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

Replacing Drum Brakes with Discs

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Old 25-07-14, 05:10 AM
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Default Replacing Drum Brakes with Discs

I've finally decided to throw the towel in with the Drum Brakes on my 401. No real stopping power even with a servo and too much Brake fade.

Has anyone had any REAL experience with replacing their drum brakes and what Discs might fit a 401 without mods?

I did spot an earlier discussion of this topic but it was not as informative as I had hoped.

Last edited by Bobbyweaf; 25-07-14 at 06:32 AM.
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Old 25-07-14, 08:00 AM
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Yep , the 405 / 406 discs will go on ( need to get them from a broken car ) but I found the 401 drums with a servo are just as good if not better ! So I would look at the operation / adjustment of what you have first.

Coopercraft do an expensive kit

Could probably make something up from E Class Merc rear discs ( same PCD )
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Old 25-07-14, 09:26 AM
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Bobbyweaf,
I converted my 400/452 to 406 hubs and discs on the front with Girling #17 caliper from a late Humber Snipe (& recently fitted Coopercraft Alloy calipers ) .
The rears I used the rear disc from a BMW 5 series c1990's but the PCD had to be changed to suit the Bristol stud pattern . The M Benz stud pattern is the same as Bristol at 113mm PCD and I know of one 401 in Australia that has the M Benz disc fitted over the STD 401 front hub , but from memory the diameter of the disc is smaller than the 406 or BMW.
You need to use a rear caliper as it has a lot of offset to fit on the outside of the 400 - 405 hub. The 406 caliper fits on the inside face of the front hub and has lesser offset.
On both front & rear I had to fabricate a caliper carrier .
I don't think it is worthwhile modifying the rears as there are problems with the handbrake operation.

If you fit discs to the front only you will have to either fit 7/8 inch diameter rear wheel cylinders OR a brake bias valve to the rear brake line, otherwise the rears will lock when applied hard.

A brake booster/servo of greater pressure than one suitable for drum brakes is essential . For drum brakes I use a servo with 820psi , for disc I use either 1270 or 1470 psi

If you have a 403 sway bar fitted you might have a problem with the caliper fouling the swaybar vertical link on full lock . The 406 has a shorter swaybar with offset vertical links to give clearance with the caliper.

For those wanting to fit disc to a 400 with the early wheels , the deeper ventilation dimples of these wheels might foul on the caliper. There is no problem using 401 - 405 wheels .

It is not a simple conversion but well worth it . I can now drive in traffic at any legal speed knowing I can brake as well or better than most modern cars .
It's a massive improvement over the standard 400 - 403 drum brakes with a servo and a big improvement on the 404 - 405 12 inch Alfins

I hope this helps
Good Luck
Geoff
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Old 25-07-14, 10:19 AM
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Thanks Lads,

Is there a makers number for the 406 Callipers? Presumably Bristol's just bought standard items from X manufacturer that fitted to their requirements.

Sadly, I could not do the job myself but if a garage is to fit them, I would want to give them parts that are a straight fit without causing any problems.
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Old 25-07-14, 10:32 AM
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Pretty sure they were the Dunlop ones that fitted the E type Jag, before they moved over to Girling for the 410 onwards....
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Old 25-07-14, 10:45 AM
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Also make sure you fit new hoses or braided ones -- I have been amazed at the difference this makes, as the old ones must expand a lot like a balloon,making less pressure to the brakes.

Bristol and most modern Merc's are 112 PCD -- I think Stefano Pasini fitted 16" Audi steel wheels to his 409 -- looks the same and more option for a bigger tyre !

But with a bigger tyre you would need power steering !!! Ahh , leave it all alone :-))
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Old 25-07-14, 11:45 AM
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Bobbyweaf,
If you are not able to do the conversion yourself it will become a very expensive operation unless you can find a 406 being parted out . If not probably best to stick with the drum brakes , as with most classic cars today they are used rarely and not to the limit .

Geoff
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Old 25-07-14, 02:44 PM
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I have (refurbed) girling discs at the front and drums at the rear, new piping and a 4:1 servo, and the braking effect far exceeds the tyres (Michelin Taxi) capacity to grip!

It would be a lot cheaper to replace your brake lines first
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Old 27-07-14, 11:16 PM
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Maybe best taking the car to a Jag specialist / restoration company and letting them take the lead and estimate the costs -- armed with the info from here ...

They may come up with something better than all of us !
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