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Discussion Starter · #1 · (Edited)
So i'm having trouble with brakes. My set up is:

SN95 Calipers and discs all the way around.
Forte's braided lines.
FFR metal lines.
Whitby Booster and Master.
Villager Reservoir.
SN95 pedal box.

I bench bled the master in the car until no air was showing in the reservoir. Then we filled the brake lines keeping the master topped off. I've bled the lines 3 times on 3 different days and there is no sign of air at the calipers. However, the brakes are horrible. Barely stops the car. So yesterday we looked at the master again. There was a lot of air at the clear plastic elbow "A." There was a single air bubble at "B." So we put 5" of vacuum on the reservoir and the bubble was quickly removed from "B" however "A" is just constantly sucking air in. We can't find any leaks in the lines anywhere. The air coming in through "A" was never ending. It's not like a little air trapped, it was like it was being pulled in. Could it be a bad master? Or could it be pulled in where the plastic slide in the master? Any ideas?

 

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If its pulling air from inside the MC then it would also push fluid out through the MC and you would have a leak. It would be wet by your booster.

Its likely entering between the plastic fitting and the MC.
 

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Discussion Starter · #4 ·
If its pulling air from inside the MC then it would also push fluid out through the MC and you would have a leak. It would be wet by your booster.

Its likely entering between the plastic fitting and the MC.
I pulled the master off the booster and it's bone dry.

Pulling a vacuum from the reservoir certainly could cause air in the rear port. The seals on the master cylinder are cup shaped, designed to create pressure, not create a vacuum seal.
I bench bled the master and then bled the lines. After a test drive I noticed air in the plastic elbows. Where would it come from? Should I bleed the master again?
 

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Pulling a vacuum from the reservoir certainly could cause air in the rear port. The seals on the master cylinder are cup shaped, designed to create pressure, not create a vacuum seal.
OK, you say he's pulling air from there as a matter of design, what's the solution for that?:confused1:
 

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Mark 3.1 (Sold)
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Air rises to the top. The bleeder needs to be at the top, and the brake line at the bottom of the caliper.
I have had 100% success using a hand vacuum pump to fill brake and clutch lines. If you have a vacuum pump, I would recommend you use it (at the caliper bleed ports) to fill your brake system.
DD
 

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Discussion Starter · #9 · (Edited)
If you have a vacuum pump, I would recommend you use it (at the caliper bleed ports) to fill your brake system.
DD
To "fill" it using a vacuum pump, you mean put the vacuum at the bleeder and pull the fluid from the master to each caliper?
 

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FFCobra Craftsman
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I don't understand why air bubbles in those plastic fittings wouldn't just rise on it's own to the reservoir and disappear. I agree pulling vacuum on the reservoir could suck air in at the pushrod but there would be no fluid leaking out. That usually ocurs w/ a worn seal that leaks both ways. I can't figure how driving the car would produce those bubbles but I am beginning to suspect a bad MC.
 

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Discussion Starter · #11 ·
I don't understand why air bubbles in those plastic fittings wouldn't just rise on it's own to the reservoir and disappear. I agree pulling vacuum on the reservoir could suck air in at the pushrod but there would be no fluid leaking out. That usually ocurs w/ a worn seal that leaks both ways. I can't figure how driving the car would produce those bubbles but I am beginning to suspect a bad MC.
That's why I am lost as well. In the "A" fitting, you can see that air as the blue fluid is absent in the elbow.
 

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I don't understand why air bubbles in those plastic fittings wouldn't just rise on it's own to the reservoir and disappear. I agree pulling vacuum on the reservoir could suck air in at the pushrod but there would be no fluid leaking out. That usually ocurs w/ a worn seal that leaks both ways. I can't figure how driving the car would produce those bubbles but I am beginning to suspect a bad MC.
Again, in theory, if you believe the piston seals are like Coleman lantern pump plungers, it could pull air if the vacuum overcame the piston seal. Here is the piston from my MC. Not quite cup shaped.


Has anyone out there ever seen this happen? I haven't. If, big if, Clemson's MC is pulling air from the piston, something is wrong with it and he needs a new MC.

When you bench bled the MC, did you leave the reservoirs connected when you installed?
 

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Well, let me see if I remember this correctly. Like in the pic Tom put up, and my crude drawing, when you apply the brakes the pressure cup will actually expand a bit on the fluid end causing the cup to have an even better seal on the outer wall of the master cylinder. This is made this way to help with sealing under greater pressure. If you put a vacuum on the reservoir you can pull small amounts of air past this cup, but if it’s air, where is it coming from it would have to get past the other seals at the other end first and i think one of those is made with a cup form to help keep that from happening? There would have to be a crack somewhere in the master cylinder or all of these seals would have to be bad to allow air in at any point along the way to get air back to the reservoir. This would in turn be indicated by a leak in fluid at that point. I’m putting my money on the air entering at the point where the plastic “plug” inserts into the master cylinder. That’s the first place I would be looking. As for the air staying at that point, it’s hard to see in the pics you posted, but it almost looks like the master cylinder is at a slight angle outward, the hoses you have connecting the master to the reservoir don’t have much rise to them, and that the plastic inserts on the master cylinder have a slight dome at the point the turn is made to go out to the hose connection. If the inserts have a slight dome here, it’s possible there is a slight vacuum here not allowing the air to move on up to the reservoir. Once you put a vacuum on the reservoir, that air bubble takes off and up the tube it goes. Here is a test for this, take some very soapy water (lots of bubbles) dab it around the master cylinder at the point where the plastic inserts meet the master, then put your vacuum back on the reservoir. If there is a leek here, the bubbles will vanish, fast……
Just my thoughts.
 

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Here is a little better (but still crude) drawing. It kind of gives an idea as to where the air would have to be coming from to enter the master. Like someone else suggested, if you have pulled the master and it is dry around the dust boot area, and you see no fluid anywhere else, I would start at the plugs on top for the reservoir. If the pressure seal was bad, it would cause a soft/spongy peddle due to a lack of pressure building, but you could only pull air here if all of the seals allowed a bypass. I guess that’s possible with enough vacuum and not have a fluid leak anywhere, but I would think if you did this a couple of times, you would develop a leak at the seals. I would start with the soap test to eliminate the adapter plugs and go from there.
Just some more of my two cents.
:confused1:
 
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