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Quick Q ref brake G valve..


Eightpot

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Im just trying to sort a braking problem out, and its probably down to the G valve -

I've taken it off and cleaned it out (LOADS of crud inside)- I can hear the ball rattling inside now, which seems odd to me, I would have thought it should be against a spring?

Anyone know if that's right or am I binning it?

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Whats a G valve???

It's to stop the rear brakes locking up under heavy braking.

I fitted one from an early Disco, having fitted a disc braked rear axle.

This Disco one would rattle when given a good shake. I took it apart to clean

it out, lots of muck and rust. It had a very light spring in it. Don't

know if the original one did.

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The valve works to block off flow (pressure) to the rear brakes when under heavy braking (basically to ensure that the fronts lock up before the rear or the vehicle is likely to spin).

The ball should be fairly free moving, given it needs to be flung forward under heavy braking, but provided with some resistance (light spring) when light braking.

The bit to watch is putting larger rear callipers on a vehicle not suppose to have them as they are still likely to lock up before the fronts (increased surface area), thinking 90 with RR rear callipers here.

I think there are also different valves for different flavours of vehicles (the parts forum can advise), so a different valve for a 90, 110, 130 on discs and the different ones for the same vehicles on drums.

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The valve works to block off flow (pressure) to the rear brakes when under heavy braking (basically to ensure that the fronts lock up before the rear or the vehicle is likely to spin).

The ball should be fairly free moving, given it needs to be flung forward under heavy braking, but provided with some resistance (light spring) when light braking.

The bit to watch is putting larger rear callipers on a vehicle not suppose to have them as they are still likely to lock up before the fronts (increased surface area), thinking 90 with RR rear callipers here.

I think there are also different valves for different flavours of vehicles (the parts forum can advise), so a different valve for a 90, 110, 130 on discs and the different ones for the same vehicles on drums.

Which is why I swapped my drum braked "G valve" for the Disco one (Disco axle fitted, would lock up the rears quite easily, now doesn't)

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The G valve on defenders is mounted at a certain place on the chassis rail to give the correct angle for the valve to sit at then only breaking force flings the wee ball forward to block the valve off, when forward motion is retartded the ball naturally wants to roll back down hill. If you look through the service manual there is a spiel on the G valve and the boas valve too, gives a nice cutaway if memory serves.

Mav

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From what I can establish, the G valves were not fitted to all 90/110/Defenders. My 1986 Ninety DOES have one, and started life with drum brakes at the rear. I have rarely ever managed to lock ANY wheels up unless the road has been wet. That said, it has always passed MOTs without concern.

I swapped to a 300Tdi Disco rear axle, and matters barely improved, other than the disks not needing the adjustment and cleaning the drums did. All calipers have been rebuilt, discs replaced, as well as the master cyl and servo with the later 300Tdi version.

I have never touched the G valve, but assume it must be working, otherwise the rear end would lock up. Agree?

I would suggest that hearing the ball moving inside would sound correct. You can exert a fair amount of inertia by shaking such a device.

Was there a spring inside? Or do you think the crud was the remains of one?

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I belive they put the G valve in to compensate between disc fronts and drum rears as genrally you can lock a drum easier than a disc due to the shear contact area.

I got spoken too during an MOT on my 90 when I had the Salisbury drum rear on and I think I'd just put new shoes in and he went for a test drive and slammed the breaks on and the back end went, I then did some hard breaking and noticed the rear end was locking up, I did as Michael did and put in a disco, disc breaked axle and that solved it, so I guess my G valve doesn't work - its still in the break circuit, now I get really even breaking front rear quite nice.

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2p...

I suggest anyone doing an axle replacement checks the parts manual with regards to the braking component numbers, specifically the g-valve (if there is one on your chosen donor), and cross references it with the one on your vehicle.

I know it is easily forgotten, and you probably won't know anything till you slam on the brakes and it gets tail happy.

Remember that an MOT brake test tests an axle at a time (unless you have an LSD in the transfer case, or some other reason it cannot be tested an axle at a time when they can use a tapley meter http://www.ebay.co.uk/sch/i.html?_trksid=p3984.m570.l1313.TR5.TRC1.A0.Xtapley+meter&_nkw=tapley+meter&_sacat=0&_from=R40) so the problem with the rears locking up before the fronts will probably not be noticeable to the tester (unless he takes it for an on road test drive).

I know on the 90, the valve is on the drivers side mounted directly to the chassis, inclined upwards + forwards (close to the exhaust on a v8 [esp. if you have a tubular manifold]).

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Remember that an MOT brake test tests an axle at a time (unless you have an LSD in the transfer case, or some other reason it cannot be tested an axle at a time when they can use a tapley meter http://www.ebay.co.uk/sch/i.html?_trksid=p3984.m570.l1313.TR5.TRC1.A0.Xtapley+meter&_nkw=tapley+meter&_sacat=0&_from=R40) so the problem with the rears locking up before the fronts will probably not be noticeable to the tester (unless he takes it for an on road test drive).

Defenders are tested this way as they don't like single axle roller rigs. So brakes are done with the meter in the footwell, or on a twin-axle roller rig of the garage has one.

At least that is how all mine have been done.

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