Bench bleeding master cylinder - Pontiac GTO Forum
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post #1 of 6 (permalink) Old 07-16-2019, 05:32 PM Thread Starter
 
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Bench bleeding master cylinder

I just replaced the original master cylinder with a new single bore master cylinder. Is bench bleeding it necessary? In the shop manual it just says to fill the master cylinder and pump the brake pedal about 5-7 times and then bleed the brakes. What’s the best/easiest way if I do need to bench bleed it? The master cylinder didn’t come with anything for bleeding it. Thanks
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post #2 of 6 (permalink) Old 07-16-2019, 07:34 PM
 
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Originally Posted by Ohio66GTO View Post
I just replaced the original master cylinder with a new single bore master cylinder. Is bench bleeding it necessary? In the shop manual it just says to fill the master cylinder and pump the brake pedal about 5-7 times and then bleed the brakes. Whatís the best/easiest way if I do need to bench bleed it? The master cylinder didnít come with anything for bleeding it. Thanks
Honestly, I never knew of the term "bench bleed" when I replaced any of the master cylinders I swapped out when I was younger. I simply replaced the master, filled it up, and had someone help me bleed all the brakes. How I ever did it without "bench bleeding" and the brakes worked will always be a mystery to me.

I think the purpose here is to make sure all air bubbles are out of the system. If the master were not level on the car, I suppose you could have a trapped air bubble. To me, bench bleeding it out of the car just ensures the master can be held level and you get a complete compression on the master cyl piston. It is also less messy because you are pumping the fluid from the master cyl well through the chambers to eliminate air and pumping right back into the cyl well - kinda like recyling it. You can see when there is no air in the master. Then you can install and then bleed your brake system like normally done.

I use a long screw driver to compress the piston. If you master cylinder did not come with the adapter fitting/clear plastic tubing, you can purchase these very cheap at most auto stores.

Here is a pretty good Youtube video to watch. It is a dual reservoir master cylinder, but the principals are still the same.

Again, you can probably install it and have a buddy help bleed the brakes, but you will most likely go through a lot more brake fluid making sure you got all the air out.
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post #3 of 6 (permalink) Old 07-16-2019, 08:03 PM Thread Starter
 
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Sounds good. Thanks for the detailed response and video. I’ll head up to the store and pick one up tomorrow.
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post #4 of 6 (permalink) Old 07-16-2019, 08:41 PM Thread Starter
 
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One more question. In the manual it says to start at the left front wheel cylinder then go to the passenger side, drivers side rear and finish with the passenger side rear. I’ve always started at the furthest from the master cylinder and finished at the one closest to it. Any idea why the manual would say to start at the closest wheel cylinder?
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post #5 of 6 (permalink) Old 07-16-2019, 09:54 PM
 
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Originally Posted by Ohio66GTO View Post
One more question. In the manual it says to start at the left front wheel cylinder then go to the passenger side, drivers side rear and finish with the passenger side rear. Iíve always started at the furthest from the master cylinder and finished at the one closest to it. Any idea why the manual would say to start at the closest wheel cylinder?
I doubt it matters. I installed four wheel disk brakes and bench bled. Then started from the farthest, everything works fine. Like Jim said, I never bench bled in the good ol days.
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post #6 of 6 (permalink) Old 07-16-2019, 10:59 PM
 
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I doubt it matters. I installed four wheel disk brakes and bench bled. Then started from the farthest, everything works fine. Like Jim said, I never bench bled in the good ol days.
X2. I was always taught to start at the farthest away brake - passenger side rear. The do the driver's side rear. Next was the passenger front, then driver's front. Still do it this way today and never any issues. I fill the reservoir up and then place the MC cap over it. This keeps any impurities out, like flies, and if the fluid shoots out as it does when you pump the pedal, it won't go all over. Just keep checking fluid levels as you go so the MC reservoir doesn't get too low and suck air.
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