Can you bleed brake master cylinder while it's in car?

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Can you bleed brake master cylinder while it's in car?

Postby 07miata » Fri Jul 03, 2015 7:19 pm

So my brake pedal is soft and after bleeding the brake lines and clutch line there's no change. So I figure the master cylinder should be bled. I've never done this before, but can't you just bleed it while it's in the car? I see lots of instructions for removing it from the car, but my limited understanding makes me think:
1. you can just leave it in, unscrew the two lines, put pressure on it like normal via one-man brake bleeder can, and watch the air bubbles come out, once they stop voila you're done
or
2. just bleed the brake lines like normal -- wouldn't any air in the master cylinder work its way out eventually and there's no special procedure needed?

I assume I'm missing something. Just trying to save myself some $$ since I have no workshop or bench vise.
Thanks for any thoughts.
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Re: Can you bleed brake master cylinder while it's in car?

Postby morrisg » Sat Jul 04, 2015 4:23 pm

Don't unscrew the lines! Do your #2 option: just bleed the brakes like normal, start at the longest distance from the master cylinder (right rear for left hand drive), then left rear, right front, left front. Be sure to keep the master cylinder half full or better throughout the bleed process. If you don't have two people (one to pump the brake pedal, one to bleed the brake cylinder) then get a speed bleeder or some other product that allows one person brake bleeding. Use the correct DOT number fluid (I think it's DOT 3 for NCs, but check your manual) and make sure it's fresh. Brake fluid picks up moisture from the air just sitting in the can and moisture boils when it gets hot.

Clutch cylinder is a separate circuit, no need to bleed for soft brake pedal.

Let us know how things go.
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Re: Can you bleed brake master cylinder while it's in car?

Postby Ryan @ GWR » Sun Jul 05, 2015 9:35 am

Air can be trapped in the master cylinder and if that is the case, no amount of bleeding the brakes at the calipers will release that air. For this reason, installation of a new master cylinder should always include bench-bleeding the master cylinder before installing it.

If the master is already installed and you suspect it has air trapped in it, I guess you could disconnect the lines and rig the master up for a bench bleed the same way you would with it out of the car, then just press the pedal a million times just like you would depress the piston with a long screwdriver if you had the master in a vice. The disadvantage there is that you are sitting in the car, so you can't watch the progress of the air bubbles through the lines - and if you haven't bench bled a master cylinder before that might not make sense, but air does not travel out of the master quickly like you're accustomed to with bleeding at the calipers. It requires a LOT of repetitions, the air will travel a quarter of an inch each time, and each time you think you've been doing it for long enough, another little bubble will appear that was trapped somewhere up inside there.

If you've got all the lines disconnected, you're just two 12mm nuts away from having the master cylinder out of the car, so there is no good reason not to pull it out at that point and it is far easier to do this with it removed where you can see everything that is happening in the lines.

-Ryan
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Re: Can you bleed brake master cylinder while it's in car?

Postby 07miata » Sun Jul 05, 2015 4:42 pm

Thanks everyone. I went ahead and bled it while it was in the car with my wife helping to push the brake pedal, just so I could see if there was actually any air in. Sure enough there were quite a bit of bubbles. Now the brake pedal is rock hard. Yippee! I agree now that I've done this, removing it from the car isn't as big a deal as I feared.
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