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Fiddle Brake woes


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Hi All,

I am looking for some advice with regards to some fiddle brakes on my trialler.

The machine has been mostly rebuilt over the last year or so and the last job was to put some fiddle brakes on it. In its last configuration it had Milner levers directly to some four pot calipers on the back (top pair of pistons for fiddles, bottom pair for foot brakes)and to be honest did not really work, at least not without massive effort and only if the ground was very very slippery.

So in order to get them to work i have the following set up;

New front calipers machined to go on the back as the old ones were mostly siezed, bottom pair of pistons in the caliper for the footbrake, the top pair for the fiddles.

Twin remote servos, one servo for each caliper (ie each side of the fiddles)

Some home made levers where each lever about 18" long pushes on a series three master cylinder.

I was hopeful that this servo assisted set up on new calipers would do the business but not so.

The set up is so poor that when pulling on one of the levers, it wont even slow the vehicle down at tickover in drive and I am pulling so hard on the levers that it is starting to distort the frame that I have made for the levers.

I really am at the end of my tether now as I cannot see any area for improvement. The lever feel is solid and do not believe that there is any air in the system.

The only thing that I can think of at the moment is that I remember someone mentioning that the cylinder diameter on the master cylinder was important but i cant remember much more than that. Would Milner levers be better?

Any help would be greatly appreciated as i'm really P£$%ed off with the whole thing especially after forking out for new calipers.

Hope someone can help!!!!!!!!!

Regards,

Dave

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I don't know of any land rover with rear calipers that have anymore than 2 pistons.

My P38 certainly only has this set up and thats alot more than a ton.

The idea of putting front calipers on the back is to to have a pair of pistons each for the fiddle brakes / foot brake

Dave

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I don't know of any land rover with rear calipers that have anymore than 2 pistons.

My P38 certainly only has this set up and thats alot more than a ton.

The idea of putting front calipers on the back is to to have a pair of pistons each for the fiddle brakes / foot brake

Dave

It sounds like the servo assist isn't, but two not working is very odd.

Master cylinder bore will affect the mechanical advantage of the system (big bore shifts more fluid, but the area is bigger so less fluid pressure for a given pedal / lever force; small bore is higher pressure but less flow - which is what disc brakes need). A way round it is to change the lever ratio, so that there is more of your bit of the lever than the master cylinder gets (if you follow). The Milner site shows levers with a ratio of about 10:1?

However, as your servos are remote, they only work on fluid pressure anyway, so they should be doing some good ...

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It sounds like the servo assist isn't, but two not working is very odd.

Master cylinder bore will affect the mechanical advantage of the system (big bore shifts more fluid, but the area is bigger so less fluid pressure for a given pedal / lever force; small bore is higher pressure but less flow - which is what disc brakes need). A way round it is to change the lever ratio, so that there is more of your bit of the lever than the master cylinder gets (if you follow). The Milner site shows levers with a ratio of about 10:1?

However, as your servos are remote, they only work on fluid pressure anyway, so they should be doing some good ...

Hi,

It certainly feels as if there is no servo assistance with them. I did wonder if it was the case that the vacuum hose (which runs from the Plenum chamber to the servos which are on the back body and then into a T piece) was collapsing stopping the vacuum effact at the servo but it all seems to be fine.

I used to not very scientific method of putting my thumb over the end and the vacuum on it seemed to be the same as the one on the main brake servo (rest of the system). I also swapped over the pipes in the engine bay and also isolated each of the servos by taking out the T piece and running the vacuum pipe directly to it. I can't imagine that its two duff servos.

I remember that the way of testing brake servos is to pump the brake pedal and then start the engine which allows the pedal to fall slightly. I tried this with the levers but did not get any movement when the engine was started. I am still at a loss.

In my mind the fact the levers did not move when the engine was started and also that it "feels" like trying to stop a disc braked car when the engine is not running leads me to believe that the problem is the servo area but I cant see where.

Anyone?

Dave

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I can only think of obvious things!

If you stop the engine and wiggle the inlet adaptor on the servos off, do they hiss loudly?

Are the slave brake cylinders angled up to allow it to bleed OK?

Have you got the brake lines on the slave cylinders in the right holes?

Have you got the brake lines on the master cylinders in the right holes? Early S3 (non-servo) MS are pull operation, later are push.

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I can only think of obvious things!

If you stop the engine and wiggle the inlet adaptor on the servos off, do they hiss loudly?

Are the slave brake cylinders angled up to allow it to bleed OK?

Have you got the brake lines on the slave cylinders in the right holes?

Have you got the brake lines on the master cylinders in the right holes? Early S3 (non-servo) MS are pull operation, later are push.

I've just tried it now and the hose does indeed hiss when i took it off the servo.

The brake master cylinders that I am using (is that what you mean by slave cylinders?) are level but have been bled with a pressure bleeder and the levers are solid.

There is only one hole on the master cylinders I am using so no chance of an error there, and the levers are working on them in the correct manner.

Dave

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For safari racers the most common way to fit them is simply to plumb into the rear brake line and use the same calipers as the main brakes. Mine have always worked perfectly like this.

Hi Steve,

From what I can work out, and now reading various stuff on the internet, it seems that usually a single pair of pistons in a standard rear caliper is not man enouugh to do the job, yours is obviously working tho?

It looks as if I have wrongly assumed that servo assistance would be enough to generate enough pressure to work "half" a caliper but a few people have mentioned on various forums that a fiddle brake to all four pistons seems to work a treat, looks as if I have been ambitious - but rubbish!

I think the best thing I can do as it stands is to sell my new (ouch) servo units and use the money to buy some fiddle brake levers from Milners and just keep it simple I guess. Frustrating though as I was pretty confident that the servo assistance would work a treat but as someone pointed out on another forum, someone they who had this system fitted needed quite a bit of revs to generate enough vacuum. Lesson learned for me I suppose unless anyone can think of a set up that will use my current items. As we speak I am scribbling ideas on a bit of paper.

More head scratching before I go to bed I think!

Dave

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Why not go back to the milners levers and either put a tee piece to the caliper pipes and have four pistons for the foot brake and the fiddle or get some defender calipers machined to fit the rear these only have one pipe as standard working on four pistons and the pad area is slightly greater . The milner levers are designed to work in conjunction with the foot brake system not as a seperate system, with the fiddle levers being piped into the foot brake system.

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You could also leave the servos in between the milner levers and the calipers which would give even better braking, if lack of vacumn at low revs is a problem you could also put a tank in the servo suction pipe to give a reserve supply. This system will give greater braking to the rears going trough the sevos on the foot brake but shouldnt be a problem on a trialer. Wouldnt advise it on road though

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I think your problem is the series 3 master cylinder. These are designed to drive shoes, not callipers - the pressure to drive a calliper is many times greater than that required for shoes (smaller breaking area etc...)

If you replace the series 3 cylinder with a 90 or 110 cylinder it should perform better.

I first discovered this on a hybrid - I had a series 3 servo/master cylinder driving the rangie brakes. The braking performance was horrendous. Replaced with a 90 cylinder and worked a treat!

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I think your problem is the series 3 master cylinder. These are designed to drive shoes, not callipers - the pressure to drive a calliper is many times greater than that required for shoes (smaller breaking area etc...)

If you replace the series 3 cylinder with a 90 or 110 cylinder it should perform better.

I first discovered this on a hybrid - I had a series 3 servo/master cylinder driving the rangie brakes. The braking performance was horrendous. Replaced with a 90 cylinder and worked a treat!

Right,

Think I seem to be getting to grips with the whole thing now and getting in straight in my head.

From what I can gather then my first mistake was using the Series 3 master cylinders as a starting point, from what I've learnt this evening they probably have large bores or something that make them not very good at driving the pistons in calipers.

Hows about this for a plan of action;

Take out my fabricated fiddle brakes / levers / series master cylinder arrangement and chuck them in the back of the workshop.

Buy some Milner Fiddle levers and plumb them in, inline but leave the servos in place for that bit of extra help when i've got some revs on?

Put a T-Piece in each side at the bakc so that all four pistons are operated at once - either when the footbrake is operated or when a fiddle brake is operated?

The only other consideration I might have is to put a brake bias valve inbetween the vehicle servo and the fiddle brake levers? However I reckon that I might be able to do without the brake bias as the footbrake is only used when descending and being an auto the revs dies down anyway so the effect of the servo wont be large, but when I need to "fiddle" round something I will hopefully have a bit more power on and the servo would then would maybe have more effect. There was also a vacuum tank present when the donor vehicle was dismantled and I think I'll just put that back on too!

Feel free to comment and pick holes as I dont intend bleeding the brakes for a fourth time after this is done!

Dave

Am i on the right track yet do you think??!!

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Hi, I am assuming that you are using a petrol engine and not a diesel? if so, you will get max vac when the engine has the throttle shut, also a normal brake master cylinder has to move enough fluid for all the brakes at once and is pressed by the mark one human leg acting on a lever that is about a foot long plus servo, if you only want to move one set of pistons, then you can have a much smaller bore cylinder with enough stroke, give it a nice leverage ratio so your arm can move it and have lots of power plus good feel of the brake.

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