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MG MGB Technical - Necessary to Bench Bleed?

Is it really necessary to bench bleed the Dual-Line non-servo master cylinders?
When I installed a new one on my 68 I first bench bled it then installed it. But like an idiot I let it run dry when bleeding the front brakes and air got into the M/C. So I filled the res. and kept bleeding it at the calipers until it looked clear. Can I assume there's no air in the M/C? Does the unit have to be bench bled again? Is there another solution to ensuring that there is no air in the M/C?

Thanks for your comments,
Sam
Sam

I haven't had a need to bench bleed, but I use a pressure
bleeder (eezibleed.)

Bench bleeding might be necessary if you're pumping the
pedal to get the air out.

If the brakes work fine, there's no air in the system. Even a small
amount of air in the MC will give a spongey pedal.
Ronald

Sam, bench bleeding a new or rebuilt MC ensures that it can push fluid through the system when on the car. If you did not tear apart the rest of the system at the same time a MC full of air may not be able to push fluid through the lines. If you ran the mc dry while bleeding you do not need to bench bleed again. just besure you get the air bubble out at the wheels.

Doug
Doug Gordon

Just today, I read an article, writen my a gentleman by the name of Tilton: as in Tilton Brakes. I figure he know a lot more than I do. This is what he had to say:

We all bleed our brakes wrong.

The proper way is to use the jar 1/2 full of fluid. Couple this on to the open niple via a hose. Gravity may do the rest, but if not, use very slow (he recomends hand pushing) pumps on the pedal. No need to do the traditional "pump, pump-hold and crack the nipple" thing.

Close the nipple on the downstroke when the air bubbles stop.

Sam:

Sorry the above does not answer your question. However, from experience, I have never "Bench Bled" an MG system, and I have had no problems.

Pete
Pete

No, you don't really have to bench bleed the M/C again...although, when installing a new (or rebuilt)
M/C, bench bleeding does save on a lot of dry pedal pushing and fiddling with the bleed screws, etc.

A good alternative to bench bleeding and pushing a dry pedal is to use a Mity-Vac to literally suck fluid
into the system via the bleed screws.

Another method often used to purge air is to reverse bleed the system - that is, pressurized fluid is
forced into the slave cylinders and calipers through the bleed screws, thus chasing out air and filling
the M/C from the inside-out. You really have to monitor the fluid reservoir closely as it tends to fill up
quickly and potentially spill over.

I've seen folks use a modified plastic pump garden sprayer for this (- it looks like a plastic jug with a
plunger on top, with a spray gun on a hose). You'll have to cut off the spray gun and hose, and adapt
a length of smaller diameter hose to fit the bleed screws as well as to conserve on fluid. Less than $20
at a Home Depot nearest you (not counting the extra bits & pieces needed to modify it).

You can also reverse bleed by rigging a homemade fluid feed jar to a source of compressed air.
About 15-20psi of air pressure is all that's needed for this. Any more pressure, and you'll risk having
a hose fly off a bleed screw and spray brake fluid all over the place.

Using your own right foot is the best way of telling if there's any air left in the system.

You should feel no soft, sponginess when you depress the brake pedal. Also, the pedal should hold
firm without any further pedal travel to the floor while being pressed.
Daniel Wong

Thanks everyone for your comments. There is not a spongey feeling at the pedal, and the car stops OK, so I guess the system is air-free. I was reading the other threads on bleeding and got a little paranoid.

Sam
Sam

Daniel,
Thanks for taking the time to outline those procedures. I've been thinking of buying a Phoenix bleeder, but I think I'll try the garden sprayer method first. I knew there must be some reason that I was saving that old plastic pump tank!

Regards, Joe
Joe Ullman

Here's a good example of how to build your own pressure bleeder from a garden sprayer.

http://www.apexcone.com/Bleeder/bleeder.html

The author (non-MG owner) has set up his bleeder to force fluid through the M/C using a modified
reservoir cap - which is a good idea especially when used for periodically flushing the system of old
dirty fluid.

Find a sprayer with the smallest container possible to minimize the fluid surface area. This reduces any
hydroscopic absorption through the exposed fluid surface.

Pushing fluid through the M/C, first, makes certain that the fluid in the system is clean (especially in the
M/C reservoir) - then I follow this with a session of reverse bleeding to insure that absolutely all of the
air is purged out of the system. I know this sounds elaborate, but when it comes to brakes, I prefer
being thorough.
Daniel Wong

Daniel, the system looks good, but I made up a reverse bleeder for even less-

Get a mayonaise jar, glass (yes glass) with plastic lid works well. Next get a length of hose that fits snugly over the bleeder nipple. Then you head to the local farm store. Get the cheapest tire stem you can find, then head to the large animal section and find a medium size syringe, usually 3 of 4 for $2. If you don't have a bicycle tire hand pump find a cheap one - small is fine.
Drill a hole in the lid and mount the tire stem (size of hole determined by stem chosen), then drill a hole in the lid a bit smaller than the diameter of your bleeder line and insert it through the lid into the jar so that it reaches to the bottom of the jar and fits the lid snugly.
Remove needle from syringe and use the plunger assembly to remove fluid from Master cylinder - when you use a new one they are as clean as you could possibly need. Fill the mayonaise jar with desired fluid. Hook the hose to bleeder, a small clamp may be necessary, and loosen bleeder. Attach the bicycle pump to the jar and pump - checking MC to see that it does not over flow, and removing fluid as needed until cylinder is full of clean fluid, working in standard longest line to shortest line method.

I have found this to work very well. Even if the holes you drilled leak a bit you will not be introducing air as the end of the hose is fully submerged in the fluid. The hand pump is slow enough that you can easily keep up with the level of fluid in the master cylinder, and the pressure is low enough that the glass jar will not explode, unless you drop it that is. Glass is very easy to make very clean and that is why I prefer the glass jar over other materials available, and the wide lid of the mayonaise jar is easy to drill holes in.

Not as high tech as the pressurized tank but works very well and uses common cheap components.
Carl Holm

This thread was discussed between 18/01/2002 and 19/01/2002

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