G Valve rebuild kit

Hi Jeff

I appreciate it's pointing out the obvious but have you already checked the flexible brake hoses in case one of them has deteriorated internally and acting as a partial check valve?

David

Thanks Chris, much appreciated.

DAVE, Thanks for the tip (not always obvious). However, I do feel that if it was flexible hoses causing the problem, then yes, they'd cause the affected brake(s) to hold on by maintaining pressure, post blockage towards the wheel cylinder.

In this case, the brake lights stay on. I know this because I have a tell-tale warning lamp fitted in the dash, wired into the brake light circuit.
That, I think indicates pressure is being held in the servo or the G valve where the brake light switch is fitted and so is before any flexible hoses (if you see what I mean).

I know it's not the master cylinder because I put a new one in, which didn't make any difference.
So, that leaves the pipework between master cylinder and servo, and pipework between servo and G Valve. I know this to be good because I replaced it all last year, made with a proper bender etc, and the problem existed before I replaced it.

Finally, that leaves only two possibilities which could maintain pressure enough to keep the brakes and brake lights on. They are the servo itself, and the G valve.

I've just replaced the air valve on the servo and tested it. The car sits in the garage with the front wheels about 4 inches off the ground, slightly nose up on hydraulic ramps.
I can operate the brakes 100 times, very fast and hard. There is a definite "thud" and rush of air heard from the servo each time, and the tell-tail brake light ALWAYS goes off immediately the pedal is released.
It only stays on, and you can feel the drag, when the car is out on the move or just stopped. The brakes release fully and the tell-tale goes off, after a few seconds or when moving off again. The latter would be a very similar symptom to collapsed flexible hoses, like you say.

No matter what I do, or how often I stamp on the brakes while stationary, I cannot induce the fault. I.E, make the tell-tale and brake lights stay on.

I did notice the hang, and the tell-tale remaining on when facing downhill, shunting around on my driveway which has quite a steep gradient. No hang is noticed when facing backwards on the steep drive.

So, in conclusion the next step must be to eliminate the possibility of a sticky G Valve.
I hope this all makes sense.

I did find a new old stock G valve for sale on a parts website for £109..ouch! But even that would have old seals and lubricant so it's probably gummed up inside by now, so I'll see what Chris comes up with and take it from there.

Jeff.

Well well well,
I was able to get out in the dry today and do several tests uphill, downhill, forwards and backwards.

The deeper I got into these tests the more interesting it became.
I now find that the "hanging brakes" happen under light braking mostly, both backwards and forwards, and on the level if I release the brake slowly.
I know all brakes are hanging because on a steep slope, I can clearly hear the fronts let go a second or two after releasing the pedal.

Now this is where it gets interesting. I switched off the engine and pumped the brake until all the vacuum was exhausted from the servo.
Then,I turned on just the ignition and let the car roll downhill. Under hard and light braking, the brakes released immediately I took my foot off the pedal.The brake light tell-tale lamp going off and the car rolling forwards.
With the engine running, I could get the brakes to hang but only if I let my foot off slowly from light braking.

In conclusion (again), I think the servo air valve hydraulic piston needs removing and inspecting.
Of course it could be a combination of servo and G Valve giving problems but I think the servo is the starting point.

I have read on another site, Lotus I think,that this problem is known to Lockheed, and they, apparently, recommend removing one of two seals from the little air valve hydraulic piston to reduce its resistance. They say its quite safe (allegedly).

So that's the next step.
I will of course update in due course.

Jeff

DAVE,
Thanks for the link and info
Unfortunately, the part number shown on the G valve doesn't match with what I found in an online old Girling parts catalogue for our earlier cars.
The number appears too high so it's possibly for a Land-rover I will check further.

To make matters worse, I stripped and cleaned the servo air valve today, including its little piston, illustrated in the photos in Chris's Post.
Cleaned and greased with Girling brake seal grease, new rubber diaphragm, slightly streching its valve spring, and a good bleed.

Road testing revealed this:

Brakes only dragging/hanging under light braking when going forwards, brake lights remaining on for a few seconds.

Rolling back on a steep hill and braking no drag.

Moderate and emergency braking, no drag.

As far as I'm aware the servo has a push through facility to bypass its action under light braking and this is when the problem occurs.

I can't see what else could be holding all four brakes unless something has collapsed or swollen in the G valve.

I'll wait until Chris sends me a repair kit and tackle the G valve.
I'm sending myself in ever decreasing circles with this one slowly disappearing up my own preverbial orifice.
My head tells me it's the Servo but the testing shows its the G valve but I would have thought the G valve would play up under heavy braking and only affect rear wheels, not all four.

To make matters worse, the fault is intermittent to some degree.

AAAHHHGGGGG.GRRRRRRR

JEFF.

As Jeff has noted the problem affects all 4 wheels, that seems to rule out the G valve as being the cause of the hanging as it only limits rear brake pressure. It is curious that the problem doesn't occur in reverse though all I can think is that light retardation going forward requires less foot pressure on the brake pedal than when going backwards because of the design of the rear brake shoes - clutching at straws! After all this time the G valve seals may well have deteriorated so that it no longer functions as intended so probably well worth overhauling.

It may be worth considering temporarily by passing the servo to determine whether it really is the culprit.

David

It might be worth considering the master cylinder. If this is not returning properly it may be holding some pressure in the line, making it appear to be a servo problem. I would think that this type of issue would be most likely with small movements of the master cylinder piston. In other words when the return spring is at its weakest and if the piston is a bit tight.

Nick