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MG Midget and Sprite Technical - brake bias bars and safety

Serious safety concerns with my self designed dual brake system...
i have dual master cylinders and a brake bias bar.
one MC for the front, one for the back.

i decided on this system after reading about it in several places and needing to bin the old codged up system that was already in place.

Reason for the twin MC setup was safety, i wanted to have a dual system and it was relatively easy to sort out using parts i could get locally.

HOWEVER.... today i went for a run, brakes ok at the start, then after a good 10miles or so on the ring road i needed to brake, it felt wrong, initially a hard pedal but then it kept on going, obviously losing fluid.

to cut a long story short.. i can paraphrase Jeremy Clarkson and say that "some poo might have come out"

I eventually got home after driving in first gear very slowely and using my not very good handbrake...

found the the cause of the problem as the brake pipe nipple leaking at rear drivers side.
Question is.. why did i lose brakes totally ? if the rears have a problem then i still should have had the fronts...the fronts still work fine. looking at the bias bar what is happening is the bar compresses the rear MC fully without pressing on the front MC, this is because there is no pressure in the rear MC as the fluid is gone... maybe there is too much travel in the empty MC before the plunger reaches its stop and allows the bias bar to transfer the force to the other MC ?

understand that this sort of set up is used my many racers... has anyone got any advice ? its there some safety backup that should be used with together with the bias bar that i dont know about..?

Andy Phillips (frankenfrog)

Got a pic of the setup Andy? Always helps with these things.

Malcolm Le Chevalier

I have the set up...and love it

It could be the front is adjusted wrong...I have mine set up around 90/10 ....some will argue this... but its more personal choice...I raced mine up and down the street slamming the brakes on till I found the bias I liked

It could be the perportion of size is wrong from master to slave...I think im using a 5/8th master for the front...I cant remember exactly

The pedal isnt adjusted correctly and ,/or to much slop play on the M should start stopping at the front 1st then as the brake is applied then the rear....worn clivis pin?

did you remember to cut out the old brass 5 way preportioning valve and re-plumb

Is there any movement in the assembly as its bolted to the bulk cant have any movement.. no rocking or twisting when the brake is applied

Air in the lines...classic default answer

It sounds like the fronts where never funtional To begin with

The best tip ...jack up the front end ...spin the front wheel by hand and have a 1/2 naked girl apply the brakes on your command...and track it down till a issue presents its self

Post some pics...I want to see and im sure others do as well

Prop and the Blackhole Midget

******Post some pics...I want to see and im sure others do as well******

I meant The 1/2 naked girl....but maybe one or 2 of the master cly system wouldnt hurt....hahaha

Prop and the Blackhole Midget

pics attached.

Prop, i think you mis-understand.. the system worked fine and i was very happy with it (as you are) and i also adjusted the bias by racing up and down the road etc...

the problem is that when you lose fluid in one of the systems then you also lose the pressure to the other MC because the bias bar applys all the force to the empty MC...

This means that the dual system, apart from allowing bias changes, is no safer than a single line system.. unless that is i've got something wrong, which isnt unheard of..

picture below of the unloaded system, rear brakes are on the bottom

Andy Phillips (frankenfrog)

now a picture showing the pedal pressed,.. the bias bar pressing the MC rod in completely as no pressure there to stop it, and the front brake MC not depressed at all.

Andy Phillips (frankenfrog)

The two brake circuits are completely isolated.
Front is MC to T-piece, then to the flexi's , then to the calipers.
Rear is MC then copper brake line to rear flexi to T-piece mounted on the diff, then to the slave cyls.

its one of the slave cyl connections which caused the problem, i am replacing the brake line today.. no problem with fixing that, but it worries me that the system isn't as safe as i thought it should be...
Andy Phillips (frankenfrog)

If one system 'dies' completely, how much travel is there on the pedal before it starts to move the other pushrod? I was wondering if you need to restrict the max angle that the bias rod can go through so that if one system leaks away you will not get the pedal touching the floor before the other pushrod starts to actuate the working brake.
r thomas

yes, thats what i was thinking.. if you look at the second picture with the pedal pressed, the bias bar is at quite an angle. Some sort of 'stop' to prevent it reaching this angle would then mean the pedal pressure would then be transferred to the other MC...

But i cant beleive this hasnt been seen before , everything i read says that this is the safe way to go to get a dual circuit system... unless , when you buy a ready made pedal box from the likes of willwood that the MC's have a shorter stroke or something ?

i put a new brake line on today, flushed the system with new fluid and the brakes work well now, to the point where i was testing them and got brake fade... but i suppose thats another story !

Greenstuff pads, less than 500mile old, new EBC discs, not slotted or drilled, fluid is a local make that says it exceeds DOT3 and DOT4, picture below... during the brake fade the pedal was firm, so i dont think its a fluid issue. after a couple of minutes sat in the shade the brakes were fine again...
Andy Phillips (frankenfrog)

picture now uploaded...

Andy Phillips (frankenfrog)

How about fitting a spacer inside the master cylinders to limit piston travel?
Chris at Octarine Services

Hi Chris, good idea too, and invisible...

but with both these solutions, there is still the niggle in the back of my mind...., if one circuit is completely lost and the piston goes so far as to hit the stop then there wont be that much pedal left for the remaining circuit, and if you've lost one set of brakes then you want the remaining ones to be as good as possible..

i'm sure there is something i'm missing...racers wouldnt put their lives at risk at 100mph+ going into a corner and lose both sets of brakes i'm sure....

having said that.. brake lines and unions shouldnt be failing either....
Andy Phillips (frankenfrog)

If you apply full braking pressure to the pedal and measure the piston displacement then fit spacers to allow just a little more travel - say 100 thou - then the spacer will ensure that there is no more than enough travel left on the good circuit.
Chris at Octarine Services

Thanks Chris, makes sense... is this something you have dealt with before ?

Prop, is it worth you checking your system to see if you have the same issue ?, bleed off some fluid from one of your circuits and see if you still get brakes on the other circuit...
Andy Phillips (frankenfrog)

No, but I have had plenty of experience of cars fitted with racing parts that were never intended to be used on a road car!
Chris at Octarine Services

Sorry about that andy...I mis-understood

I cant do that anytime soon..the car is to buried at the shop...its going to take some time to dig out all the work junk around it...

but yes, mine would do the exactly the same thing in your situation

Im using the tilton set up... of there bias bar, pedals, masters and housing...and Its almost exactly like yours

I dont think this is an issue to loose any sleep over...its rare to loose a brake line or a fitting, granted once it happens it is a bit of a mind fu+k...I know, when my crankshaft exploded in the old engine I went way mega overboard to make sure that will never happen agian

If it does happen, you got 3 choices...get a tow, baby as you did with the parking brake or wind out the bias bar so it can run on just one of the MC

Im glad you posted this... because ive never thought of this good to know
Prop and the Blackhole Midget

You can sorta see mine in the corner


Prop and the Blackhole Midget

One thing I did You might consider ....

I replaced most of my solid tin lines with stainless steel braided lines on AN made installation a brease and they have the added security that nothing will go wrong with the lines till my 6th generation grandchild is an old man...I still have a few sections to replace, and it looks alot nicer then hard brake lines

Prop and the Blackhole Midget

Hi Prop,
Yes, until it happens you think youve got a safe dual circuit system.

As a 'get you home', you could have a spacer that fits into the bias bar tube.. wind the bias bar all the way to the working MC as you said, then put the spacer in the tube from the other side (spacer like a thick half a washer). Then that would fix the bias bar at a 90 degree position and should work the remaining MC better...

All ok if you know the system is failing and have chance to stop and sort it out...

I think Chris's spacer in the MC idea is the neatest solution , it means stripping the MC's out and rebleeding the full system etc, but worth it for peace of mind..

Naked girls coming soon, once i can get my car down to Pattaya for the weekend...
Andy Phillips (frankenfrog)

I think I can see what's caused the pivot to travel so far off centre. IS it just the photo, or are the two MC push rods pointing away from each other? Ideally, there should be much less of a gap between the centre pivot and the clevises. This gap is what limits the pedal offset in the event of a circuit failure, and it shouldn't get any where near the threaded rod touching the pivot.

Adjusting the system to maintain the same ratio but with the gap closed up will stop this happening in the future. The gaps where I've circled in red should add up to about 1/4" to allow the bias but not more.

S Overy

PS, having a large washer either side of the pivot is a good idea too, as it helps prevent the surfaces marking each other or catching up some how.
S Overy

Hi Simon,

So if i understand correctly.., the thing that stops excessive travel of the bias bar in case of one circuit failure is that the clevises bind on the side of the tube that is welded to the top of the pedal..?

i think the photo makes it look worse than it actually is, but there is some adjustment i can make to the two arms, i previously understood it that there should be 1/4"gap each side of the bias bar to prevent binding, but you say this should be a total of 1/4" across the two gaps so 1/8" each side. i'll have a go at adjusting that this weekend...

Thanks for your advice.
Andy Phillips (frankenfrog)

Yup, you've got it. The system binding in the event of a failure is the only way it can maintain a decent amount of braking in the remaining circuit. Simply closing those gaps down evenly should sort it all out.
S Overy

Don't fit a spacer - travel restriction into your master cylinder. The master cylinder needs all the stroke it can get to deal with the brake fade (assuming its fluid vapourisation) that you are reporting!
Simon is correct regarding the gaps on either side of the bias bar, pack it out with nylon or PTFE washers so it doesnt bind even at full stroke, but make sure the pushrod displacement on each circuit is similar(to account for fade)or you could get the brake balance changing when you dont want it to!
G Waite


One extra un-related tip

Make sure you enclose the assembly ....I used a bread pan and door seal to enclose my set up...the hole for the peddles sucks alot of hot molten air straight onto your legs and feet...and in summer, it burns really bad

Prop and the Blackhole Midget

Hi All,
I think Simons tip is the way i'll go at first, and try it out by bleeding some fluid off from one circuit.

The fluid in the front circuit is looking a bit dirty so i'll kill two birds with one stone and test the bias bar mod whilst flushing new fluid through.

However that wont be this week as shes at her first beauty show in a bangkok shopping centre for the next 5 days...(seperate thread coming for that)

I think the brake fade was due to pads/disks rather than the fluid boiling as the brake pedal was firm, just that the pads didnt seem to be biting.
I have the 3M black sticker fitted that was supplied with the greenstuff pads, i think that helps in reducing heat transfer to the caliper, and also the brake fluid has a 500 deg boiling point.

prop, i have my feet nearly melted whenever the fan comes on with the air going through that hole just next to the clutch pedal, someday i'll get around to making a cover to block it off...
Andy Phillips (frankenfrog)

This thread was discussed between 15/06/2013 and 19/06/2013

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