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6 cyl Bristol cars Type 400 to 406 - restoration, repair, maintenance etc

Early 2 litre disc brake conversion

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Old 23-06-23, 03:12 PM
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Default Early 2 litre disc brake conversion

I'm pleased to say Graeme Payne has found some 406 hubs for me to go with the 406 stub axles I already had so I have the front setup ready for discs on the 400.

My next challenge is to sort the rear brakes. as 1" wheel cylinders can't be found I am considering getting larger ones sleeved. It should be straightforward to turn down the actuator but the challenge lies with the spreader for the 1" cup.
I know I could fit a proportioning valve but that's far too easy...

I have a new vacuum tank but it's missing the one way valve that the hose clips onto. Any help tracking down something suitable would be welcome.
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Old 23-06-23, 04:22 PM
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My 405 has 3/4 inch rear cylinders which are much easier to find. It was originally upgraded to discs by Bristol who presumably fitted smaller cylinders then, although the service notes just say disc brake conversion. In some later service notes it mentions fitting smaller rear cylinders.
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Old 23-06-23, 06:56 PM
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That's good to know. Any ideas where to get them? I looked a couple of years back and couldn't find any.
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Old 23-06-23, 08:16 PM
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I know that the repair kit (seals and cover) is Lockheed KL 71543 as that is what is on an old Spencer Lane invoice that came with the car's papers.
Those kits fit Hillman Minx, Husky and Rapier rear wheel 3/4 inch cylinders ( 1955-67 ), so perhaps that would be a good starting place. But that is for a 405 - I don't know what you'd need for the 400.
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Old 23-06-23, 08:47 PM
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Thank you for that. Your reply led me to further searches that now show 7/8" bore units being remanufactured in the USA. I feel a purchase coming on. Will report back.
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Old 23-06-23, 09:01 PM
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Better still I just found them available from Abingdon MG parts Ltd. Seemingly plenty of stock and £40 for the pair delivered.
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Old 24-06-23, 04:44 PM
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Default 400 disc conversion.

I did this years ago with a complete 406 front end.

Pretty straightforward, but it can be difficult to bleed the brakes, and I think thatís because my servo is on the left side of the car under the Bonnet with a long pipe run to the master culinder. My 403 had a factory fitted servo+ discs, and that was in a pod on the right side under the Bonnet. It also had a vacuum tank, not needed nowadays. I had SLJ fit a reduction valve to stop the rear brakes over braking.
After years of ownership I found my 403 had different sized rear cylinders. Could explain a 180* spin on heavy braking during a high speed run early one morning.
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Old 24-07-23, 03:14 PM
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According to Powertrack the rear brake cylinders for the 400/1/3 are 1 1/8" and pattern cylinders are available.
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Old 24-07-23, 04:07 PM
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Thank you, Peter.

1 1/8 rear wheel cylinders are ideal if you have standard drum brakes all round, but not if discs are fitted to the front. More efficient front brakes transfer much more load to the front wheels under heavy braking. The nose of the car dips and the rear lifts. If too much braking effort is applied to the rear brakes the rear wheels are far more likely to skid.
I had an excellent practical demonstration of this years ago when I first bought my 403 with front discs. I fitted new brake parts all round including 1 1/8" cylinders on the rear per the motor factor's look up. A couple of days later I met a tractor coming towards me on a narrow country lane. The back end of the car made a spirited bid to overtake me but I managed to straighten up in time. The change to 1" cylinders made all the difference.

I have no experience with servo assisted twin leading shoe front conversions but these may well have a similar issue although not so severe.
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Old 24-08-23, 08:47 AM
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I am now wondering whether it is necessary to fit a vacuum tank with a newer servo. Not having vacuum wipers or washers and now using a slightly more modern servo with built in non return valve I can't immediately see the need.

Also I'm interested to know what ratio servos others use with disc brake conversions. I have a 4.25:1 on my 403. It balances the light throttle pedal pretty well but it's probably more assistance than necessary. I fitted a 3:1 on my 401 and that seemed ok for the short distance I drove it prior to sale. I think the 401 had a 406 pedal box fitted so it would have had more mechanical advantage than an original 401.

All this leads me to the decision that 4.25:1 will probably fit the bill for a 400 with original pedal box. Opinions, please!
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Old 24-08-23, 10:19 AM
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Has anyone considered a brake bias valve gadget in the brake like to the rear wheels? It limits the rear brake fluid pressure based on the braking force applied (has a weight in it that flies forward under braking and shuts off the supply of brake fluid after a certain level of force and nose dip are reached?) there was one in a 70s Vw I had.
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Old 24-08-23, 10:31 AM
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I think most of those who fit any form of front / rear splitter will use a proportioning valve. The type I think you are referring to is mainly used in vans that will have very variable loads in the rear. I have a Mercedes sprinter with one of these.
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Old 24-08-23, 12:36 PM
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The last missing piece of my disc brake conversion jigsaw is a cap for the 403 master cylinder to connect to a header tank. Does anyone have a spare or a suggestion of where to find one?

http://www.bristolcars.info/forums/a...1&d=1692884139
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File Type: jpg IMG_4405.jpg (111.6 KB, 14 views)
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Old 26-08-23, 06:51 AM
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Quote:
Originally Posted by David C View Post
I am now wondering whether it is necessary to fit a vacuum tank with a newer servo. Not having vacuum wipers or washers and now using a slightly more modern servo with built in non return valve I can't immediately see the need.

Also I'm interested to know what ratio servos others use with disc brake conversions. I have a 4.25:1 on my 403. It balances the light throttle pedal pretty well but it's probably more assistance than necessary. I fitted a 3:1 on my 401 and that seemed ok for the short distance I drove it prior to sale. I think the 401 had a 406 pedal box fitted so it would have had more mechanical advantage than an original 401.

All this leads me to the decision that 4.25:1 will probably fit the bill for a 400 with original pedal box. Opinions, please!
I have a 403 with a factory fitted Dunlop disc brake conversion, done in 1958.

Vacuum tank - a very knowledgeable friend pointed out that the vacuum on a 400/401/403 is produced by only the front two cylinders, not the whole engine, as these cars have no full engine installed manifold. I've (so far) dispensed with the vacuum tank, but my 403 is not on the road yet, so I don't know whether this will work. Easy to put the tank back if it doesn't,

Servo ratio - Powertrack matched my Bristol fitted servo as a 1 : 1.9, and they have supplied me with a modern equivalent. Seems that a disc braked car needs less assistance than a drum braked one. Time will tell once I get the 403 back on the road.
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Old 26-08-23, 06:53 AM
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Quote:
Originally Posted by David C View Post
The last missing piece of my disc brake conversion jigsaw is a cap for the 403 master cylinder to connect to a header tank. Does anyone have a spare or a suggestion of where to find one?

http://www.bristolcars.info/forums/a...1&d=1692884139
Bristol did this conversion on my 403. They simply drilled the cap and welded an adaptor to it to take a tubing connector.
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Old 26-08-23, 08:00 AM
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My 403 was also converted to Dunlop discs by the factory in 1958. It has the cap I pictured so I imagine they used what they could get hold of.

I had endless issues with the Dunlop callipers sticking and finally changed to Coopercraft. It was a huge improvement.

I will be interested to hear how you get on. 1:1.9 seems a little low to me.
I'm struggling to find a ratio for the original 406 servo 4257-551.
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Old 26-08-23, 11:59 AM
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Mike and David ,
I have front Disc brakes on one of my 400 and the other has discs front and rear. These are mostly 406 parts, but with some variations .

I disagree that drum brakes need more boost with the servo than a disc brake car . It needs the opposite !!! If drum brakes are over boosted they will easily lock the brakes, even at touring speeds.

Here in Australia all the remote Servo (Booster) are rated in psi .
The common booster for drum brake cars are a PBR VH44 at 820 psi ,
Disc brake front or F&R use either a VH44J at 1270psi or VH40 at 1470 psi.

I fitted the VH44J at 1270 psi to both of my 400's .
I also fitted a Jaguar vacuum tank (from memory ex a Mk9 or 10) to both , which is very similar to to the one fitted to 406 .
The brakes on both are very good and not over boosted .

It is a major improvement to the braking system needed if one actually wants to use the car .

Does anyone know how the ratings of servos in UK eg 1:1.9 or 1:4 compare with psi valves ??

Geoff
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Old 26-08-23, 01:54 PM
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Input and output pressures for PBR boosters can be found here:

https://d31wxntiwn0x96.cloudfront.ne...4_16032018.pdf

This shows your VH44J as 560 in, 1270 out or about 2.25:1
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