Rear Brake

This area is for discussion/information on the "modern" AJS motorcycles manufactured in China from 2006 onwards.
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ajscomboman
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Re: Rear Brake

Post by ajscomboman »

Dudley if by pumping the pedal went solid it is a definite indicator that there is air somewhere in the system, usually in or near the master cylinder it's not that common to have an air pocket in the caliper itself regardless of what people say. The other area for air pockets is a slight upward kink in the hydraulic hose. My Tiger is a bugger to bleed on the front discs because of this issue and a Lexmoto scooter I've just done had the same issue, very spongy with lots of travel. In the end I removed the caliper, wedged the pads, straightened out the hose and reverse bled the system pushing the fluid back into the master cylinder. Job sorted and a solid feel on the lever. Bleeding it in the conventional manner proved to be a waste of fluid and time.
56G80S
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Re: Rear Brake

Post by 56G80S »

I've been following this thread as I have a similar situation with the Yamaha Diversion. LIke Dudley, I rarely use the rear brake. When a pal mechanic finally drops off the 4DOT brake fluid he promised I'll try and follow Rob's approach. Looking at some websites the reverse bleed approach is the way to go.

Johnny B
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dave16mct
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Re: Rear Brake

Post by dave16mct »

Sometimes tying the lever closed overnight with the mastercylinder top loose works.
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Mick D
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Re: Rear Brake

Post by Mick D »

ajscomboman wrote:if by pumping the pedal went solid it is a definite indicator that there is air somewhere in the system,
Probably, but not the only possible cause:

If the self adjusting system in the caliper was faulty you will see the same effect - the self adjust relies on the piston seal sliding on the piston, if it sticks then the self adjust doesn't work, rapid pumping of the pedal can break the sticktion and restore function. The observed partial contact of the friction material tends to support this.

Regards Mick
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ajscomboman
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Re: Rear Brake

Post by ajscomboman »

Mick D wrote:
ajscomboman wrote:if by pumping the pedal went solid it is a definite indicator that there is air somewhere in the system,
Probably, but not the only possible cause:

If the self adjusting system in the caliper was faulty you will see the same effect - the self adjust relies on the piston seal sliding on the piston, if it sticks then the self adjust doesn't work, rapid pumping of the pedal can break the sticktion and restore function. The observed partial contact of the friction material tends to support this.

Regards Mick
But highly unlikely on a brand new caliper. I'd agree it can happen on calipers with pistons that are corroded or have stood for a while but Dudley has used his not so long ago.
39speedtwin
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Re: Rear Brake

Post by 39speedtwin »

Rob how did you reverse bleed?
I have never done this, possible cause is air between the rear brake cylinder and the front calliper.
Once I have tested it on Wednesday if still spongy I will bleed the front using the rear cylinder by conventional method.
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Re: Rear Brake

Post by Mick D »

ajscomboman wrote:But highly unlikely on a brand new caliper. I'd agree it can happen on calipers with pistons that are corroded
Not so, (the opposite actually) - a corroded piston tends not to retract, the pads wear more quickly and the brake may squeal / leak.

It's all speculation but on a new bike it shouldn't be air in the system unless the system has been disturbed since purchase.

Regards Mick
Stan Palmer
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Re: Rear Brake

Post by Stan Palmer »

I am watching this carefully in case something really good comes out. Bleeding small hydraulic cylinders has been a problem for years on my sons enduro bikes. We have tried all the ruses people suggest, including standing the bike near vertical for the rear. The one way bleed valve system didn't work for us. The reverse flush with a syringe showed promise, but the master cylinder is too small to allow a good whoosh of fluid fast enough to displace the air. We always get there in the end, but could do with a sure fire method. On hot days on fast tracks the brakes can go off mid event, I can only presume boiling of the fluid. We darent try to fix them mid race as it might put him out altogether.
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ajscomboman
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Re: Rear Brake

Post by ajscomboman »

Mick D wrote:
ajscomboman wrote:But highly unlikely on a brand new caliper. I'd agree it can happen on calipers with pistons that are corroded
Not so, (the opposite actually) - a corroded piston tends not to retract, the pads wear more quickly and the brake may squeal / leak.

It's all speculation but on a new bike it shouldn't be air in the system unless the system has been disturbed since purchase.

Regards Mick
Talking at cross purposes I think as I was agreeing to the sticktion. I agree that corroded pistons will stick and not retract once freed.

If pumping it made it solid only for it to go soft again then it is more than likely air. Just because a bike is new it doesn't mean it was ok on arrival.

Dudley, there are various calipers in use for split braking, the most common is the front caliper with 3 pistons and 2 bleed nipples and possibly a union with 2 take offs. Some have a remote splitter hidden and others have a master cylinder with 2 hoses taken directly off it. Usually you find the top and bottom piston is for the front caliper and the middle piston is linked to the rear and has a separate nipple. I'd need to see some pictures.
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Re: Rear Brake

Post by SPRIDDLER »

Some years ago I had a campervan and the footbrake pedal would 'give' as the pressure limiter for the rear brakes activated to ensure that they didn't lock up under heavy braking.
Bambi G3 trailer.jpg
I still haven't resolved the rear brake issue on my Chinese 250 Mutchless (un-adjustable long pedal travel - mentioned several posts above) so following this with interest.
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