bleeding the breaks... info please
Race Director
Joined: May 2003
Posts: 10,956
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From: California
I think this may be it. I have had the car since almost new, and the original brakes worked great... until one day I had to slam on them and i heard and felt some sort of pop... and ever since then no mater what I do or replace I have that long peddle travel, I have even replaced the peddles themselves. cay you ben the rod? is there a clip somewhere i may have broken? GOD, if their were a C3 brake expert in so cal I would go to him ASAP... got any ideas?
Team Owner
Joined: Mar 2002
Posts: 24,160
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A good, properly functioning proportion valve (misnomer.. it's only a switch)
Have you ever eliminated this valve/switch from any cars? It seems to be a problematic component from what I've read.
Team Owner
Joined: Mar 2002
Posts: 24,160
Likes: 78
I think this may be it. I have had the car since almost new, and the original brakes worked great... until one day I had to slam on them and i heard and felt some sort of pop... and ever since then no mater what I do or replace I have that long peddle travel, I have even replaced the peddles themselves. cay you ben the rod? is there a clip somewhere i may have broken? GOD, if their were a C3 brake expert in so cal I would go to him ASAP... got any ideas?
Burning Brakes
Joined: Jun 2008
Posts: 1,086
Likes: 1
From: Michigan
I don't know about the newer C3's but I'm trying to recall from 35 years ago when I had a '70. It seems the master cylinder was divided into two halfs with a dam between the halfs. The piston also had two plungers. The idea was that one half of the MC and one seal on the plunger operated the front brakes and the other half and other plunger operated the rear brakes. The idea was that if one half ran out of fluid or there was a leak in the system, the othe half (front or back) would still operate so you could stop the car.
If he blew a seal wouldn't he be stopping on just one set of brakes?
If he blew a seal wouldn't he be stopping on just one set of brakes?
Team Owner
Joined: Mar 2002
Posts: 24,160
Likes: 78
I don't know about the newer C3's but I'm trying to recall from 35 years ago when I had a '70. It seems the master cylinder was divided into two halfs with a dam between the halfs. The piston also had two plungers. The idea was that one half of the MC and one seal on the plunger operated the front brakes and the other half and other plunger operated the rear brakes. The idea was that if one half ran out of fluid or there was a leak in the system, the othe half (front or back) would still operate so you could stop the car.
If he blew a seal wouldn't he be stopping on just one set of brakes?
If he blew a seal wouldn't he be stopping on just one set of brakes?
You're correct on your interpretation of the piston. But what happens when the front seal goes out is the back seal is now being used to pressurize both lines. That's what gives you excessive pedal travel.
Race Director
Joined: Mar 2006
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From: Florida
No, there's a junction block (the switch/proportioning valve) bolted to the frame that both lines dump into.
You're correct on your interpretation of the piston. But what happens when the front seal goes out is the back seal is now being used to pressurize both lines. That's what gives you excessive pedal travel.
You're correct on your interpretation of the piston. But what happens when the front seal goes out is the back seal is now being used to pressurize both lines. That's what gives you excessive pedal travel.
And if you have 1 master seal go out and also a calipers seal go out on the other half, then you would have no brakes at all.
The proportioning valve or switch combo is soley for regulating the hydraulic pressure for front and back and much more so on a car with rear drum brakes. The off centering of the valve is just a switch that alerts you to a brake failure. You could subsitiute an adjustable valve for the rear system and control your own proportioning.
The dual cylinder master effectively is what creates 2 separate brake systems, a front and a rear system complete with separate plumbing from the master to the paired calipers.
Team Owner
Joined: Mar 2002
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It's that alone which makes me leary of eliminating it though I know several guys that have.
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Race Director
Joined: May 2003
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From: California
I heard that there is a peddle or rod adjustment, but I can not find or see any..
p
Race Director
Joined: Mar 2006
Posts: 14,112
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From: Florida
The properly working proportioning valve/pressure differential switch/combo valve, (is commonly misnamed) is not designed to separate the front and rear systems in case of failure. That is done with the master cylinder and the 2 separate circuits alone.
On some valves but not all, the spool can block off the failing side, but that is independant of the operating side. There should never be any fluid passage from front system to the rear system thru this valve.
that is why this is not correct.
On some valves but not all, the spool can block off the failing side, but that is independant of the operating side. There should never be any fluid passage from front system to the rear system thru this valve.
that is why this is not correct.
Race Director
Joined: Mar 2006
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From: Florida
I have never seen a rod bend from use, but anything is possible.
Race Director
Joined: Feb 1999
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From: In Dreams There Is Truth Ohio
I am going to make this a sticky so no more bleed questions are asked.
For what it's worth...I start the bleeding process with air and then let the gravity take over....
For what it's worth...I start the bleeding process with air and then let the gravity take over....
Last edited by GDaina; Mar 20, 2009 at 03:10 PM.
Team Owner
Joined: Mar 2002
Posts: 24,160
Likes: 78
The properly working proportioning valve/pressure differential switch/combo valve, (is commonly misnamed) is not designed to separate the front and rear systems in case of failure. That is done with the master cylinder and the 2 separate circuits alone.
On some valves but not all, the spool can block off the failing side, but that is independant of the operating side. There should never be any fluid passage from front system to the rear system thru this valve.
that is why this is not correct.
On some valves but not all, the spool can block off the failing side, but that is independant of the operating side. There should never be any fluid passage from front system to the rear system thru this valve.
that is why this is not correct.
You are however correct that there is no fluid passage front to rear through the valve. That was my misunderstanding.
Last edited by wcsinx; Mar 20, 2009 at 03:36 PM.
Race Director
Joined: May 2003
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From: California
To test the mc installed on the car, remove the lines from mc, get some inverted flare plugs or make them out of welded up brake lines, and stand on the brake pedal. It should be very hard, stay there and be at the top. If not at the top, try and look under the dash and see how much rod movement there is before it engages.
I have never seen a rod bend from use, but anything is possible.
I have never seen a rod bend from use, but anything is possible.
Team Owner
Joined: Mar 2002
Posts: 24,160
Likes: 78
Not really
So replacing the MC had no effect on the pedal travel?
And this only happened after slamming on the brakes, right? I ask because excessive pedal travel can also be caused by mismatched boosters and MCs, but that would've been apparent immediately upon installation. Pull the MC off. You don't need to crack the hard lines, just take it loose and push it far enough away that you can see the booster output rod. Now get someone to sit in the driver's seat and push the brake pedal. As soon as they touch the pedal, you should see that rod move. If not, then we'll go from there.
So replacing the MC had no effect on the pedal travel?
And this only happened after slamming on the brakes, right? I ask because excessive pedal travel can also be caused by mismatched boosters and MCs, but that would've been apparent immediately upon installation. Pull the MC off. You don't need to crack the hard lines, just take it loose and push it far enough away that you can see the booster output rod. Now get someone to sit in the driver's seat and push the brake pedal. As soon as they touch the pedal, you should see that rod move. If not, then we'll go from there.
Last edited by wcsinx; Mar 20, 2009 at 04:24 PM.
Race Director
Joined: Mar 2006
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From: Florida
It isolates failure to the front or rear in the case of a slave cylinder failure. But if the forward seal of the MC goes out, then yes you are pressurizing both lines with the rear seal. The 2 hydraulic circuits are no longer separated at that point.
You are however correct that there is no fluid passage front to rear through the valve. That was my misunderstanding.
You are however correct that there is no fluid passage front to rear through the valve. That was my misunderstanding.
That valve has nothing to do with failure other than to power an idiot lite on the dash.
For the dual system operation take a look at this and pay particular attention to the parts that say
Primary failure and
Secondary failure
Also note that there are at least 1 additional seal(s) whose only purpose is separating the primary and secondary chambers in case of a failure of the other.
http://www.autoshop101.com/forms/brake02.pdf
Race Director
Joined: Mar 2006
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From: Florida
Race Director
Joined: May 2003
Posts: 10,956
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From: California
.... but I have looked and see NOTHING to adjust.
I have had the whole dash apart.... where is this secret adjustment? thx
Team Owner
Joined: Mar 2002
Posts: 24,160
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1. To turn on the dash light
2. To shut off the low pressure circuit
This often causes difficulty in bleeding the brakes when people slam the pedal down, throw the piston off center, and then can't figure out why they're getting no flow to the front/rear.
Go read Tom454's post on the first page. He explained it quite well.
Originally Posted by Tom454
A good, properly functioning proportion valve (misnomer.. it's only a switch) will not allow you to bleed the brakes one line at a time. The principle of operation is such that when one line/caliper leaks (or is open for bleeding), hydraulic pressure (M/C-pedal pressure) will cause the valve's internal piston to slide to one side, activating the dash brake light, and closing off the opposite (front or rear) fluid circuit, rendering that closed off circuit un-bleedable. When this happens, the closed off half of the valve will allow little or no fluid to be bled through that circuit (front or rear).
For the dual system operation take a look at this and pay particular attention to the parts that say
Primary failure and
Secondary failure
Also note that there are at least 1 additional seal(s) whose only purpose is separating the primary and secondary chambers in case of a failure of the other.
http://www.autoshop101.com/forms/brake02.pdf
Primary failure and
Secondary failure
Also note that there are at least 1 additional seal(s) whose only purpose is separating the primary and secondary chambers in case of a failure of the other.
http://www.autoshop101.com/forms/brake02.pdf
Here is a picture of the MC piston ...
Notice there are 2 and only 2 seals. If the front seal goes out, then you have lost the isolation of the front and rear circuits.
Last edited by wcsinx; Mar 20, 2009 at 09:04 PM.
Team Owner
Joined: Mar 2002
Posts: 24,160
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Again, if it were an adjustment issue, why did it only show up after a panic stop? I really think you're barking up the wrong tree here.