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MG Midget and Sprite Technical - Brake master cylinder

Brakes still somewhat spongy on the mk 3 despite several bleedings using various methods. Thinking of upgrading the master cylinder. Would this help matters and if so, what do you recommend?
Oggers

Hi Oggers
As is apparent from my own postings, I am no expert. But I would have thought changing the master cylinder would only help bleeding problems if there is a fault in your current maste cylinder, which is of course possible I guess.

Graham
Graham V

I DREAM of sponginess, Oggers. Been trying for months to get a brake pedal, and still no luck, though there are two suggestions people have made, that I've yet to try.

Availability of attractive assistant has been scant.
Nick and Cherry Scoop

My assistant is nowhere near attractive but very helpful (I have a fellow MG friend on the brakes). Maybe lowering your standards/expectations would help.
J Bubela

Nick - but I need decent brakes - with about 600kg and 140 or so HP under the bonnet....
Oggers

Assume you've tried eezibleed or something similar?
Graham V

I have tried Guy's python several times! - which was excellent. Some air still in there - somewhere. My thinking was the original master is affecting things - somehow
Oggers

Oggers,
Have you driven it at all since bleeding? I have always found that having got a reasonable pedal, but if still a little spongy, the thing to do is to go for a drive (so long as you have enough brakes for this to be a safe opttion). After 10 or 20 or so miles then bleed again.

I believe that what happens is that small air bubbles in the system get trapped in various connections and corners and won't bleed out. The vibration and use of the brakes seems to shake these loose so that they can coalesce into larger bubbles which then can be bled out.
GuyW

On my morning walk with the dog I met a neighbour who asked me how the rebuild was going, and on the subject of bleeding hydraulics he said he always jacks the rear end up for bleeding the rears, and front end up for bleeding the fronts. Is this common practice? (I'll try ANYTHING)
Nick and Cherry Scoop

Nick

I’m surprised you didn’t ask him round ;o)
Dave O'Neill 2

I did sort of hint, Dave, but he has family duties today.
Nick and Cherry Scoop

Guy

Indeed I have, and have bled again twice. Essentially, I have to tap on the pedal and release before I need to brake properly and in order to obtain a firm action high up the in the pedal stroke.

Perhaps I just need to bleed more fluid through - especially at the rear. I am though perhaps too conscious of the danger of draining the master reservoir and the mess of the whole process.
Oggers

I assume you have the rear brakes correctly adjusted. Minimum clearance and with the handbrake off!
GuyW

Guy

Yes - pretty sure I have done that, but will check..

Many thanks

Oggers

Someone could no doubt do the maths, but the rear cylinders take quite a lot of fluid displacement to move the pistons as the rear shoes operate. If there is too much travel from excessive clearance then the foot pedal cannot cope and one ends up pumping the pedal for a second stroke.
GuyW

Guy

I like the logic, but it is not no much of an initial full depression of the pedal, more an inch or so, release it, then it firms up, more or less from the top of the stroke when depressed again. It does seem like trapped air - somewhere, but for sure will check the adjustement of the rears.

Zetec midget is a bit of monster and needs decent braking
Oggers

Worn pedal bushes then?
GuyW

Oggers I believe Guy is on the right track here. If the rears are requiring more fluid displacement than the master can supply in a single stroke (at the pedal) it will affect the entire system. There are some options open.
One: reduce the bore of the rear cylinders. That will increase their stroke or reduce their displacement requirement at the cost of reducing the applied effort at the end of the shoe and consequently reducing the braking effort at the rear.
Two: reduce the displacement requirement by minimising the shoe clearance by careful adjustment as Guy rightly alluded to.
Three: consider a brake servo if you don't have one. The Zetec should have no trouble supplying more than adequate vacuum. In the case of a Zetec engined monster this is what I might favour. The maths are simple and the car the engine came from will certainly have had servo brakes.

There's also the option of converting to rear discs. I don't favour it personally but it's certainly possible.

I know you are a technically competent person so I apologise if this is all too obvious.
Greybeard

You might wish to try cutting a length of wooden broom handle, (or PVC pipe) a suitable length to wedge between the seat and the brake pedal (fully depressed). Pump the brake pedal a dozen (or more) times and set the post to hold the pedal down. Then leave it overnight. In the morning open the bleeder/s. Back to front.

Good luck mate,

Lazza
LC

Thanks for the support Greybeard, but at this stage l wouldn't be looking for design changes like smaller rear piston or rear discs. Or even a servo. There is something wrong with the present set-up as it is. My car will lock all 4 wheels on a dry road at 40mph and the only upgrade is l am using Mintex linings.

It may simply be there is still air in there - and that is all that is wrong. But otherwise l would look for things that either reduce the efficiency of the pedal stroke: worn pivot and cotter pins. Or that require more fluid for the brakes to come on. The common problem is rear brakes that have become too slack. Another is excessive wobble of the front discs which push the pistons out so there is further for them to go before they begin to work. It's worth checking these for run-out
GuyW

Just a thought Others -a P.O. hasn't fitted the front calipers on the wrong sides have they? This results in bleed nipples not being at the top and impossible to bleed properly!!
GuyW

Oggers, did you say an inch or so? At the pedal? To me, that's miraculously efficient.
Nick and Cherry Scoop

Just a sudden thought, is there a brake bias fitted that might be causing problems, perhaps set up with too much bias or causing a problem in another way.
Nigel Atkins

Chaps

This is all excellent stuff. Yet to check rear adjustment, will do so today. I do still think there is trapped air in there somehow. As aforestated, I think the initial inch or so of travel compresses whatever air remains, relasing the pedal allows the pedal to return to the top of the stroke before the air expands again, and before it does, I obtain a firm stroke second time around. This seems to eliminate pedal pivot point and worn cotters.

Front calipers are Wilwoods which are quite large and with bleeds all up top - but maybe air in there somewhere.
Oggers

what's the model & specs of the Wilwoods? Have you done the sums to work out how much more fluid displacement is needed for the Wilwoods over standard calipers? If not then this is the root of your 'problem' which is actually a 'feature'. You should perhaps be looking at a triple cylinder pedal box then you can size the masters to the respective front and rear displacement needed.
(apologies if this has been covered earlier in thread or elsewhere).
David Smith

Nick, have you tried advertising for an attractive assistant in the Hereford Times?

I remember using Larry, sorry Lazza's method when renewing my system and it worked well. Nothing to lose?
Jeremy MkIII

Oggers
Are you 100% sure you don't have a weeping connection somewhere, I had the slightest weep from both front banjo connections which frustrated me for weeks before I found and fixed it. The symptoms were identical to the ones you describe.
Have you tried pressurising the master cylinder with an easibleed then trying the brakes, if the problem has gone then you may have a leak, if the problem is still there then its possibly a displacement issue as described above.
My understanding is that the symptoms you describe would only occur if the leak was very slight - anything more and you wouldn't be able to get any improvement on the repress of the pedal and the pressure of the easibleed would just force the fluid out the system via the leak in fairly short order.
I could be completely incorrect in my thinking - but it seemed to work for me!


S G Macfarlane

Jeremy, I have to say that Lazza's method makes no sense to me. But then, nor does my lack of success so far.

I will try it, though if I pump a dozen times, the pedal won't go down at all.
Nick and Cherry Scoop

>>the pedal won't go down at all<< but over how long(?).
Nigel Atkins

David

I think you have it! I completely forgot about the larger 4 pot pistons. Mind you so did the PO! Is there an easier solution rather than a 3 MC set up? Larger master that fits original brake pedal and outlet pipe would be ideal - maybe somthing from an MGB or similar?
Oggers

Not sure of the thinking behind it either Nick but it worked. Others with a lot more technical knowledge will have an explanation.
Jeremy MkIII

"Is there an easier solution rather than a 3 MC set up?"
- perhaps, but it'll take some trawling around to find something, or a bit of luck in finding someone who has trodden the same path before you.
The Spridget pedal box is fairly shallow, hence the m/cyls have their mounting flanges set at 45 degrees. Girling do a range of proprietary cylinders, one of which will probably fit the bill but the flanges are aligned vertically. It may be possible to fit one of these at 45 degrees but you'd need a remote reservoir to get enough fluid into it.
http://race.parts/Catalogue/Braking/Master-Cylinders/Girling-Master-Cylinders
gives an idea of what's available.
David Smith

I offer here the proof that I'm attempting Lazza's method (egged on by Jeremy).


Nick and Cherry Scoop

Nick, I like the innovative use of the tennis ball!
Bill Bretherton

That's saved my head a few times now, Bill.

Question for Lazza:-

In the morning, how do you bleed it? Remove the pressure and then pump normally? Or leave the pressure in place for the first bleed?
Nick and Cherry Scoop

This is weird.
This morning the pedal was still nicely off the floor, holding pressure.
I cracked the n/s rear open briefly to let the pedal go to the floor - two bubbles - then dismantled the apparatus.
And without further bleeding, I HAVE A PEDAL!
Nick and Cherry Scoop

Hi Nick,

Sorry for the delay on my response, I've been away from my computer for a few days, but glad to hear you have it sorted. I'd like to add that I am not the originator of the method. Probably first tried in the early 1970's when a neighbor (w/Sunbeam Alpine) was always lending a hand in the garage ;-)

Cheers,

Lazza
LC

Wahey!

She's looking good Nick - the tennis ball is a neat trick, I've used foam pipe insulation for similar purposes.

For explanation, here are a few suggestions
https://forum.wscc.co.uk/forum/topic/65562-wedging-brake-pedal-down-overnight/

My favourite is the Air Bubble Fairies, seems to make the most sense...
Jeremy MkIII

Thanks, Larry. I will bleed the system when my attractive assistant returns from Wales this afternoon, and that will be interesting: perhaps I will somehow manage to re-introduce the air bubble.

Thanks for that thread, Jeremy.
I agree.
Definitely Fairies.
Nick and Cherry Scoop

This thread was discussed between 06/07/2018 and 12/07/2018

MG Midget and Sprite Technical index

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