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Air in Master Cylinder?

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Old 05-16-2018, 01:37 PM
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Air in Master Cylinder?

Hope someone can help with this as I am a bit stumped.

I'm having an issue with both rear calipers which I purchased recently along with new hoses. When I try to bleed the rears, I am unable to get any fluid out of the bleeders. The brake pedal does not depress all the way to the floor when I release/open the bleeder valve on each caliper.

When I loosen the brake hoses to the calipers, fluid does come streaming out when depressing the brake pedal, on both passenger and drivers side.

My question is , is this a symptom of the Master Cyl having air trapped or am I overlooking some other problem?

edit: to add further, the fronts also have rebuilt calipers and bleed fine. I'm suspecting air in the MC since I noticed the reservoir was empty - had to have have drained when the old calipers were removed and the car just sitting for a length of time

Last edited by x55Cam; 05-16-2018 at 02:24 PM. Reason: added more info
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Old 05-16-2018, 09:42 PM
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Re: Air in Master Cylinder?

Have you tried removing the bleeders and see what happens?
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Old 05-17-2018, 12:53 AM
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Re: Air in Master Cylinder?

Originally Posted by firestorm View Post
Have you tried removing the bleeders and see what happens?
Just bleeding the calipers without the bleeders? I actually thought about trying that. Will give that a shot, running out of possibilities but that's a good way to get more air out of the system, if it's the issue.
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Old 05-17-2018, 07:32 AM
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Re: Air in Master Cylinder?

If the MC was empty for the rear, the quickest and best way to overcome this dilemma is to take 10 minutes and remove the MC and bench bleed it and then reinstall. You can try to bleed till the cows come home for the next ten years and will NEVER get the air outta the MC.
Try it. Its quick and very easy to do.
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Old 05-17-2018, 01:51 PM
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Re: Air in Master Cylinder?

Yeah, I tried bleeding while in the car and the rear is just not pushing out the volume of fluid like one would expect.

Edit: and when I do that paper clip test for the combination valve to check to see movement, it doesn't push out the clip.

Last edited by x55Cam; 05-17-2018 at 03:23 PM.
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Old 05-19-2018, 04:43 PM
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Re: Air in Master Cylinder?

Ok, here's an update and what appears to be the 'fix', although there are some peculiar observations that I will explain in a bit.

Had to remove the master cyl from the car and bench bleed it to make sure the air was completely out. As was witnessed doing it this way, air was still inside. Bench bleeding made the difference. Pushing the plunger in the MC took about 20 times before the air was completely removed. Still not sure why putting it on the bench as opposed to leaving it in the car and performing this operation made the difference?

Before the MC bleeding, while having a partner hold the brake pedal, I was still able to turn the rear rotors by hand, no wheels or rims on. After the MC bleed, and re-bleeding the rears. even with a pry bar wedged in , using 2 lug nuts to protect the studs, the rotors did not budge an inch.

Anyway, it did help with allowing the backs to be bled. But my experience with the way I usually notice when bleeding brakes was a bit odd for this car.

- When opening the bleeder valve of the rears, either side, the pedal did not go all the way down to near the end of it's max play. Not even close. Never experienced this before when bleeding brakes on other vehicles I have owned. Does it have something to do with the proportioning valve? I did replace the spring in the proportioning valve for a stiffer one, as has been suggested in other threads, during this whole process of trying to figure this problem out, if it ever was a problem.

- The test where you insert a paper clip in the proportion valve to detect movement when depressing the brake pedal, failed miserably, if this test is a legitimate way to determine if the valve is working properly? Has any one tried this?

Interesting experiencing with bleeding IROC brakes with the J65 option, in any case.

Last edited by x55Cam; 05-19-2018 at 05:27 PM. Reason: spelling
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