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sutty_32

Low rear brake pressue

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Morning all,

I'm after a bit of advice from people's experience.

My disco failed its MOT at the weekend on rear brake effort, the % was too low compared to the front. The numbers for each side weren't too dissimilar, and although the rear pads and discs aren't new, they don't look old enough to be causing the issue (but I will change them anyway just to be sure).

We figured it is likely to be low brake pressure and the MOT guys thoughts were it would be the pressure valve that reduces the brake pressure to the rear brakes to stop them locking up.

These are quite expensive and just wondered if anybody knows of a way to test if this is faulty, or if its the brake master cylinder as I think this is split between front and rear system. Would swapping the front and rear lines to the master cylinder show up any fault in this?


Thanks

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Swapping the front & rear on the master cylinder might show up the problem - by the back locking up before the front. It might be worth replacing the master cylinder seals just to rule it out as they are low cost. At the very least, it will narrow down the problem.

I've not heard of the PRV's failing - but it's always possible. It should be possible to find a used one - or possibly even service your existing valve?

Si

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Cheers Si, I didn't realise you could buy seal kits for them so I'll give that a go. Whilst its in the post I'll swap the lines round tonight, even if it doesn't lock the tyres (I have 33x12.5's on) and I don't fancy going very quick as if I'm right, I won't stop very well. I should at least be able to see how much the front nose dives at low speed.

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I have a feeling that the valve concerned is just a delay. If you brake gently, the front & rear come on together and the difference in capacities of the calipers gives the 60/40 braking ratio. When you stamp on the brakes, it partially shuts off the back and restricts the flow, so the front comes on before the back.

IIRC it's just a spring loaded ball bearing in a tube. when the spring is compressed by the flow, the ball sits against a seat, restricting the flow.

I suspect that if the spring were weak or the ball has jammed - it woul cause this problem. You can disassemble the valves (the end unscrews) and may be able to clean / replace components?

Si

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1st check the Master Brake cylinder (MBC) is doing it's job. It's cheap and easy to do.

Whip off the MBC, mount it in a vice. Rig up a closed loop.. for example:

http://forums.lr4x4.com/index.php?showtopic=83293&hl=

I recently had a problem with the rear circuit (led to rusty rear discs), which was caused by a failed seal (even though it was a brand new Lucas MBC from Paddocks). Sent mine back - got a warranty replacement - works fine now.

You might need to invest in a bit of copper brake pipe, and a pipe flaring kit kit.. (less than £10) but a worthwhile investment for LR owners anyway!

Look for evidence of Air Bubbles in the rear circuit when pumping in a closed loop

Good luck!

Nick

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Thanks, was going to start stripping down tonight but thats a good check of master cylinder. I've got a flaring kit already, just need a bit of copper pipe. I guess I can even do this on the car as a very quick check!

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Bit late, but when I had this problem it was my rear flex pipe. The MOT guy said it maybe this, swopped it and MOT next day and passed.

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Thanks for all the replies, I rebuilt the master cylinder and put some new discs and pads on the rear and it passed fine. Might have been a mix of both, but the brakes to feel nice and firm now!

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Thanks for all the replies, I rebuilt the master cylinder and put some new discs and pads on the rear and it passed fine. Might have been a mix of both, but the brakes to feel nice and firm now!

Congrats ! Very satisfying I'll bet.. :i-m_so_happy:

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