Vacuum bleed brakes through MC cap?

[BDCarrillo]

Had a scary thought and my google-fu didn't turn up anything useful...

Right now I'm replacing the brake master cylinder, and of course the front/rear brake lines have to be removed. Conventional wisdom says to bleed all of the lines after putting in the new master, but that means pulling air from the master all the way out to the wheels. IE, the only place with air right now is an inch or so of tubing right at the master cylinder, so why pull those bubbles down every single brake line...

Sooo... why not just vacuum that air out through the master cylinder cap? I rigged up an old cap to hook up a hand vac pump, and with the new master on the bench it does pull fluid up through the lines under vacuum, meaning I shouldn't be fighting any valves.

Any thoughts on this?

 

[Black dawg]

Dont even have to pull fluid up through the lines...I bleed stubborn clutch masters this way. Most always, I can just install the master fill with fluid then put slight vac to res. Fluid will fill up the vacuum. Sometimes takes a pump or two of the pedal between vacuuming, but it does work.

 

[79jasper]

No reason it shouldn't work. But you won't know if you're fittings are sucking air.
Most pressure bleed from the master.

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[riotwarrior]

Any thoughts on this?

Ya ...full bleed flushes out old fluid...simple nuff to do...easy to for 1 person done plenty 1 person bleed jobs.....since making tool....never needed 2nd person.

Sure feels good knowing fluid and contaminents are flushed.

JM7.3CW.....BWTF DIK

 

[DaytonaBill]

No reason it shouldn't work. But you won't know if you're fittings are sucking air.
Most pressure bleed from the master.

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I do! I use this...

I'd rather bleed all new fluid in there anyway because this makes it stupid easy!!!

That blue stuff is racing DOT 4 fluid, before the Gov prohibited them from using the blue dye. I used the blue brake fluid the first time because I figured that would make it easy to tell when the fresh stuff was flushing the old stuff out.

Now the next time I flush it, I'm using the same stuff, only it's gold colored and again, that will tell me when the gold has flushed out the blue stuff...

Anyhow, if you suck through the cap, that means the calipers in front is retracting and you need to pump your brakes while running, before you drive off. Otherwise the pedal will go to the floor, maybe all of the way down to the floor... Not good for a test ride...

 

[IDIoit]

We just did the brakes on my sons truck.
Entire brake system is bran new.
We used this to bleed it.

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[BDCarrillo]

Well the theory was sound, but it proved to be fruitless.

When vacuuming the master cylinder, it's very difficult to see if the air has bubbled out into the reservoir, and you leave gunky old fluid in the line.

I ended up doing a traditional vacuum brake bleed, but might re-rig my "old reservoir cap with a nipple glued in" into a pressure bleed system.