Bentley 3 litre vacuum brake servo query (picture)

Julian West · Joined: 27 Aug 2012 Posts: 29 Location: Bayern, Germany

Earlier this year I came across an article about fitting a modern vacuum brake servo to a 3 litre Bentley. Does anyone know where this was published or have a copy of it that they could let me have? A photograph of the vacuum connection to the inlet manifold for later cars that were fitted with the original Dewandre servo system would also be appreciated.

John Murch · Posted: Tue Dec 16, 2014 7:09 am Post subject:

Sorry to say that I can't really help with your questions, do you have an hydraulic brake set up?
I would think that the easiest vacuum take off point would be that usually utillised for the autovac.

I think my car is fairly stndard (picture taken a long time ago after an engine rebuild, it doesn't look quite like this anymore!)
John

Laurie Fox · Guest

Google "Clayton Dewandre" for quite a lot of general information.

Laurie

William Highley · Posted: Tue Dec 16, 2014 6:51 pm Post subject:

[img]http://www.bdcl.org/forum/userpix/42_Bent9_2_1.jpg[

I had a Feeney and Johnson servo fitted to my 4½. As you can see in the photo the vacuum pipe is connected to the Autovac connection on the manifold.

What make of servo are you thinking of fitting? Somewhere I might have some information on the Feeney and Johnson servo.I could scan the information if it is any good to you. I think they are still being produced although the rubber bellows are very expensive now.

Regards,

William Highley

William Highley · Posted: Tue Dec 16, 2014 6:54 pm Post subject:

Sorry about the picture. I reduced it down to 197 kb. Is that still too large?

Regards,

William

Laurie Fox · Guest

William

See www.bdcl.org/forum/viewtopic.php?t=2446 . If still in trouble email the picture to me and I will get it up for you.

Laurie

Laurie Fox · Guest

Here are William's pictures.

Laurie

William Highley · Posted: Tue Dec 16, 2014 7:55 pm Post subject:

Thank you Laurie for uploading the photos for me onto the Club Forum.

I have found an attachment showing a diagram of the Feeney and Johnson servo. If anyone would be interested in it I could email it to them. I was sent it from "Steering Developments" who still have replacement parts and they may also possibly sell the whole unit.

Regards,

William

Laurie Fox · Guest

William

Or email it to me and I will put it up here.

Laurie

Laurie Fox · Guest

William has sent me two .pdf files. Click www.bdcl.org/forum/userpix/files/BentleyServoLayout2.pdf to see both.

This arrangement moves the brake operating lever to a position corresponding to the depression of the brake pedal provided there is enough suction. The drivers foot will only feel the back pressure from any pedal return spring which may be installed. There is no feedback from the brakes themselves.

Laurie

Julian West · Joined: 27 Aug 2012 Posts: 29 Location: Bayern, Germany

Thank you all for the photographs and diagrams, which are most helpful.

Concerning the 4.5 litre engine fitted with the Feeny & Johnson system, did the bore of the hole in the inlet manifold need to be enarged from its original size? Also is the inlet manifold connection arrangement for the Feeny & Johnson servo essentially the same as for the original Dewandre system?

John Murch · Posted: Thu Dec 18, 2014 9:58 am Post subject:

http://www.steeringdevelopments.co.uk/driving-controls-aids/vintage-bentley/
In the event of failure of the 'bellows', it would appear that you still have standard brakes, but would have to depress the pedal rather further, it that correct? I imagine that would not be a problem unless you had rather more play in the system than necessary.
All very interesting!
John

William Highley · Posted: Thu Dec 18, 2014 10:41 am Post subject:

This is the link to Steering Developments which is the company that bought Feeny and Johnson. I spoke to Neale Boulton who was most helpful when I needed some parts.

http://www.steeringdevelopments.co.uk/

RE. the bore in the hole in the inlet manifold. The servo was originally fitted in the late 1960s by my father . I wasn't at home when it was done. I think it highly unlikely that he would bore out and re-thread the hole in the inlet manifold. I have always assumed that he screwed a brass union into the existing hole. The brass union had a take off which the vacuum pipe was pushed over and fastened with a jubilee clip. The other end went to the Autovac. Whether this union was part of the kit he bought from Feeny and Johnson I don't know. I am not familiar with the original Dewandre system so I am afraid I am unable to advise you on that.

The only problem we had with it over the years was that the rubber bellows would eventually perish although they have only been replaced once since the unit was originally fitted so not too bad! If you have to replace them now I am afraid they are very expensive. If you buy a second hand Feeny and Johnson servo do make sure the bellows are in good order.

The other problem I had was that when the brakes were partially applied the car would "pop back" through the exhaust. If you put your foot hard down on the brakes and kept it there it was OK . It was only when you pressed the brakes partially on that it happened. If you richened the mixture on the dashboard control knob there was no back firing so it was obviously weakening the mixture when the brakes were partially applied. When I spoke to Neale Boulton at Steering Developments. He said it was either the bellows had a hole in them or the soft valve seat in the control valve needed replacing. The bellows looked OK so I replaced the soft seat in the valve unit and it cured the problem.

I don't see a link on the Steering Development site any longer for the servo unit so I hope they still have parts for them. The firm essentially adapts cars for disabled drivers.

Kind Regards,

William

William Highley · Posted: Thu Dec 18, 2014 11:00 am Post subject:

Julian,

This is a picture of the engine prior to the servo being fitted. I have tried to enlarge the relevant area and you can just see where the junction of the autovac connection before the servo was fitted. In the later picture you can see that a brass union had been fitted to enable the vacuum pipe to be connected. Whether it came as part of the kit I don't know.

Regards,

William

John Murch · Posted: Thu Dec 18, 2014 12:08 pm Post subject:

William, see the link in my previous posting, it refers to the Bentley specific kit, so I assume they must still have spares.

Laurie Fox · Guest

John

I have now looked at the link you provided but find it a bit odd. I wonder why they show a picture of a Derby which already has a very good mechanical servo?

Laurie

William Highley · Posted: Thu Dec 18, 2014 2:45 pm Post subject:

Laurie,

The open car is a 3 litre and I think the closed car looks like an 8 litre . I was surprised by the price for the servo unit which is nearly £1200. I wonder if that includes VAT? It seems rather a lot for what it is. I know the rubber bellows were around £200 & they may be more now.

I noticed that Ben Collings had a used Feeny and Johnson servo for sale on his website a while ago. What the bellows were like I don't know. I can no longer get into his website. There may be others for sale as well. I wonder if they are now being taken off cars as the bellows are so expensive to replace.

Thanks for the link John. It seems that they are still able to supply them.

Regards,

William

Christopher Carnley · Posted: Thu Dec 18, 2014 2:48 pm Post subject:

The servo is actually an adaptation of the Magneti Marelli brake booster patented in 1928.

This patent has been cited in several subsequent US and other patents, including the Ayers patents of 1959.

You may see them fixed onto lorry axles as brake boosters, may be obtained from your local Motor Factor, and adapted to the vehicle at very low cost.

Pictures of the original 1930 installation, (courtesy of the chesty Stephen Blakey as picture posting is beyond me) to follow.

(Don't tell Feeny etc that I told you!)

Laurie Fox · Guest

Willam

Yes, you are quite right. The illustration shows an 8ltr, not a Derby. Senile decay on my part!

Laurie

Stephen Blakey · Posted: Thu Dec 18, 2014 11:22 pm Post subject:

John Cobbing took me to the pub today to try to cure my worsening man-flu with steak 'n ale pie and beer. Worth a try anyway. On the way we dropped in at Wayne Huckle's to admire the original bodied H J Mulliner 4½ saloon that he and the team have completely rebuilt (with its original body). I interrogated him about servos and he showed me John Ball's car which Kevin drove back from France as reported in the review.

I attach a few pictures for those not familiar. There's no Autovac on this car hence no T piece on the vacuum tapping. The servo installation is pretty reversible as the only modification to the car is a single new bolt-hole in the gearbox cross member to mount the pivot of the intermediate arm shown on the drawing. He guessed at about £1400 to £1500 for parts and confirmed that they are still made. These installations are a service he provides (removing overdrives is another.....)

It's a matter of personal judgement about the desirability of having that hanging there, à chacun son goût and all that. I reckon it's one of the lowest points on the car, however there's still more ground clearance than on a modern. It's a load more stuff to require maintenance and potentially go wrong. I've always found the standard brakes on the Cobbing 4½ to be very good. They respond most gratifyingly to a good shove on the pedal. Perhaps it's my rider's thighs?

Chris, your image files have shrunk on the way here. Could be the wet weather? Anyway there's not enough pixels for them to be intelligible. Could you please re-send full size?

Christopher Carnley · Posted: Fri Dec 19, 2014 10:03 am Post subject:

That is the Magneti Marelli patent vacuum servo, as adopted, for light trucks in the 1930,s.

The image shows how very slender are the frames of the 4 cylinder cars, necessitating the tension rods to lower the neutral axis.

The contemporary R-R cars had the same design; industry, vogue; defect.

Julian West · Joined: 27 Aug 2012 Posts: 29 Location: Bayern, Germany

Thanks for the photos of the vacuum servo installation as fitted to a 4.5 litre car. Does anyone have experience of how effective this servo is when fitted to a 3 litre car, which has much poorer brakes than the wrap around ones of the 4.5 litre?

John Murch · Posted: Mon Dec 22, 2014 9:24 am Post subject:

In the Technical Facts at 3.16 c) xiii it states 'On no account should the Dewandre Servo unit be fitted to 3 or 4½ Litre cars'.
Can anyone tell me why this might be?
I note that the self-wrapping front brakes were fitted to the 4½ for the 1929 model from XR3332 together with a reinforced front axle.
So is there a suggestion that it is only safe to fit a servo onto the reinforced front axle.
The Dewandre unit looks different to the F & J version, but would seem to operate in much the same way.
http://www.computeroutpost.com.au/item/9781184147051/
I suspect that this might have been covered at some point in the 'Review', perhaps someone with an index could have a look.
John

Robert Craven · Posted: Mon Dec 22, 2014 10:17 am Post subject:

I hope Julian gets some answers to his question about servos and 3 litres since I'm contemplating having one.

Stephen Blakey · Posted: Mon Dec 22, 2014 12:48 pm Post subject:

I think the general idea is that you have the same brakes as before only with a reduced pedal effort.

Interesting point about not fitting a servo to the early cars.

If you're seriously interested, perhaps you should phone the chap who fits them?

Julian West · Joined: 27 Aug 2012 Posts: 29 Location: Bayern, Germany

The article about fitting vacuum servo brakes to a 3 litre may well be in Classics Monthly rather than Practical Classics.

Peter Keevil · Posted: Wed Dec 31, 2014 3:05 pm Post subject: Feeney and Johnson servos

Having had at various times both 3litre and 4.5 litre cars I can vouch for the effectiveness of the F&J system on both types of car. Two points of interest though. 1. Make sure that you fit a good size vacuum reservoir and 2. Include a non-return valve in the system which will protect the installation and if the tank is big enough, allow for up to 5 applications of the braking system even if the engine fails.

Julian West · Joined: 27 Aug 2012 Posts: 29 Location: Bayern, Germany

Peter,

Thanks for the advice about fitting a vacuum reservoir with a non-return valve to the system.

John Murch · Posted: Thu May 05, 2016 8:09 am Post subject:

Julian, did you get any further with your brake servo enquiries, or were you able to improve your braking efficiency with careful adjustment of the system?
John

Julian West · Joined: 27 Aug 2012 Posts: 29 Location: Bayern, Germany

John,

My Bentley has now been fitted with an F&J vacuum servo servo system including a vacuum reservoir.

The braking is now much improved and is proportional to pedal movement rather than to applied pressure. As well as enabling power braking even after the engine has stopped, the reservoir also eliminates the change in engine revs that otherwise occurs when applying vacuum brakes when stationary. Driving the car in London traffic is no longer the nightmare that it was before.

However, as your Bentley stops well without a servo, you probably don't need one. My problem was that I was unable to apply sufficient pressure to the brake pedal for an acceptable amount of retardation. Having a fixed bench seat and short legs didn't help.