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mahon257

Brake Bias (front/rear)

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For the last 2 MOT's the examiner's have both given my car advisories for rusty rear discs. This week, it finally dawned on it that something's amiss after a trip to JSF to get the brakes bleed after I'd rebuilt the n/s inner wing (another story).

Couple of years ago, I swapped the standard Disco front brakes (solid discs and callipers) for defender ones (wider, vented discs, bigger callipers). The consequence seems to be... less braking force being applied to rear brakes - hence disc's have gone rusty through lack of use.

I replaced the callipers and discs on all 4 wheels at the same time with new parts - so it shouldn't be a fault with the rear callipers themselves...

Does anyone know if it's possible to change the brake bias? I'm told the rear's should do around 20% of the brake load (factory settings). I've spotted a valve of sorts on the brake hoses (near the ABS unit), not sure how to adjust it - or if it's even possible...

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Note to self....

Apparently I need a "Brake Bias Proportioning Valve Pressure Regulator For Brake Adjustment" (fleebay).

or a "Brake Proportion Valve ,Adjustable Prop Valve, Brake Bias Adjuster Knob Type"

No adjustment possible with existing "Pressure reducing valve" (ABS) - so have to install extra gear to reduce pressure to front brakes... and shift pressure to rear.

ABS Brake pipe layout attached.

post-7658-0-93977500-1359125016_thumb.gif

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Rusty rear discs are pretty common on RRC/Discos, so I would be starting there really, unless you can actually prove using maths and a bit of work that you have incorrect brake balance due to mis-matched components (calipers).

The only difference that could cause this (I think) is the surface area of the pistons in the front caliper, do you know that these are different and not just the same with a spacer down the middle of the caliper?

I'd replace the discs and see what your balance is like after that, it may sort the problem entirely, otherwise you are potentially just masking the problem by shifting pressure rearwards.

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Rusty rear discs are pretty common on RRC/Discos, so I would be starting there really, unless you can actually prove using maths and a bit of work that you have incorrect brake balance due to mis-matched components (calipers).

The only difference that could cause this (I think) is the surface area of the pistons in the front caliper, do you know that these are different and not just the same with a spacer down the middle of the caliper?

I'd replace the discs and see what your balance is like after that, it may sort the problem entirely, otherwise you are potentially just masking the problem by shifting pressure rearwards.

Cheers for the reply... I'm not 100% sure that the Defender Brakes I fitted have a different surface area (pistons). I do know that the braking force was at least doubled when I fitted them (nearly popped my eyeballs out LOL). I think there were (at least) two types of the defender callipers, that I could have fitted, in the end I went for the ones with a single brake line - which matched my brake pipe layout. From memory there were also "2 pipe" callipers available.

Good to know it's a common issue (i.e. I'm not alone on this), Following on from your points... this leads me to think the following:

Pistons surface area differences between front/rear callipers - this "feels" right to me (he says, using "the force"). As everything on the wheels was renewed at the same time (disc, callipers) I feeling inclined to doubt it will be some sort of wear issue. The car has done less than 1,000 miles since the new brakes were fitted.

Pressure Reducing Valve - might be seized - though they seem to be quite reliable from reading around? wonder how you would test it? Looks like they're £30 new. Could be worth a punt.

Fitting a brake bias valve - could be a nice "customisation". My truck weight 3.2 tonnes (lots of gear added). Sounds like I'll need to find rolling road to get brakes tested (and remove front prop if not a 4x4 rolling road), if I decide to fit this though. Could be expensive!

Not sure which way to go yet... I'll mull it over for a while! Thanks again.

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Solved this..

The problem was one of the 2 plungers in the master brake cylinder was allowing fluid to pass (i.e. broken seal). The plunger/piston/seal in question was the one that operated the rear brake circuit... hence the rusty rear discs! There was no noticeable loss in brake performance as the fronts obviously do most of the work anyway.

The Master Brake cylinder (Britpart) was less than 2 years old.

I can't put the blame on the part entirely, as when I installed it 2 yrs ago I just threw the Brake fluid in and bled the brakes (I assumed that would be good enough). This time around, with another new MBC, I rigged up a pair of "U" shaped 3 inch copper brake pipes and bled the master Cylinder in a vice (with the fluid being returned to the reservoir with each pump of the cylinder). Took ages to evacuate all the air bubbles (interestingly the problem was the rear circuit again).

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