Here is a link to a thread that pretty much covers all of the parts and issues.
https://www.thesamba.com/vw/forum/viewt ... sc&start=0
Ignore my crazy sketches. Its just thinking out loud. A large portion of what I have been experimenting and designing for 411/412....has less to do with putting power assist brakes on it....and is mainly about installing a newer, easier to find, equivalent master cylinder that gets the MC out from under the dash.
The biggest problem with the stock MC....is that its under the dash. If and when there are leaks and if and when you have to replace one....it gets fluid everywhere on the carpet, rusts the pedal cluster and screws up the fuel pump relay and any wiring around it.
That being said...the stock MC is excellent and perfectly sized and balanced for these cars.
Since that time two years ago....I have come to the conclusion that I can actually improve a couple of things on the stock master cylinders and they will perform much better. A couple of items about type 4 master cylinders.
Just remember this....except for being great for a rebuildable core....ALL NOS master cylinders...are 100% junk....100% junk....100% junk. They are not to be used except for great core pistons and bore and even then with age over 20 years they may have some pitting.
But...even if you do not have an NOS core...as long as you have an MC that has less than .003" pits in it.....you can rebuild it better....and make it last a lifetime.
All of this may sound like a lot if hassle...but otherwise your options are few or non existent. Either send the MC out and have it stainless steel sleeved....about $150 or so....and if it does not come with new seals....then buy a type 3 piston and seal rebuild kit from someplace like Rockauto...toss the pistons and just use the seals and flap valves to rebuild what you have....or
1. Rebuild your stock one so it can last a lifetime by plating the bore.
2. To do this....take a decent old core....buy a cheap 360 grit hone and hone it until all pits and grooves are gone. Then measure it. You are not allowed to have more than .005" large bore than the piston heads. Its considered worn out and will cause problems if the bore is more than .005" larger than the piston heads.
3. Then buy 600-800 grit 3/4" flexhone....and hone it until its smooth as glass. Very fine hone marks.
4. Then buy a small electroless nickel plating kit from Caswell plating. Plate the bore until its .003" larger than the piston head (this takes about 1.5 hours in the plating bucket. The .003" spec will be at best factory level of new.
Then lightly hone with the 600 grit hone for a very fine tooth...and rebuild with the original pistons using type 3 seals.
Between the plating kit, the piston seal kit and the two hones....you will have about $270 invested.....BUT....that cylinder will NEVER rust again inside or out and will be very wear resistant...and you can do 2-3 cylinders. The pistons will never wear out.
I may plate a handful close to Christmas. Ray