Air in Master Cylinder and D2D Conversion - Pontiac GTO Forum
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post #1 of 7 (permalink) Old 09-01-2015, 09:55 AM Thread Starter
 
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Air in Master Cylinder and D2D Conversion

Hello, I installed RSD's Drum to Disc conversion kit and their complete brake line kit on our 68 GTO (non-power). After the installation, I bled the brakes at all wheels and the air seemed to be all out, but the pedal braking is spongy. I did the bench bleed procedure on the Master Cylinder before I put it in, but I had difficulty installing it since the proportioning valve and brake lines were coming in contact with the wheel well. I finally got it installed, but my guess is in the process air may have got back into the master cylinder. Is there a procedure to get the air out of the master cylinder without removing it from the vehicle? If I simply continue bleeding at one wheel like the driver's side front closest to the master cylinder, will the air eventually come out, or would I need to disconnect the lines on the master cylinder connected to the proportioning valve and effectively do a bench bleed on the vehicle? Apart from the fact brake fluid would leak out the side of the master cylinder, I really don't want to disconnect or remove anything if at all possible in fear it would allow more air into the system. Any suggestions would be deeply appreciated. Thanks!
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post #2 of 7 (permalink) Old 09-01-2015, 10:05 AM
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You can gravity bleed, start at the passenger side rear then the DS rear then PS front then DS front.
You always start from the furthest point not the closest.
The best way would be to pressure bleed the whole system to get out all the old fluid.
Run a hose from the bleeder valves into a clear water bottle and watch for the bubbles if you gravity bleed.

If you bench bled it properly you don't have air in the master unless you let the reservoir run dry.
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post #3 of 7 (permalink) Old 09-01-2015, 11:25 AM
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Originally Posted by Goat Roper View Post
You can gravity bleed, start at the passenger side rear then the DS rear then PS front then DS front.
You always start from the furthest point not the closest.
The best way would be to pressure bleed the whole system to get out all the old fluid.
Run a hose from the bleeder valves into a clear water bottle and watch for the bubbles if you gravity bleed.

If you bench bled it properly you don't have air in the master unless you let the reservoir run dry.
I totally agree. I would add to loosen the lines at the MC and bleed them first. I do this all the time by myself. Loosen the lines then pump the peddle but only like 1". Do that about 5-10 times, then proceed like above with the exception of leaving the MC lid off. Also I've seen people put the calipers on the wrong side, the bleeder should be at the top, or above the hose. Good luck!!
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post #4 of 7 (permalink) Old 09-01-2015, 11:42 AM Thread Starter
 
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Thanks guys. I originally did as you indicated, working from the furthest (passenger's side rear) and to the closest, and the air all seemed to be out. I don't think there's any more are in the lines, just trapped in the master cylinder. The brakes work but the pedal pressure is spongy. That's why I thought maybe to continue at the driver's side front, the closest to the MC where I left off, keep pumping and bleeding, until the air would finally come out of the master cylinder and out the front driver's side caliper. Does that make any sense? I'm concerned loosening the lines at the MC going out to the proportioning value might suck in additional air. Thanks again.
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post #5 of 7 (permalink) Old 09-01-2015, 11:49 AM
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Don't pump, that can take a 2' single air bubble and brake it up to 100 smaller bubbles. Just open the bleeder and let it gravity bleed, don't even touch the peddle.


I'm concerned loosening the lines at the MC going out to the proportioning value might suck in additional air.


That's why you only pump the peddle an inch or less. If you do it quickly it won't suck air in, and when you tighten those lines they should be leaking fluid, thus no air.
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post #6 of 7 (permalink) Old 09-02-2015, 07:44 AM Thread Starter
 
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Thanks again! Ok, so I'm going to try to gravity bleed air out of the MC by first loosening the line going out of the front reservoir to the proportioning valve while I have someone slightly push in on the pedal (1" or less), and then retighten each time before the pedal is released back up. Then after I'm finished with the front line, I'll do the same with the line going out of the back reservoir to the proportioning valve. Does this sound like it should work? Thanks.
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post #7 of 7 (permalink) Old 09-02-2015, 01:02 PM
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Originally Posted by mwatson1647 View Post
Thanks again! Ok, so I'm going to try to gravity bleed air out of the MC by first loosening the line going out of the front reservoir to the proportioning valve while I have someone slightly push in on the pedal (1" or less), and then retighten each time before the pedal is released back up. Then after I'm finished with the front line, I'll do the same with the line going out of the back reservoir to the proportioning valve. Does this sound like it should work? Thanks.
You are over thinking it, a hose into a clear plastic bottle and gravity bleed from the rear slave and watch for the bubbles.
Leave the cap loose on the master and keep it full.
Repeat on the front.
You will have fresh fluid in all the lines and all the air will be removed.
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