Bubbles when brake bleeding with vacuum pump

[Gryphon Rider]

This happened with my car but might also apply to a motorcycle. When bleeding with a hand-operated suction pump, if I pump quite fast I get bubbles coming out the bleeder nipple. If I have my helper on the car's brake pedal using the standard bleeding technique immediately after, it only takes a couple of cycles and the fluid is coming out bubble-free again. I can make it produce bubbles again by creating vacuum again.

Is the vacuum causing the brake fluid to boil/cavitate, or might I have a mystery leak that allows air to be sucked in, like perhaps past the slave cylinder seals, which may not be designed to withstand negative pressures? The fluid in the master cylinder reservoir was not allowed to get too low.

[ptgb]

When I use my Mityvac to change the fluid, I was getting small bubbles as well.

To cure it, just take some thread tape and wrap it around the base of the bleeder nipple. Usually is sucks a small bit of air around the hose connected to the nipple.

[IamGCW]

When I use my Mityvac to change the fluid, I was getting small bubbles as well.

To cure it, just take some thread tape and wrap it around the base of the bleeder nipple. Usually is sucks a small bit of air around the hose connected to the nipple.

+1  (I use the yellow thread tape, it is thicker)

Gil

[9Ball]

agreed it is probably coming from the bleeder nipple...try opening just enough to get fluid to flow but not enough to allow air to enter from the threads.

[The Anvil]

What the other guys said. It can also get in between the nipple and hose. Air being much thinner than a liquid it will pull in pretty easy under heavy vacuum. If you still get air try a ziptie on the hose where it meets the nipple. Also, don't use old, brittle hose. Use newer, flexible stuff.

[hubcapsc]

I think it is likely the hose at the nipple too. I use a tight hose, and make sure the hose
goes straight onto the nipple without it being able to get cocked when I'm pumping.

-Mike "cross posted to alt.sex.aluminum.baseball.bats"

[Attic Rat]

When you are using a helper the fluid is coming out under pressure thus no bubbles. When you use you a vacuum to pull the fluid out then you are pulling air around the bleeder valve thus bubbles, This is pretty common

[98valk]

same here, its coming from the bleed screw threads. I put teflon tape on the threads and bubbles went away

[Gryphon Rider]

Thanks for your responses.  I can see that the bubbles are coming out the centre of the bleeder, not the interface between the hose and the bleeder.  You all are likely right that air is getting past the screw threads.  I had already finished bleeding the car when I posted, but I'll certainly keep the thread sealing tape on hand for the next time.  I had no idea there were different thicknesses/types of thread tape; I've only used the thin white teflon stuff, and then only for wrapping threads before connecting fittings.

[pancho]

Different thickness thread sealing tape is a new one to me too.  Learn something new here most every day.

[98valk]

Different thickness thread sealing tape is a new one to me too.  Learn something new here most every day.

the thicker yellow tape is/was for gas, oil, etc connections.
supposively the white tape was reformulated a few yrs ago so it can be used for gas, oil, etc now. one of the junior engineers in my office had looked it up and told me this while working on his race camaro. remember the tape isn't a sealant and doesn't provide the final seal, it is actually a lubricate that allows the taper thds to engage more deeply without gauling to seal, the thds cause the sealing not the tape.

[whitestroke]

When using a vacuum to bleed brakes and air is leaking by the threads it should not let air into the system, correct?

[98valk]

When using a vacuum to bleed brakes and air is leaking by the threads it should not let air into the system, correct?

correct.

[Earl in Pensacola]

I lowered the mity vac's  vacuum and the bubbles stopped.  try doing the "bleed" a little slower.