Front Disc Brake Conversion with Stock Master Cylinder?

[six6dub]

I'm getting ready to put in a 4" narrowed beam with disc brakes conversion. My question is do I need to upgrade the stock master cylinder? I recently replaced the MC when doing a 4-wheel brake job and I'da rather not got to the larger 19mm dual circuit if I don't have to. Follow up Q: if I am able to to stay with the stock 17mm MC, do I need to add a larger reservoir to it (the one that sits on the MC)?

[Patty66]

I think the concern is with the drum brake wheel cylinders needing a residual pressure valve vs the disc brake calipers not needing residual pressure.
Or more so, if residual pressure is present, the calipers would stay clamped... though idk how much.
Also, if residual pressure is not present... the wheel cylinders would require you to pump the brake pedal to expand them.
This is just my understanding on this... could be wrong ofcourse.

[jmartini]

When I got my 66 the previous owner had disk brakes on the front (which I returned to stock drum brakes). They had changed the master cylinder to a dual circuit. However, they kept the original single circuit brake fluid reservoir and put a funky T connection down by the master cylinder, which really takes away the additional safety that a dual circuit master cylinder provides should you develop a leak.

I had to change that master cylinder last year and so I installed an appropriate 2 line brake fluid reservoir at the same time. If I were to do this over again, I’d most certainly pull the gas tank to do it. It is not an easy task working under the bug.

I know you said you would prefer not to change the master cylinder but you will get additional protection by doing that. Here's my blog post on changing it out, just in case you go that route. https://1966vwbeetle.com/replacing-the-master-cylinder/