After my sva fail for brakes I was told that my effciency was only 50%.
60% is required to pass.
When I drove the car I could get the front brakes to lock, but the rears were not as good.
I was advised to calculate the correct master cylinder sizes using a spreadsheet. I did that and I found that instead of using two 0.75 masters, it was better to use a 0.625 on the rear circuit.
According to the calculations, this would give me a 60/40 split front to rear.
Now when I drive the car, it will stop relatively quickly, but the fronts no longer lock.
I can hear the tyres just start to grip at the front when I brake.
The fact I can't lock the brakes is worrying me.
As the discs and pads are new, could this be a bedding in problem?

Not much help but I'd be worries too. I can lock my brakes with ease, in fact, when I'm giving it some I often get a chirp from a wheel that goes over a manhole cover etc.

i was always told to put the smaller cyclinder at the front, not the rear. Different displacement sent the fluid different ways.

Other people on here have said the smaller cylinder to the rear
Effciency is all of the brakes readings combined, divided by the calculated car weight divided by 100 (the sva man told me).
But if 2 or more wheels lock then that is a pass too.

A smaller cyl' will generate a higher pressure and that will give greater brake output.

If the rear brakes are under performing then a smaller diameter cyl' will increase the performance of the rear brakes.

If you push hard enough you'll ALWAYS lock one set of wheels. Just depends which set lock first and how hard you push - should always be the fronts first (as you already know!).

[Edited on 5/7/06 by JAG]

Think I have sussed it.
I think there is still air in the rear circuit.
Because the rear cylinder has too much travel on it, the fronts are not getting enough bias.
When I put the handbrake on and try, the master cylinders are firmer and I can see the front calipers moving more.
Anyone good a bleeding brakes

quote:

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If you push hard enough you'll ALWAYS lock one set of wheels. Just depends which set lock first and how hard you push - should always be the fronts first (as you already know!).

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Air in the system YES but wrong size m/cyl' NO.

Like I said one axle will ALWAYS lock first depends how hard you push.

Just bled the brakes and pedal is better. I can see all calipers move when the brakes are depressed. The car stops on a sixpence, but I still cannot get them to lock.
Hopefully it is just pads that now need some bedding in.

I changed my previa front brake pads (servo fed) and it took a couple of days and more than 40 town miles before they felt like they did before.
This car has only 2 miles on the clock!
Hopefully I can get some miles in tomorrow before the retest.

[Edited on 5/7/2006 by nitram38]

quote:

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Originally posted by JAG
Air in the system YES but wrong size m/cyl' NO.

Like I said one axle will ALWAYS lock first depends how hard you push.

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I'm not a gnu, but consider it this way.

"When I drove the car I could get the front brakes to lock, but the rears were not as good." This means the effort you put into the system was being directed more to the front of the car, locking the fronts well before the rears.

"Now when I drive the car, it will stop relatively quickly, but the fronts no longer lock." This is showing that the input you are putting in, is being more evenly spread between front and rear.

So to lock the brakes now is going to take more physical effort?

I guess you are not using a servo? It always takes me a moment to get used to just how hard I have to press the peddle in the Indy. Especially when I've just got out of my 2 ton German Estate with it's massive servo and brake assist thingy.

Good luck with the test anyway.

HTH

Mike

Finally I have solved the problem!!!!!!
The bias bar centre was too low on the pedal.
Because the pedals are floor mounted, the master cylinder pushrods were being forced downwards in a cyclic motion so that they would give way and actually stop providing continous pressure.
I solved it by moving the bias point up higher so that the pushrod started at an angle, but as the pedal was pushed the pushrods levelled off and the force was horizontal to the cylinders.
I only raised it about 8mm.

What a difference. Now I can lock the fronts without the pedal bottoming out and stand on the brakes if I have too!!
It took me 3 hrs to take them out and cut and re-weld the bias tube.

I can go to bed happy now!!!!!!!!

Nice to hear it's sorted!! Hope mine work out fine as i've used the same spreadsheet . Sounds like your beast stops pretty well now then...

Liam