[Healeys] Drum to disc conversion - master cylinder change and a few questions

[Kees Oudesluijs]

Phil,

If you have a single brake system you better convert to a dual system 
front/rear first.

I think I would change to front disk brakes only for the time being 
leaving the rest as is and see what will be needed from there.

The brake pedal travel will increase because of the larger piston 
surface area so it may or may not be necessary to change to a bigger 
diameter MBC (which will increase pedal effort). In any case it will be 
necessary to overhaul the MBC if it has been in operation for any length 
of time as it may fail due to the ring of crud formed at the end of the 
pistons travel.

If the rear brakes lock up to quickly fit rear wheel cilinders with a 
smaller diameter or an adjustable pressure limiting valve on the rear 
brakes. Another possibility may perhaps be to have the brake servo on 
the front brakes only.

The pressure needed for the operation of front callipers also depends on 
the total surface area of the calliper pistons. It may be well worth to 
use after market 4 or 6 piston callipers from e.g Willwood or BCC 
instead of the original A-H callipers.

Kees

Op 16-1-2023 om 23:14 schreef Philip Fish:
> Hi all,
> I'm a long time Healey owner having had my BT7 for over 30 years.
>
> Recently I had a complete failure of a brake front drum (photos below) 
> on my 1956 Jensen 541 (a 4ltr Austin powered contemporary of the 
> Healey) resulting in a total loss of my brakes. Not an experience I 
> want to go through again. As a result I am looking to do a drum to 
> disc conversion using predominantly Healey parts
>
> My car uses several components in common with the BN1, in particular 
> it uses the same king pin/swivel axle set up. I have pretty much go 
> the parts needed for the conversion (Denis Welsh Heavy Duty swing 
> axles, AH 3000 steel wheel BTC101 hubs, taper bearings, disks and hub 
> adapters etc) but I have some concerns about what changes I might need 
> to make to the rest of the hydraulic system, particularly what changes 
> I will need to make (if any) to the master cylinder and rear wheel 
> cylinders.
>
> I was wondering what the list subscribers have done to the MC and rear 
> wheel cylinders when you've converted the front drums to disks on your 
> early cars. I believe that the front calipers need higher pressures 
> (900psi) to operate than compared to the rear drums (400 psi) and was 
> wondering if I need to fit some for of brake balance valve to stop the 
> rears locking up before the fronts. Do you change the bore of the rear 
> wheel cylinders when doing a conversion as an alternative? I have also 
> read on the web that there may be a check valve in the MC which I 
> might have to remove. Apparently with a drum brake set up the check 
> valve retains some residual pressure after braking to prevent any air 
> getting into the wheel cylinders, but with a disk brake set up this 
> could cause brake drag.
>
> I would hate to do the conversion only to have worse brakes than my 
> twin trailing shoe front drums that I have now.
>
> Currently I have two 5/8" wheel cylinders on the rear, four 1 1/4" 
> wheel cylinders on the front and a 3/4" master cylinder with a pretty 
> small reservoir. The system also incorporates a Lockheed 3:1 brake servo.
>
> Any advice would be appreciated.
>
> Phil
>
> https://joc.org.uk/jensen-models/541-series/
>
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