rear brake master cylinder question


Information and questions on GL1200 Goldwings (1984-1987)
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superbill
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rear brake master cylinder question

Postby superbill » Thu Aug 25, 2016 4:46 pm



Hello all! I haven't posted on here a while, sorry for that. My Wing has been running pretty good till this issue.But I have an odd question to ask in regards to the rear brake master cylinder. I am currently cleaning and rebuilding it and am ready to put it back together however... I am wondering about the pressure control valve.

According to the repair manual ..."A pressure control valve is incorporated in the rear master cylinder. When refilling, take extreme care to keep foreign matter out. The pressure control valve cannot be disassembled. Do not disassemble."

So, after it was all cleaned out and dry, I put my mouth to it and blew, out of curiosity, to make sure everything was cleared out. (Don't judge me ..) There are 2 outlets that go from the master cylinder to the 2 brake lines. When I blew on the master cylinder, one outlet put out a lot more air than the other one.

My questions are these: Is that normal that one outlet lets out more air than the other? If this is normal, then why? How would I know if the pressure control valve is working or not?

The reason I'm rebuilding the rear master cylinder is because the rear brake lever was always soft. After measuring the rear rotor, I found out it was WAY below specs so I assumed that was the reason. But now, after the "blow test", I have more questions. Any help would be appreciated.



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twocams
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Re: rear brake master cylinder question

Postby twocams » Thu Aug 25, 2016 7:27 pm

Is that the pressure control valve built into the master cylinder? If so, sounds like it maybe working not letting to much air through. :shock:

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superbill
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Re: rear brake master cylinder question

Postby superbill » Thu Aug 25, 2016 7:33 pm

Yes, the pressure control valve in the master cylinder.

Could it possibly be that the one outlet that is letting out the least amount of air goes to the closer rear brake caliper, which wouldn't need as much pressure. And the one opening that's letting a lot more air out, needs to let out more because the connected front caliper is a lot farther away?

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twocams
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Re: rear brake master cylinder question

Postby twocams » Thu Aug 25, 2016 9:11 pm

Very possible. Is that the way you took it apart?
I went to a few auto schools some 40 yrs ago when I was in the Air Force. But seems to me the pressure control valve was first used with vehicles with rear drums and caliper fronts. That was to make sure the drum brakes were starting to apply before the front caliper was applied.

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superbill
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Re: rear brake master cylinder question

Postby superbill » Fri Aug 26, 2016 6:33 am

I took it apart like any other brake master cylinder. I always clear them out after cleaning with compressed air. But on some things I like to blow air through them myself so I can get an idea of how much air pressure is actually coming out. It's hard to tell sometimes with compressed air since it's so fast.

Still hoping someone has more info about the difference in pressure coming out of the 2 openings on this rear master cylinder... anybody?

superbill
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Re: rear brake master cylinder question

Postby superbill » Sun Aug 28, 2016 1:18 pm

Anybody out there have anything to add? Still hoping for some help here... thanks

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82aspen
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Re: rear brake master cylinder question

Postby 82aspen » Thu Sep 08, 2016 1:26 pm

superbill wrote:Yes, the pressure control valve in the master cylinder.

Could it possibly be that the one outlet that is letting out the least amount of air goes to the closer rear brake caliper, which wouldn't need as much pressure. And the one opening that's letting a lot more air out, needs to let out more because the connected front caliper is a lot farther away?


i'd think more goes to the front cos that's where most of braking happens

curious what the big deal is here? I'm not clear on your main concern?

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82aspen
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Re: rear brake master cylinder question

Postby 82aspen » Thu Sep 08, 2016 1:40 pm

superbill wrote:So, after it was all cleaned out and dry, I put my mouth to it and blew, out of curiosity, to make sure everything was cleared out. (Don't judge me ..) There are 2 outlets that go from the master cylinder to the 2 brake lines. When I blew on the master cylinder, one outlet put out a lot more air than the other one.

My questions are these: Is that normal that one outlet lets out more air than the other? If this is normal, then why? How would I know if the pressure control valve is working or not?


with linked brakes, the majority braking (with the pedal) is at the front, the rear needs to be proportioned so it doesn't lock up

superbill
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Re: rear brake master cylinder question

Postby superbill » Thu Sep 08, 2016 4:10 pm

Thanks 82aspen. That was just what I was thinking.... that one of the two outlets from the rear master cylinder would allow more fluid to be moved toward the front. And the other outlet that goes to the back brakes would send less fluid to them. Or put another way, that the 2 outlets would have two different levels of pressure.

My main question/worry was if what I was seeing at the rear master cylinder was correct.... 2 different levels of pressure coming out of the rear master cylinder. The manual states that the pressure valve in the rear master cylinder was not rebuildable. So I was wondering if what I was seeing was correct. I had no way of knowing how somebody would know if the pressure valve was bad or not.

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Re: rear brake master cylinder question

Postby Rusty Bike » Thu Sep 08, 2016 4:20 pm

Seat of the pants reasoning tells me that 82aspen has it right. Is the hole with less flow going to the rear?

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82aspen
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Re: rear brake master cylinder question

Postby 82aspen » Thu Sep 08, 2016 4:32 pm

i'm not aware of a test procedure for the rear m/c pressure control valve other than perhaps something like the rear easily locking up, there are no repairs as you mentioned, but, it can act up,

the main prob given is, one speck of dirt can be a game changer, foreign matter can come into the picture via neglect (old dirty fluid)

When refilling, take extreme care to keep foreign matter out

if in any doubt, about all you can do is clean flush, try/compare with another used one, you can also de-link bypassing the pressure control valve feature

a soft pedal is most often air in the lines, but original rubber lines & old seals from m/c to calipers can also all contribute
Last edited by 82aspen on Thu Sep 08, 2016 4:43 pm, edited 1 time in total.

superbill
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Re: rear brake master cylinder question

Postby superbill » Thu Sep 08, 2016 4:41 pm

Yes Rusty Bike, the less flow outlet leads to the rear.

And 82aspen, as you say, there is no real way to test the rear master cylinder other than comparing to another one. So with both of you agreeing with my perception that there should be 2 different pressures coming out of the rear master cylinder, I'm going to go ahead and reinstall it and assume it's working ok.

Thanks to both of you for your opinions/help. That was what I was looking for.




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