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MG MGB Technical - Vacuum Brake Bleeding

As a former SCUBA diver, I know that bubbles under pressure are smaller than at sea level, this perhaps explains why using an Eezibleed at lower pressure gives better results.

Has anyone ever tried applying a vacuum to a hydraulic system? If you connected a household vacuum cleaner through a suitable adaptor/trap to the reservoir cap, would the fluid “boil”? And therefore extract air? In any event, the bubbles would become bigger and may be dislodged towards the MC. I’m looking forward to any of your experiences or comments on this, and if there are no serious discouragements, I will try it and let you know.

Some of you out there seem to be aircraft technicians, how do you bleed an aircraft hydraulic system? Perhaps there are some tips you can share?

I have heard that it is best to bench bleed an MC before fitting, but please tell me how do you do this? My MC has no bleed nipples.

Happy New Year to you all, looking forward to replies.

Thanks - Paul
Paul

Don't know the answer to your question, but I am curious as to whether anyone has experience with a Mityvac. I was thinking of purchasing one.
Greg Bowman

Interesting idea, but I think you will find that applying vacuum to the system will just suck in air past the wheel cylinder seals which are designed to keep fluid in - not air out......

In my experience, the best way to bleed is to pass enough volume of fluid, quickly in order to shift trapped air. The best technique is to have an assistant press down on the pedal hard while you undo the bleed nipple - this results in a rush of fluid / air. It is also important to shut the nipple BEFORE the pedal hits the floor so that air is not sucked back into the system. This procedure repeated three or four times per wheel usually shifts all the air.

If not, then vigorous pumping of the pedal will break the air into smaller bubbles and if you then repeat the bleeding process you can drive out the aereated fluid.

Most problems over bleeding are caused by too little and too slow!
Chris Betson

Gregg, To answer your question, I have been using a Mityvac for brake and clutch systems bleeding for several years. It normally works well and gives equal or better results than using an assistant to pressure bleed. Regards, Clifton
Clifton Gordon

When ford motor company assemble the brake systemon the production line the fill the system by first crearing an almost perfect vacuum on the system and then fill them full of fluid. no bleeding required. and we never had problems with leaking wheel cylinder seals. So I dont think the vacuum as drawn dy a vacuum cleaner would do any harm. Interesting thought though
B R pasmore

To bench bleed the M/C manually:

1) Mount the M/C on a bench vise - by the flange - not the cylinder body.

2) Install a couple of short steel brake lines on each outlet port (about 2-3 inches, or so, in length).

3) Install clear vinyl hoses (fish aquarium hoses) to each end of the short steel brake lines and run
them into an empty container.

4) Fill the reservior with fluid.

5) Push (by hand) the M/C piston all the way home - and hold it in. Clamp one of the vinyl hoses shut
using a needle-nosed plier - or a surgical hemostat. Release the M/C piston, slowly, allowing fluid to
be drawn into the cylinder. Release the hose (pliers).

6) Repeat Step 5 until all air bubbles are gone.

7) Go to the other hose and repeat Steps 5 & 6 for that hose.

You're ready to go after all the air has been expelled from the cylinder. Wrap the cylinder in a towel
before carrying it to your car to install it as you do not want to drip any fluid anywhere.

You will still have to bleed the entire brake system after you install the M/C - but bench bleeding the
M/C saves you from much of the initial dry pedal stomping and empty (air) bleed screw turning.

For me, I always use a Mity-Vac to pull new, fresh fluid into the system as it eliminates the need for
bench bleeding altogether. I then follow this up with a few strokes of conventional pedal bleeding
as I find that it helps seat the seals - and the moving piston(s) and rushing fluid helps further dislodge
any remaining stubborn bubbles.
Daniel Wong

Ah, Daniel. I have, for many years now, been reading your posts. After reading each one, I have had the feeling that I, and perhaps the rest of us, should say a loud "Thank You". I have never read one which was not impressive both for intelligence and knowledge. I think we all greatly appreciate your taking the time to post. I, most certainly, do. So, let me say thank you for this post (one of the best systems I have read about and superior to my, previous, method). Thanks for taking the time to post it. Les
Les Bengtson

Aircraft hydraulic system bleeding is accomplished in several different ways. bleeding the brakes is usually accomplished either by reverse flow (pumping fluid into the brake bleeder fitting and into the reservoir, or by pumping up and then steadily depressing the master cylinder or power brake valve, and bleeding out into a container. A lot of variations to this, as some AC are equipt with dual MC's in each cockpit, etc. The main systems are usually bled by cycling the components, while connected to an external hydraulic pressure source. I use the mytyvac on my cars with good results. The main thing to remember is that the brake fitting is not air tight when open, you can draw in air, which will appear as fine bubbles. Givng the bleed fitting a good coating of grease wll give an airtight fit. The other thing is that it depletes the reservoir quickly...if you empty the reservoir, you'll be pulling air into the system and you'll have to start all over. Start with the furthest away wheel (by tubing length) and end up with the closest.
R. L Carleen

I think that each one of us has found a way
of bleeding that works best given our
individual technique. Personally, my results
with the Mityvac were mediocre.

An important advantage of pressure or vacuum
bleeders is that it avoids long pedal strokes --
I read recently that pedal-based bleeding results
in pushing the seals into pitted areas of the
bore, leading to early failure.

I now use the gunson's eezibleed exclusively, and
have replaced both brake and clutch MCs on my
midget and B. I have not had to bench bleed
the brake MCs. Recently, I replaced the dual-line
MC on my 79 B, and used the Eezibleed, again, with
no need to bench bleed. Only one Eezibleed session
was necessary. However, there are some tricks
with the eezibleed.

1. Use less pressure than recommended in the
instructions. I use 10 PSI maximum.

2. Don't screw the cap on the MC on too tightly --
this will cause leaks (as will using the recommended
max pressure of 20 PSI.)

3. Stuff _lots_ of paper towels around the cylinder
being worked on to catch any leaks that do occur.
Ronald

I've never found it necessary to bench-bleed anything, just install dry, fill, and bleed. The bleeding process has been no different when new and dry to anytime else. A case of YMMV, I suppose.

I agree about not overtightening the Gunsons cap on the MC, doing so causes the rubber gasket to pop out from between MC and cap and this is what causes the leak. I also find that while the Gunsons is quick and easy for filling the system and bleeds the rears OK it doesn't the fronts, both my cars need someone standing on the pedal while I rapidly open and close the caliper nipples to get the last air out.

For the clutch I have only ever used the Gunsons to reverse fill the system from the slave nipple, using even lower pressure of about 5psi, after which it was immediately ready for use needing no conventional bleeding at all.
Paul Hunt

This thread was discussed between 02/01/2003 and 06/01/2003

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