Air in Rear brake - MVAgusta.net
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post #1 of 19 (permalink) Old 10-26-2019, 06:43 AM Thread Starter
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Air in Rear brake

Every few months I have to bleed the air out of my rear brake just so I can hold the bike on hills.


I know it's common on these bikes, but brakes are simple, there must be a solution

I've bled the air out of mine till they're nice and firm and working well 6 times, it only lasts a few rides before there's air in the system again and I'm putting all my weight on the pedal to hold me on hill starts.
The air has to be getting in there from a faulty point somewhere.

Has anybody had any luck with a master cylinder re-build kit ? (can you get them ?)

2016 Brutale 800

Currently in the shed: XV750 Custom Cafe, 2019 CB500x. 1974 Honda CL450. Yamaha IT175 J. and a couple of Chinese bikes.
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post #2 of 19 (permalink) Old 10-26-2019, 09:42 AM
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Lots of dialogue on this One67, in previous discussions, and I don't think with much success. Mine's still rubbish, but I'm always curious as to what people try.
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post #3 of 19 (permalink) Old 10-26-2019, 10:05 PM Thread Starter
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I spoke to a mechanic a while ago, he said he thinks the master cylinder is too close to the engine and the fluid is getting too hot.
So I flushed out the old fluid and replaced it with dot 5.1 high temp brake fluid. Only lasted a few rides, so no difference.

I checked the temperature of the master cylinder after a ride, and it was only 42 degrees C, So it's definitely not over heating.

Is it a Brembo master on the rear ? or a Nissin like the front brakes.

2016 Brutale 800

Currently in the shed: XV750 Custom Cafe, 2019 CB500x. 1974 Honda CL450. Yamaha IT175 J. and a couple of Chinese bikes.
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post #4 of 19 (permalink) Old 10-27-2019, 04:18 AM
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Mine (TV) was non existent till I had it serviced 2 weeks ago and twas fixed there , I believe it had new seals fitted I can try and find invoice for more advice ?
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post #5 of 19 (permalink) Old 10-27-2019, 04:19 AM
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one67,

Can't comment on your model. However MV rear brakes aren't crash hot, depending on your point of view. Which is either you don't use them at all, use them track wise to steady the bike or as a good percentage of normal braking. In the latter case they're pretty crap.

Check the pads, they wear really fast on the rear. This sees the (piston/s) protrude heaps, making bleeding hard to keep constant. Renew the pads so the caliper pistons are more seated in the calipers. Push the pistons back into the calipers and then bleed. Wedge the brake pads as per the Knurl's recent post.

Yes the rear master cylinder gets engine/exhaust heat heat but you're onto that.

Cheers

Deano

2009 1078RR Brutale
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post #6 of 19 (permalink) Old 10-27-2019, 08:13 AM
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Quote:
Originally Posted by one67 View Post
I spoke to a mechanic a while ago, he said he thinks the master cylinder is too close to the engine and the fluid is getting too hot.
Your mechanic is not correct.

If you look at what has been done with past rear brake master cylinders, you will find previous modes have the master right next to the catalytic convertor. This does impart quite a bit of heat, yet those rear brakes do not become air bound as frequently.

The B3 I have, 2015 800RR, has the exhaust routed to the opposite side of the motor and there is quite a bit of air space for heat dissipation between the master and the oil sump. It is not heat.

It is annoying, I know. I have had better longevity with removing the rear caliper and bleeding it in a position where it is held so that the bleed valve is the highest point.

Is your bike an ABS version?

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post #7 of 19 (permalink) Old 10-27-2019, 10:26 AM
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Silentservice is right, your mechanic is an idiot.

Also, never use DOT 5 on the street. 5 is meant exclusively for racing, where you flush and R&R your systems regularly.

Go back to Dot 4, check for leaks and bleed the rear caliper with a MityVac or similar vacuum pump and hold the caliper upside down so all of the air moves more easily to the top of the caliper.

I have never had a problem with mine. It is stock with correct Dot 4 and the reservoir is in the stock locale.
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post #8 of 19 (permalink) Old 10-27-2019, 10:33 AM
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Quote:
Originally Posted by laducsp View Post
Silentservice is right, your mechanic is an idiot.

Also, never use DOT 5 on the street. 5 is meant exclusively for racing, where you flush and R&R your systems regularly.
DOT 5 is very different from DOT 5.1

DOT 5.1 is fine, although it tends to attract moisture faster than DOT 4.
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post #9 of 19 (permalink) Old 10-27-2019, 03:30 PM
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see tech bulletin
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File Type: pdf 8645_Service Bulletin n° 189 - Rear brake bleeding procedure.pdf (656.5 KB, 38 views)

Faster and Faster untill the fear of death subsides.....
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post #10 of 19 (permalink) Old 10-28-2019, 12:21 AM Thread Starter
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Quote:
Originally Posted by Pablotv View Post
Mine (TV) was non existent till I had it serviced 2 weeks ago and twas fixed there , I believe it had new seals fitted I can try and find invoice for more advice ?
Thanks Pablo, let me know if it holds it's pressure. I'm interested to know if they did replace the seals in the master or the slave.

The mechanic I spoke to is not my mechanic, as I've been doing my own spannering for over 35 years.
I don't think he's an idiot either, he's just clutching at straws and trying to be helpful like the rest of us.

I'm not having any trouble bleeding the brakes, I can get all the air out till the pedal is as firm as it should be.
It just doesn't stay that way and continues to draw air in from somewhere.

I want to know where.

2016 Brutale 800

Currently in the shed: XV750 Custom Cafe, 2019 CB500x. 1974 Honda CL450. Yamaha IT175 J. and a couple of Chinese bikes.
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