OK, time to relight this thread!
So I was working on my shop mate’s vintage race 510, and we were plumbing brake lines. Low and behold, I got to measure everything.
Stock Datsun brake tubing ID = 3.2mm = (8.042 mm square cross section)
Brass block "restrictor" above transmission = 2.4mm (4.524 mm square cross section)
Inlet to stock Datsun rear wheel cylinders = 3.0mm (7.069 mm square cross section)
We were re-plumbing the lines in the car. Since the entire rear drive train was out, we felt it was time to replace the original brake lines, as they didn't look very good. Just for interest sake, the ID of the AFTERMARKET brake tubing is 2.6mm (5.309 mm square cross section)! So the restrictor, as it were, will not be used and the brake line will now go through the cockpit from the firewall to the rear seat area.
After doing the math; the 2.4 mm brass block orifice has barely over half the cross sectional area of the stock brake line. However the real story is the inlet of the rear wheel cylinder. The brass block orifice is about 63% the size of the wheel cylinder orifice.
Just food for thought.
Of course all you guys running rear discs have other issues and fittings.
Keeping in mind that the actual flow through the rear brake line at application is nearly irrelevant, making me think that the orifice does little, if anything at all. I’ve had some experience with hydraulic systems when working with the Navy. In a stock 510, my feeling is that by the time you stab the brakes in a panic stop, the hydraulic system will have equalized in pressure (fron t to rear) long before the spongy front suspension would hit full compression, making this "restrictor" irrelevant in the overall system dynamics, at least in regards to helping stop the rear wheels from locking. So, I honestly think we are back to square one. There is no proportioning valve in a PL150 – we know this, and the "restrictor" isn't much of a restriction when you think about how little fluid is actually traveling through the line.
The car in question is a 69 model 2 dr, and I’m sure it was the original part. However, this has been a race car since the mid 70’s, and it has been in the very capable hands of Andy at Specialty Engineering for that time. So, anything could have been changed. However I still have the fittings that came out of the Bronze, and I will one day make a point of checking them, as well as any other brass block orifice I can get my hands on, just to make this legitimate and not another internet myth.