Post by dave1800 on Sept 19, 2013 13:46:55 GMT
In the Autumn edition of the LOCI magazine, Tony Hobs raised an important safety issue regarding the G valve as found on Mk1 and Mk11 Landcrabs. Failure of this valve can lead to the rear wheels locking even under relatively light braking with potentially fatal results. Having had a rear wheel lock without warning (not actually the G valve) I can assure you that travelling down the road at 30mph at right angles to the intended direction of travel is quite interesting 
I have agreed with Michael Sanchez (editor of LOCI) that I should bring this to the attention of the readers of this forum ahead of any future article in the LOCI magazine. I am sure many of you are fully aware of the functions of these valves and apologise for spelling them out; however I would not be surprised if newer owners are not familiar with them. A valve failing or failed will normally not show up in a MOT unless the car is driven. I would imagine the most likely cause of failure is if the brake fluid has become contaminated and internal corrosion prevents the ball bearing cut off valve from functioning. While this was usually a reliable component, degradation may take place over the years and it is certainly worth checking before an accident occurs.
Here is a copy of the note I drafted to Michael today; please let me know if you spot any errors or can add to my description and experiences.
Chris: if you think it a good idea maybe you would wish to post a link on the Wolseley Forum?
Dear Michael
Tony Hobbs made a comment about potential dangers from failure of the Girling G valve in the Autumn LOCI Magazine.
I am not sure if you wish to add to what he has already written or whether you would like to pass this note to him for his comments? I believe he has raised an important issue that could potentially be life threatening.
When a valve fails completely then the effect is immediately obvious but it may be helpful to look at my personal experience back in the mid-seventies.
It would be very interesting to hear others' experiences and if you have no objections I would like to float this on the Landcrab forums?
Background
In order to maximise the braking capability of the 1800, BMC introduced a Girling G sensing valve to provide an early form of anti-lock brakes.
The Mk1 cars used a G valve mounted at either 13 degrees stamped on the bracket or 17 degrees up to A17S 15245A on the right hand inner wing. This sensed the level of braking (G) and at a predetermined level the deceleration forced a ball bearing to close the main feed to the rear brakes but allowed for the pressure in the rear brake lines to continue to increase via an integral differential pressure reducing valve but more slowly than to the front brakes. This had the effect of preventing rear wheel lock up. With more load in the rear the front of the car rises which in turn makes the sensor actuate at a higher deceleration. It was used with the Girling Powerstop Mk2 Servo.
This was simplified for Mk11 cars with a full cut off to the rear brakes once a predetermined G had been reached. It was mounted at 25 degrees in the same place as the Mk1 valve. It was used in conjunction with the Mk2b servo.
With changes to regulations about rear wheel lock up, the Mk111 cars used a simple pressure reducing valve located close to the rear wheels together with the Mk2b servo (UKcars).
Those of you who have driven the full range of 1800s may note that the brakes on Mk111 cars do not appear as good as the Mk1/11 as the pressure applied to the rear brakes under gentle to moderate braking is less and the cars tend to dip the nose more not benefiting from the pull down of the rear suspension as the rear brakes bite offering some degree of anti-dive.
As you will be aware rear wheel locking can result in instability under hard braking with a tendency for the rear to “fish tail”.
My experience
Many years ago I suffered problems with premature rear wheel lock up on a Mk11 car. The rear drums and cylinders were stripped down but nothing untoward was found and the rear flexible hoses replaced as these can sometimes create what are effectively non return valves if they decompose internally. The G valve was suspected, but on removing it, the ball could be heard to be running freely. However as this was a real safety issue it was changed but made no difference. A thorough check of all the suspension, steering joints and alignment revealed nothing. Eventually the problem was cured by changing both rear brake cylinders. They had shown no signs of sticking or wear or internal damage when the drums were removed.
I was advised by the local BMC agents that they were aware that if rear brakes were adjusted too tightly that on applying the brakes hard there was insufficient time for the G valve to actuate and rear wheels could lock under hard braking on Mk11 cars only. This may have been more of an issue with warped drums or other reasons for high spots on the rear brakes.
A few years later I had a similar problem with an Austin Ambassador which had no G valve that locked one rear wheel under light load. A change of wheel cylinders again resolved the problem even though there was nothing apparently wrong with them.
Options
If the G valve is faulty and no replacement can be found or it cannot be repaired, I believe the safest option is to fit the pressure reducing valve as fitted to Mk111 cars. If this valve is not readily available there are many others that can be used including some that are adjustable so can be matched more precisely to the 1800. Before resorting to these measures it is important to check all of the rear brake components including hoses and ensure they are not adjusted too tightly in an attempt to try and raise the brake pedal height.
David
I have agreed with Michael Sanchez (editor of LOCI) that I should bring this to the attention of the readers of this forum ahead of any future article in the LOCI magazine. I am sure many of you are fully aware of the functions of these valves and apologise for spelling them out; however I would not be surprised if newer owners are not familiar with them. A valve failing or failed will normally not show up in a MOT unless the car is driven. I would imagine the most likely cause of failure is if the brake fluid has become contaminated and internal corrosion prevents the ball bearing cut off valve from functioning. While this was usually a reliable component, degradation may take place over the years and it is certainly worth checking before an accident occurs.
Here is a copy of the note I drafted to Michael today; please let me know if you spot any errors or can add to my description and experiences.
Chris: if you think it a good idea maybe you would wish to post a link on the Wolseley Forum?
Dear Michael
Tony Hobbs made a comment about potential dangers from failure of the Girling G valve in the Autumn LOCI Magazine.
I am not sure if you wish to add to what he has already written or whether you would like to pass this note to him for his comments? I believe he has raised an important issue that could potentially be life threatening.
When a valve fails completely then the effect is immediately obvious but it may be helpful to look at my personal experience back in the mid-seventies.
It would be very interesting to hear others' experiences and if you have no objections I would like to float this on the Landcrab forums?
Background
In order to maximise the braking capability of the 1800, BMC introduced a Girling G sensing valve to provide an early form of anti-lock brakes.
The Mk1 cars used a G valve mounted at either 13 degrees stamped on the bracket or 17 degrees up to A17S 15245A on the right hand inner wing. This sensed the level of braking (G) and at a predetermined level the deceleration forced a ball bearing to close the main feed to the rear brakes but allowed for the pressure in the rear brake lines to continue to increase via an integral differential pressure reducing valve but more slowly than to the front brakes. This had the effect of preventing rear wheel lock up. With more load in the rear the front of the car rises which in turn makes the sensor actuate at a higher deceleration. It was used with the Girling Powerstop Mk2 Servo.
This was simplified for Mk11 cars with a full cut off to the rear brakes once a predetermined G had been reached. It was mounted at 25 degrees in the same place as the Mk1 valve. It was used in conjunction with the Mk2b servo.
With changes to regulations about rear wheel lock up, the Mk111 cars used a simple pressure reducing valve located close to the rear wheels together with the Mk2b servo (UKcars).
Those of you who have driven the full range of 1800s may note that the brakes on Mk111 cars do not appear as good as the Mk1/11 as the pressure applied to the rear brakes under gentle to moderate braking is less and the cars tend to dip the nose more not benefiting from the pull down of the rear suspension as the rear brakes bite offering some degree of anti-dive.
As you will be aware rear wheel locking can result in instability under hard braking with a tendency for the rear to “fish tail”.
My experience
Many years ago I suffered problems with premature rear wheel lock up on a Mk11 car. The rear drums and cylinders were stripped down but nothing untoward was found and the rear flexible hoses replaced as these can sometimes create what are effectively non return valves if they decompose internally. The G valve was suspected, but on removing it, the ball could be heard to be running freely. However as this was a real safety issue it was changed but made no difference. A thorough check of all the suspension, steering joints and alignment revealed nothing. Eventually the problem was cured by changing both rear brake cylinders. They had shown no signs of sticking or wear or internal damage when the drums were removed.
I was advised by the local BMC agents that they were aware that if rear brakes were adjusted too tightly that on applying the brakes hard there was insufficient time for the G valve to actuate and rear wheels could lock under hard braking on Mk11 cars only. This may have been more of an issue with warped drums or other reasons for high spots on the rear brakes.
A few years later I had a similar problem with an Austin Ambassador which had no G valve that locked one rear wheel under light load. A change of wheel cylinders again resolved the problem even though there was nothing apparently wrong with them.
Options
If the G valve is faulty and no replacement can be found or it cannot be repaired, I believe the safest option is to fit the pressure reducing valve as fitted to Mk111 cars. If this valve is not readily available there are many others that can be used including some that are adjustable so can be matched more precisely to the 1800. Before resorting to these measures it is important to check all of the rear brake components including hoses and ensure they are not adjusted too tightly in an attempt to try and raise the brake pedal height.
David
