Replacing Drum Brakes with Discs

[Geoff Dowdle]

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