Air in front brake system - BMW Luxury Touring Community
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post #1 of 5 Old Sep 3rd, 2013, 4:20 pm Thread Starter
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Air in front brake system

I flushed the brake fluids last January, 8 months ago. I made the mistake of letting the front brake reservoir run dry and pumped air into the system. After flushing a quart of fluid through the system, the brakes were still spongy. So, I pumped it up and strapped a zip tie on to hold it in place. The next morning, the brakes were operating as they should.

2000 miles later - I took a trip and ended up on Tioga Rd. (10,000ft). On the way down the hill, the brakes all but disappeared. When I grabbed the front brake, it would engage, but the handle would quickly bury itself into the hand grip and was forced to pump the brake continuously. 2 days later, I was staying at about 150ft above sea level and for the next 4 days would daily grab the front brake handle, just to see if it was still acting up. On day 4, the brake was back.

I had no ABS flashing and think the problem is residual air in the brake line coupled with an extreme altitude change.If the problem hadn't have disappeared, my next step would have been to reverse flush the front brake. Because without pulling off the tupperware to check the path of the brake line, it seemed to be the simplest way to remove any residual air. I'm wondering if any one else has run into this.
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post #2 of 5 Old Sep 3rd, 2013, 6:36 pm
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Re: Air in front brake system

What year is your bike?
On my 05 there are separate circuits for the wheels and the controls, so this may be your issue...or not, depending on the model you have.

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post #3 of 5 Old Sep 4th, 2013, 1:19 am Thread Starter
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Re: Air in front brake system

It's a 99 and has separate brake circuits, With only a few more months to go until the next winter service, I'll re-bleed it. I'm fairly sure it's altitude related and won't be riding over 7000' until then, It's fine at sea level and the mountain passes around the tahoe basin,

Forgot to mention, there was no fluid loss. I did consider the possibility of a failing master cylinder or a bit of debris in the system

Thanks for the reply, TB
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post #4 of 5 Old Sep 4th, 2013, 9:19 am
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Re: Air in front brake system

it's quite possible the master cylinder piston will not return when lever released, on my 85 similar problem replaced piston, cleaned bore, piston was sticking part way in, the lever does not pull the piston out

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post #5 of 5 Old Sep 4th, 2013, 5:49 pm Thread Starter
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Re: Air in front brake system

Gary, thanks for the info. If the problem re-occurs, I'll reverse flush and then look into the master cylinder.
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