I own a fiat 500 L and have bought a Ricambio disc brake conversion. Are there any guides on how to do this, and is it necessary to upgrade the master cylinder too?

I have a number of colleagues with front disc conversions on their 500s who have retained the original master cylinder. If you DO want to go to 'twin-circuit' you will have to modify the front of the car slightly to allow fitment of the master cylinder (it will be longer than the original master cylinder), plus, you may have to fit a 'remote' brake light switch (possibly no room at front of master cylinder or, not fitted on car of cylinder origin). This can be done a number of ways, including a 'banjo-bolt' with built-in pressure switch. You may also have to make new metal brake pipes into the new master-cylinder. If you find that you have problems getting a good hard pedal, you may find that you have to go down a size of rear wheel cylinders; the front discs will use more of the brake fluid available from 1 depression of the brake pedal to get full movement of the caliper piston.
A word of warning--by fitting front discs you will improve the braking ability of the car a fair bit--especially the front brakes. With standard section (3-1/2in or 4.00in) wheels, you will find that the front brakes will lock-up much more easily--ESPECIALLY in the wet! Drive carefully.

With standard section (3-1/2in or 4.00in) wheels, you will find that the front brakes will lock-up much more easily--ESPECIALLY in the wet! Drive carefully.

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Fair point...you have implied that upgraded wheels/tyres should perhaps be considered alongside a brake-disc upgrade, but it's interesting that after the effort and expense of making this modification, you might end up with a car which is possibly inherently less safe.

Prompted me to start another contrarian thread.