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

[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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