The Lotus Grand Tourers

Rear discs on my car are not shiny and lightly scored circumferentially as I would consider normal.
Brake pad wear is hardly noticeable since I bought car around 6000 miles ago.
Disks look slightly mottled. Not rusty and not pitted, just a bit speckly. Both faces of both discs look the same. Calipers were rebuilt 18 months ago and slide freely.
On the plus side there is light brake dust around and it just passed an MOT today, but I'm sure something is not 100%.

It's almost like the rear gets no significant pressure until you really stand on the pedal. I note the MOT tests front axle, then rear independently, so can't consider differential pressures between the two circuits as far as I can see. By the time the rears showed a pass, the fronts probably had long been locked under a typical driving conditions I suspect.

Any knowledge on how the G valve works and whether it can be ,'calibrated' by turning orientation. I note mine sits at quite an angle.

My only other thought was Master Cylinder seals on the rear circuit, but no sign of leaks, fluid level never changes and as I say, it did pass an MOT today.

Thoughts please.

I don't know for sure Tony, but I suspect the Lotus factory one is a simple piston with larger diameter on the outlet side to reduce pressure. This type are more commonly used in OE applications and are not adjustable.
If it is sticking then that might cause you to have to stand on the brakes before getting any rear brake.
The adjustable ones work on a different principle, and have always been a bit of a mystery to me. There is quite a good explanation on the Wilwood brakes site https://shop.wilwood.com/blogs/news/how ... valve-work
, and that would suggest that an adjustable one would never give the symptoms you describe as they only work under heavy braking - they're "invisible" under light braking. If it was jammed then you would always get full pressure to the rear brakes.

I didn't have to press the pedal hard to activate the rear brakes when I tested mine with the engine running and gears engaged on axle stands, so it does sound like something is not quite right. Mine is also mounted on an angle.

malrig wrote: Tue Apr 08, 2025 15:41 I don't know for sure Tony, but I suspect the Lotus factory one is a simple piston with larger diameter on the outlet side to reduce pressure.

that would be hard to bleed.

A typical brakevalve lets the fluid through until a set pressure is attained, then it prevents the pressure from increasing to prevent locking the wheels under heavy braking

I thought the 'G' valve was essentially a steel ball in a cylinder that was slightly inclined at the front of the car.
At the start of braking the brake fluid flows through the cylinder and activates the rear brakes 'strongly'. As the car decelerates the ball moves to the front of the cylinder (against gravity) and so progressively blocks the flow of fluid to the rear brakes. This has the effect of putting more braking force into the front brakes as the cars weight shifts onto the front wheels - and so prevents the rears locking up.

My G valve was so corroded the ball was stuck in position. In my case leaving the valve open all the time. Yours could be stuck mostly closed?
If you take the valve off the car and shake it, it should rattle as the ball moves within the cylinder (that does involve draining the brake fluid of course)

Im with lozza on the method of operation. Have you still got rubber flexies on, they could have 'cats arsed' although sticking brakes is an indicator for this.

Bash

My G valve was so corroded the ball was stuck in position.

Thanks Lozza. I'd read somewhere that they were virtually indestructible and just didn't go wrong so very useful to know you had problems.
Did you repair the old unit or was it too badly corroded?
Also looks like the end cap can be removed ( after a fluid drain) without removing from car - Does that give access to all the operational bits?

I also read that bleeding can be a bit awkward.

My research had led me to same conclusion on operation, which is why I wondered if they could be "calibrated " on the car by altering the angle of installation. I couldn't find detail on our particular valve, but I did find similar diagram for a Left / right balance change with a central sprung supported ball in a Tee Piece. Here cornering G would have moved the ball, reducing pressure to one side.

The left right ball valve trick is where some F1 cheating was identified by a certain team recently. Not practical in a road car as it only works if you are already braking hard into a corner.
I've done a lot of playing with my brake 'feel' recently. Initially, changing the master cylinder did improve the feel. The old one wasn't showing any signs of a problem but it had done 140k miles. I went for the landrover alternative and an adaptor on one of the pipe outlets.

Bash

Have you got a picture of your g-valve Tony? I think I may have kept mine from the Excel - in the depths of the shed somewhere? Are they all the same on the excel or did they change with the year? Mine was an '89 MY. I'll have a look-see.

Pete

There’s only 1 part number for all years, so should all be the same.

Found it! I'll take it to bits over the weekend to see what's what

Pete