nitram38
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| posted on 28/6/06 at 07:02 PM |
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Brakes question
After my SVA fail at Mitcham yesterday, one of the failures was my rear brakes at only 50% efficency. (MOT requires 50% and sva 60%).
I have a dual brake bias setup and rear calipers from a 214 rover. The master cylinder is brand new and the calipers are reconditioned. Discs and
pads are new, but I can see that the pads are having an effect on the disc surface.
I have checked the bias bar and all is working as it should. The front cylinder engages just before the rear cylinder and then both move together.
The pedal does not run out of travel and feels firm.
Now the questions:
Could it be that the brake bias needs to go more towards the rears?
Or could it be that the self adjusting caliper needs to be wound out a bit? (suggested by my brother).
I thought that the caliper/hand brake adjuster was automatic.
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pewe
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| posted on 28/6/06 at 07:18 PM |
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nitram, in this month's Practical Performance Car (July issue just out) there is a tech query on a similar subject. Kev Leaper suggested the guy
look at the bore sizes on the master cylinders. Suggest you either buy a copy or drop kev an email (kev@ppcmag.co.uk) I think he'd be interested
in your project in any case. Cheers, Pewe
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nitram38
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| posted on 28/6/06 at 08:06 PM |
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pewe thanks for the tip, but I this is confusing me.
Both cylinders are the same size and the fronts must be bigger than the rears and they work fine.
Neither of the mastercylinders reach full stroke and both feel firm.
If the fronts recieve enough pressure, then surely the smaller rear calipers must be recieving more than enough pressure as they are smaller.
I could understand it being the other way around, the smaller caliper good and the larger fronts in-efficient.
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MikeRJ
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| posted on 28/6/06 at 08:18 PM |
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quote:
Originally posted by nitram38
I could understand it being the other way around, the smaller caliper good and the larger fronts in-efficient.
Exactly the opposite: the smaller the pistons in the caliper, the less force they exert for any given hydraulic pressure (though they move further for
a given volume of brake fluid).
Going back to basic O-Level physics (shows how old I am)
pressure = force/area
re-arranging gives:
force = pressure * Area
So the force with which the brakes pads are clamped onto the disc is proportional to how hard you press the pedal (obviously) and the area of the
piston.
Sounds like you need to move the bias toward the rear a touch. However don't forget that new discs and pads are often very inefficient when
first fitted, they can need anything up to 100miles of normal driving before bedding in fully.
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froggy
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| posted on 28/6/06 at 08:23 PM |
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beat me to it, if youve not done many miles the brakes will be a bit weak as the pads on the rear take longer to bed due to the lower proportional
pressure going through them , when i re test cars failed for rear brake probs they often only just scrape through with new discs/pads as i know they
will get better with a few miles on them
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John Bonnett
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| posted on 28/6/06 at 08:31 PM |
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Hi Nitram,
I'm just wondering whether you may have air in the rear circuit. With a disc set-up there shouldn't be much master cylinder piston
movement at all. I have a similar arrangement on my car and the pedal is solid with little pedal travel.
Are the discs clean of all varnish and any other corrosion protection finishes?
There is no doubt that brakes improve once the pads are bedded in. This is a problem unless you can take the car to somewhere like Bruntingthorpe for
a morning.
I'm sure you have thought of all these things yourself so please forgive me if you have, it's just sometimes fresh thought helps.
John
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nitram38
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| posted on 28/6/06 at 08:47 PM |
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All the brand new discs were cleaned before fitting, but the car has only done a few miles, so this is a possible cause.
The brake pedal does not move far, just enough to take up the slack in the bias bar and then I get a solid pedal.
I have wound the bias another turn and half towards the rears as I was only 10% off the 60% required.
Legally I cannot drive it unless I have an MOT booked etc, but I plan to attach my adjuster cable (disconnected for SVA) and take it to my local
friendly MOT tester
I should be able to adjust it on the cable without dismantling the car.
I will not do this at the sva because he just looked for a roll pin and did not notice the locknuts. Otherwise he may have asked me to adjust the
bias completely to the rears for the "worst case senario" as per the sva book.
I suppose I will be taking it the long way to give the brakes a bit more stick first!
The funny thing is that the brakes feel good and positive without snatching and they also feel progressive.
I just want to ask you guys first incase I have over looked something blindingly obvious 
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