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Discovery 2 Axles - Brake Headache


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Hi All,

Currently scratching our heads over an issue that has been plaguing us for a while. We run a car on D2 axles with a D1 brake set up, plumbed in as per Rave. It's a Rover V8 running an edelbrock.

The trouble we are having is a sinking brake pedal when the engine is running.

It's had all new components including master cylinders (various different types/makes), new servos, SS brake hoses, fully renewed copper pipes, bias valve (again multiple including a new one currently) and a full set of D2 standard pads/calipers/discs on it. It's also had a good 15 litres or more of fluid through it through various means (gravity, old school pedal pumping, easi-bleed and mityvak) and the master cylinder was bench bled each time prior to fitment.

The brakes with no servo are rock hard but with very little stopping power. The brakes with servo assistance are spongy to say the least but stop on a sixpence. Pedal travel is about half way as is normal, however if you hold at that or hit it hard it will sink to the floor within seconds.

We have also tried the servo vacuum off both the Edelbrock take off and also the standard fitment on the performance intake. All of which run at the same hg. as a standard V8 Discovery 1/P38 does according to the Mityvaks vacuum gauge on our other vehicles.

Any body have any advice on where to look next? The brake stop well but when you're hurtling across a field on an event and the brake pedal hits the floor despite stopping immediately, it doesn't exactly fill you with confidence.

We have tried over the years even plumbing it in with a mix of D1/D2 set ups on the plumbing to no avail.

Thanks in advance.

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Just a thought but does the d1 master cylinder move enough fluid? Maybe the d2 cylinder moves more fluid in one press of the pedal so you're having to push further for the d2 calipers to actually operate.

Just a suggestion as you seem to have covered most other possibilities.

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Flexing callipers could be the cause of the spongy pedal? David at Llama4x4 mentioned it to me when we were talking about the possibility of using stainless throughout. He had a customer that couldn't get a decent pedal with D2 callipers and full stainless lines. After swapping out the callipers it was perfect.

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  • 3 weeks later...

I've previously had issues with bleeding Disco 2 callipers, carp pedal even when you've run gallons of fluid through.
In my case the problem was solved by removing the callipers and bleeding them with the nipple pointing upwards. Big gob of air out of the bleed nipples that were previously giving clean fluid, and the pedal came up well.

I don't think there's much difference in sizing between the D1 and D2 master cylinders, and I would also suggest that the callipers on the D2 require less fluid to shift as they've got half the number of pistons that you'd find on a D1

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and I would also suggest that the callipers on the D2 require less fluid to shift as they've got half the number of pistons that you'd find on a D1

That doesn't depend on the number of callipers but on the area of the callipers! ;-)

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2 big ones does the same as 4 small ones.

The same on Mini 2 pot and Metro 4 pot callipers: they have exactly the same total piston area. It doesnt make any difference.

That said, I dont know the diameters of the D1 and D2 pistons so in that case I dont know the difference :-)

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In hydraulic terms a pair of opposed pistons works exactly the same as a sliding design of the same size. The bore/housing in the sliding caliper acts as the second piston.

So a twin piston sliding caliper with 40mm pistons will work and displace exactly the same fluid as a 4 piston opposed caliper with 40mm pistons.

When comparing sliding and opposed calipers, you can simply compare one half of the opposed caliper to the sliding caliper.

D1 pistons are 41.2mm.

D2 pistons are 48mm.

Which is quite a difference.

Defenders use 46mm though, so maybe the Defender master cylinder would be a better match if your not already using it?

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In hydraulic terms a pair of opposed pistons works exactly the same as a sliding design of the same size. The bore/housing in the sliding caliper acts as the second piston.

I know that ;-)

Difference is 36%.. Which is a big difference indeed!

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