's much appreciated.

Hi

 

Yes you push the piston on I use a piece of wooden dwell  but you can use a screwdriver. 

Same technique open nipple push in close nipple. 

I also place a peace of wood 12mm wide in between the pads to stop the pots coming out to far.

Lineaway’s got you covered. Burp the banjo bolt at the top of the master cylinder with the system in place. Yeah it’s counter to what the manual says but it works because it’s the highest point in the system. Also pay attention to the centering of the disc between the pads. The pads only retreat a tiny distance when the brake is released and it’s possible to have an asymmetric actuation. In a “normal” system the disc is strong enough to make the actuation self centering but the disc on a trials bike is so flexible the system may not properly self center. Put a finger on the disc while actuating the brake. If there is any deflection of the disc a slight pressure opposite side while actuating the lever will help balance the caliper. Beta rear brakes have always been a pain but burping the banjo bolt helps immensely. 

From an earlier discussion

Cheers Dan, that's very helpful. I've just ordered a banjo bolt with a built in bleed nipple that should help do what you described.

On 9/24/2020 at 8:49 PM, clintrandall said:

Cheers Dan, that's very helpful. I've just ordered a banjo bolt with a built in bleed nipple that should help do what you described.

I don’t want to be too negative too soon, but will it fit? The one I tried fouled the air box. If you get a success, please give a reference to the part. I’ll be keen to get one myself.

I eventually got the air out of mine by removing the complete system from the bike a creating a straight uphill run for the air. A fair bit of dismantling, but worth it.