brakes pressure release value

[yogibear47.lee]

where to find it on a disco 1

[BogMonster]

In the engine bay IIRC, though rocket science should not be required to follow the brake pipes from the round bits on each corner till you find it

[JimAttrill]

Are you sure the Disco has one? Does the Defender have one, because I can't say I've seen it if it does?

What's it for, anyway?

[BogMonster]

Reduces brake pressure to the rear brakes to prevent premature lock-up of the rear wheels.

The D1 (or some of the D1s) has one, because the one on my old 1996MY went wrong and you couldn't lock the rear wheels even on loose gravel!

[JimAttrill]

Oh, those things are normally at the back of the car, actuated by a rod so that it cuts down the brake pressure when the back lifts. Can't see how it would work in the front of the car, but with Land Rovers you never know

[BogMonster]

No idea - I know the ones you are on about, found on most Japanese 4x4s I have looked at, and with what appears to be a much more logical way of operating!

From RAVE:

"The brake pedal is connected to a vacuum assisted

mechanical servo which in turn operates a tandem

master cylinder. The front disc brake calipers each

house four pistons, these pistons are fed by the

secondary hydraulic circuit. The rear disc brake

calipers each house two pistons, these are fed by the

primary hydraulic circuit via a pressure reducing valve."

and

"3. Pressure reducing valve

To maintain the braking balance, pressure to the rear

axle is regulated by a Pressure Reducing Valve (PRV)

This PRV is of the failure bypass type, allowing full

system pressure to the rear axle in the event of a front

circuit failure. It is sited on the left inner wing [fender]."

So I don't know if it actually varies the brake pressure applied to the rear according to deceleration, the phase of the moon or what Land Rover's technical people had for brekkie, or perhaps just reduces it by a set proportion regardless of whether the vehicle is empty or full of lardy Americans in the back. According to various RAVEs I have, it is fitted to Discoverys, Defender 90s and "some Defender 110s if required to conform with market requirements".

[First_Fleet]

Aussie Spec. D1 Facelift has one where mentioned ^ ^.

[JimAttrill]

"perhaps just reduces it by a set proportion regardless of whether the vehicle is empty or full of lardy Americans in the back"

I reckon the above is the right guess.

There was a big hoo-hah on one of the 4x4 forums here about the danger of fitting air helper springs to Jap rice-burning pickups. The air springs lift the back, which tells the braking system that there is little load, so the brakes don't work properly. There was some mumbling about invalidating the insurance, etc.

[BogMonster]

Yeah I can see that could have some interesting effects on the braking performance

I suppose I now have a rice-burning pickup

[FridgeFreezer]

Isn't this the same brake bias valve as fitted to most RR's? No rocket science to it, it just reduces the pressure to the rear brakes, it's not linked to anything else. Why they didn't just fit a master cylinder that was proportioned correctly I don't know