Just thought I'd add my 2cents to this discussion since I've done a lot of research and development on this topic lately with the HD master cylinder project. I just wanted to clear up a few things that I've noted reading thru this thread to maybe help out others.
First off near the top the recommendation of using -4AN lines because the -3AN lines aren't adequate is not correct. The AN system was developed by the US Army/Navy and has absolutely nothing to do with the metric system so saying -3AN is 3mm is inaccurate. The Dash Number is multiplied by 1/16ths of an inch to get the inside size of the flow tubing so -3AN is .1875" (3/16) or 4.8mm in metric INSIDE the tube for flow. -4 is huge at 0.25" Inside or 6.35mm
The factory hard lines that run from the master cylinder to the brass block and from the brass block to the transmission flex line point are all 3mm or 1/8" Inside diameter. The -3AN braided SS lines are 3/16" or 4.8mm inside and flows 50% more fluid than stock so no need to go huge with -4 lines.
Next since the factory hard lines on either side of the brass block in discussion here are only 3mm internally the largest size you need to drill this block to is 1/8" (3mm) and it will act like it's not even there. Going bigger like shown above is likely to destroy your brass block as you can see in the photo that almost all of the Inverted Flare sealing surface has been obliterated by drilling it to 5/32" and you're now relying on the treads to hold back fluid pressure instead of the invert flare union. RadioFlyer you may have gotten away with this, it may be slowly weeping fluid past the threads at both sides of the block already or may begin weeping down the road... only time will tell.
My recommendation is to drill this to 3mm MAX as it won't remove as much of the inverted flare sealing area.
Final note which I've not seen discussed or noted is that while everyone is fussing with this brass union block no-one has stopped to question the orifice size at the outlet of the factory master cylinder. The factory master cylinders outlet is only 2mm and exactly the same size as the brass block. So without eliminating the restriction at the MC you could add a sewer pipe connecting the MC to the slave but it won't get you any faster clutch release with this 2nd restrictor still in play.
Hope that adds to the discussion, helps save a few people destroying the brass block and clears up the miss-information early on in this thread.
Best Regards
Ryan@RJM
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