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MG MGB Technical - Brake bleeding

Just some info I thought would be useful: I have recently rebuilt the master cylinder on my 1974 MGB GT. This required a full brake fluid refill and bleed which I’m pretty certain hadn’t been done for some time as I soon found that both front bleed nipples where blocked and had to be replaced. I started using a gunsons easy bleed kit, starting bleeding the rear wheel furthest from the master cylinder and working towards the front closest brake. Lots of dirty fluid and air came out...Result a soft pedal which could be pumped up to a hard pedal. So I started again....some tiny bubbles this time. Result ...no difference. Third time tried the easy bleed and pumping the pedal....Result no difference. End of day one!
Read on several forums to bleed the brake servo and to bleed by using a long bleed pipe and pumping fluid around the system back into the master cylinder....Result ....no difference!!! End of day two - need more brake fluid.
Read that there is an old mechanics trick to pump the soft brake pedal until hard and then to jam a piece of wood between the pedal and the seat. Cut a piece of 4x2 and jammed it hard onto the pedal.

Result next morning...Rock solid brake pedal...Better than ever!!!!!

I think I could have saved myself a lot of time and frustration if I had used the wood jam trick after the first bleed. I don’t know what the science is, but it works!
Just though I would let others know.
David Brown

Recommended many times - wedge the pedal down overnight. Primarily for clutches as you get more pedal travel though.

For brakes I low-pressure bleed with the EeziBleed first, then do a high-pressure bleed with someone pressing down on the pedal as hard as they can while I rapidly open and shut each caliper nipple in turn. That always blasts more air out, and after that they have been fine.
paulh4

David,
yes it works, as you've found. Not all mechanic's tricks, for this and other things, always do and many mechanics have their own variations.

Guy on M&S side reckons driving the car for about a 100 miles to literally shake the air bubbles up and then bleed again for even better result. I've yet to try but sounds a good idea.

I'm not a fan of anything other than the jar method with or without an assistant - just make sure the end of the hose in the jar is immersed in old (or new) brake fluid to prevent more air getting in or an air lock.

My brake and clutch kit below, I like to keep things simple if I can (as I only understand simple).

You may want to keep with the easi-bleed but if not I can send you my simple kit notes just email me.






Nigel Atkins

The "science" behind it is that pumping the pedal up and down can aerate the fluid and no amount of bleeding will solve that as new fluid just gets aerated as well.

By holding the pedal down for a while the air bubbles gather together and can either travel up to the master cylinder by themselves or gather where they can be bled out.

When bleeding brakes or clutch I allow the initial bleed to happen under gravity alone - just open the nipple and let the fluid flow.

Once you have clear fluid at each nipple then get your attractive assistant to put their foot hard on the pedal and crack open the nipple to allow a fast spurt of fluid / air out. Close nipple while fluid is still flowing. This way no air gets back into the system from the nipple and aeration is minimised.

The tube and fluid bottle method is flawed because the thread of the nipple is quite coarse and will allow air back into the cylinder even if the end of the tube is submerged in fluid.
Chris at Octarine Services

I have learned that any agitation of brake fluid at all introduces bubbles. If you take your bleed jar out into direct sun or hold it up to a bright light, you will see many, many tiny bubbles, almost foam throughout.

Bleeding is really a two day project. I don't know if having the pedal depressed overnight makes a difference but having the fluid in the system overnight gives all of those tiny bubbles a chance to rise to a high point as one.

When you pour fluid into the master cylinder reservoir, it needs to rest before being pumped into the bore, lines, calipers and wheel cylinders. Use a bright light and you will see the aeration in the reservoir after pouring. It will give you an idea of how much air is being introduced into the system. Never-ending unless it all sits overnight.
Glenn Mallory

"I don't know if having the pedal depressed overnight makes a difference "

The only difference is that when the air collects at the top of the line, as you release the pedal, that air is pushed into the master cylinder and escapes.

I have an emergency bleeding procedure for clutches which deliberately uses the foaming. This works even for totally empty systems - fill the master cylinder and pump the clutch pedal vigorously for a minute, then hold the pedal down for a minute and repeat until you get a solid pedal. Usually takes 5 or 6 repetitions to get a working clutch.
Chris at Octarine Services

On brakes, I use one of those bits of tube with a cut in them (a quid or two off ebay), which means generally I can do the job unaided. Bit of plastic bag under reservoir lid when removing old component so as not to lose much fluid. If just changing, say, a wheel cylinder, remove plastic bag, check fluid level, two or three pumps does the job.

However, as Chris references access to an attractive assistant is so much better, particularly if all corners are being attacked, or you need to make a complete fluid change.
Peter Allen

I should have put I think the piece of wood overnight works and not necessarily mechanics' tricks.

Chris made a good point about nipple thread but with the jar method you just put some grease round the top to act as an air seal. I've not noticed any creep down the thread.

I too like to do as much by gravity as possible, I'm not in a rush and I can wander off as many things in life are more interesting to me than working on a car.
Nigel Atkins

Glenn,
I'm not against your system but it would take ages with my fluid changes as I like to give a very thorough flush-through with plenty of fluid as I don't like to keep any after final top-up so I can't think how many times I refill the reservoir. If I had to each refill overnight it'd take the best part of a week.
Nigel Atkins

I find fluid leaks down the threads ... especially when the nipple on a new wheel cylinder is blocked and I see fluid dripping down from behind the back-plate! Fortunately shoes OK.
paulh4

One lesson I learnt many years ago doing this job is to ensure that the handbrake is adjusted in accordance with the WSM.

If this is not done then there is extra travel in the foot brake (rear shoes) which bleeding will not cure.

Best Richard.
Richard Thompson

Richard, I don't follow your logic - the shoe position is set by the adjuster, the handbrake lever should not move the shoes off the slave cylinder rams when correctly adjusted. If the handbrake is over tightened it will move the shoes closer to the drums, not further away.
Chris at Octarine Services

I'm a gravity fan, living upside down on the bottom of the world you learn to appreciate gravity

If I'm doing a fluid change I suck the old fluid out of the reservoir and fill it right to the top with new, leave it for a minuite or two to settle then just by magic ,any remaining old fluid finds it's way to the top and can be sucked away, refill to the brim
Then with the car elevated I fit 4 clear plastic hoses about 2ft long to the bleeders,The longer the better, they syphon better, open all the bleeders and let the fluid run, keeping the master cylinder topped up
Usually the front hoses come clean first and the bleeders can be closed, 'sometimes' the rears need a little help to flow so the pedal can be pushed up and down very slowly till the new fluid comes through--slowly, slowly is the winner here
When the new fluid shows then the bleeder gets locked off
At the last dealership I worked we had 2 different colours brake fluid ,clear and blue/green, so if you had a car with say clear fluid, the next time round you would go blue/green so that it's easy to pick up when the new fluid has come through
In Australia there is a real fetish on brake fluid changing, most cars(owners)get it done every 2 years some anually
willy
William Revit

It's only the shoe return springs that push the pistons back into the cylinders. This means that the first time you apply the foot brake after fitting a new wheel cylinder, or after having pushed the pistons fully back for some reason, there is a longer pedal travel while it pushes the pistons out to meet the shoes. But after that the pistons stay in contact with the shoes.

With a slack handbrake cable the shoes will push the pistons further back into the cylinders than they should, leading to a long pedal.

This can make people think that the system needs bleeding. However on the single-circuit system at least there is a fluid return restrictor in the master outlet, and the check for air in the system is to pump the pedal rapidly. If it goes short and hard, but goes long again when left a few seconds then that's the indication that there is air in the system.
paulh4

I don't agree - the shoes sit on the piston ends and push the pistons fully into the bore, there is no slack.

Quick pumping of the pedal so it goes hard just moves the shoes and pads into contact with the drums and discs - as soon as you stop the shoes and pads move back away and the pedal stroke becomes longer.

This is not a test for air in the system - that is indicated by a soft or spongey feel to the pedal not its length of travel.

Some free movement, an inch or so, is perfectly normal. I would be worried if there was no or little movement as that would indicate pistons that were sticking and brakes that would be binding.
Chris at Octarine Services

Pull the handbrake up with the pedal released and the shoes will move out but the pistons stay where they are, i.e. the shoe webs move out of the piston slots to some extent.

As for when the handbrake is released it depends on wear in the levers, the shoes can either push the pistons fully into the cylinder (my roadster) or they may still be out a little bit (my V8). The tighter you have the handbrake set i.e. the fewer the clicks when it is applied the more likely it is to NOT push the pistons fully back when released. After being applied for the first time, it is only the shoe springs, as far as is allowed by the handbrake cable, that pushes the pistons and fluid back. A slack handbrake cable as well as the lever needing more clicks to be apply the brakes, results in a longer pedal, but not spongy.

As for bleeding, if you have air in the system as well as being spongy the pedal will travel further as it is compressing the air, so more fluid is pushed out of the master. The fact that a few quick pumps brings it up harder as well a shorter, when you have the restrictor as on single-circuit systems, then goes longer and spongy again after a couple of seconds confirms it. The fluid does not move back into the master immediately so you are 'storing' some in the lines which is keeping the air compressed, for that couple of seconds.

The restrictor has a large passage which pushes a spring out of the way when you push the pedal (pic 1) for a large fluid flow. When you release the pedal the spring closes off the large port and fluid can only return via the much smaller channel (pic 2), and much slower.






paulh4

This thread was discussed between 10/06/2020 and 12/06/2020

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