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MG MGB Technical - Bleeding the clutch, don't do what I did.

Recently it was necessary for me to bleed the clutch hydraulics because I inadvertently let the reservoir go dry. That was my first mistake.

My second and my much larger mistake was how I bled the system. I used a Visibleed system with the check valve emptying into a plastic cup. While under the car controlling the slave bleeder screw, the pressure of my wife pumping the pedal caused the check valve to rise up out of the cup and spray fluid everywhere... including into my eyes.

15 minutes in the shower staring into the water spray, calls to the poison hotline then 3 hours in the emergency room later and I have abrasions on my eyes. Pretty dumb. A week on antibiotics and all should be well, except for the forthcoming bill.

What should I have done differently. I should have used a deeper receptacle than a plastic cup. I purchased a two quart plastic pitcher with a small opening in the top. The depth and restricted opening kept the check valve in the pitcher when I got back to the task. I also wore goggles, the type you wear when cutting metal.

Hopefully by documenting this experience I can help others can avoid a similar situation.

Brian Denis

What a suck holiday lesson. There are loads of posts about bleeding the MGB clutch. I used a pressure bleeder and still had to revert to my better doing pedal work without a bleeder. Did have an enclosed bleeder container I got from the company who made the pressure bleeder. Otherwise you can punch a small hole into a screw on cap to a large plastic bottle with the hold landing in liquid so there's no chance of pulling air back into the line accidentally.

However, it appears that this whole weird dance is a result of the slave not being built as the factory intended. It should've had a slot which would alleviate all the nightmares: some do reverse bleeding, some remove the slave and let the 'micro bubbles' go back up the line. You push in the pin, etc. Moss has bleeder videos.

But look at this. Might be worth redoing the slave to not worry about bleeding when renewing.
<i>This is the correctly manufactured Clutch Slave Cylinder with the slotted internal bleed hole for MGA MGB. For years the aftermarket companies have reduced cost by changing the internal flow of the MGA and MGB clutch slave cylinders. This causes a large amount of headaches when trying to bleed the slave cylinder, the air is unable to easily escape. With this OE designed cylinder it has the correct slotted bleed hole and the correct support for the mounting base which gives the cylinder stability. We have also gone one step further, to keep stability in the rubber cup we had the seal support made out of an alloy for longer life and stability. Cross Ref.# GSY106, LL13901</i>

from the bp northwest parts site....

Big question is why did it go dry? Only happen if there's a leak somewhere.


Brian, The M/c can't empty unless there is a leak. Sort that out first before you risk the bleeding process again!!
Allan Reeling

Thanks for the responses. I will be handling the leak, which is coming from the slave cylinder, but I was wanting to get back on the road ASAP with the nice weather finally here in New England.
Brian Denis

My OE slave cylinders have the bleed aperture right at the top of the cylinder where it meets the back wall. Even though the 4-cylinder bleed screw exits downwards this is no different to a significant amount of the pipework especially for the clutch and even the brakes.

I've bled my V8 clutch and brakes today as part of the slow rebuild. I choose to fill and bleed the clutch in reverse i.e. with an EeziBleed on low pressure connected to the slave. A few second with an empty reservoir and you see the fluid level rising, so stop them, top off as normal, and I immediately had full travel and no grinding in reverse. As the V8 cylinder bleed nipple points upwards I chose to do it with the slave detached from the bell-housing and hanging down.

Similarly I fill and low-pressure bleed the brakes with EeziBleed, but from the reservoir this time. That always leaves the pedal soft and long, which pumps up after a couple of strokes, but goes long again a couple of seconds later. This shows there is still air in the system. Then I do high-pressure bleeding, which is the only time the pedal is used. Someone presses down hard on the pedal, while I rapidly open and shut each caliper nipple in turn, and that always blasts an extra lump of air out. Venting into a tall, narrow coffee jar!

Having the nose higher than the tail definitely helps both clutch and brake bleeding. But if either system is still problematical leave it overnight with the pedal wedged down as far as it will go, nose higher than rear, then when you release it next morning and air should have floated up to the top of the pipe by the reservoir, and be flushed into the master as you release the pedal.

I have bled the clutch on MGB, midget and Elan using the reverse filling technique Paul describes. Has never failed to give a full travel pedal on the first attempt. The Eezibleed uses a tyre as a pressure source and in my experience you want the pressure VERY low for the reverse technique. About 5 psi is my preference.
Mike Howlett

Indeed very low, although I've found 15 for example is OK. If you go too high it can blow the connections apart, and if the seal on the master cap adapter won't hold the pressure it will overflow.

I had the opposite problem. I've had my EeziBleed for donkey's years and the seal on the EeziBleed reservoir has gone soft and pops out of position if you screw the cap down too hard, or leaks anyway if you don't. The upshot is that I had to re-inflate the tyre about half-a-dozen times before I got fluid through the first and longest run. Reminded by a pal, I contacted gunsons and they are sending me a replacement seal FOC.

This thread was discussed between 04/07/2017 and 06/07/2017

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