I dug through my achieve for research, and my scanner sucks. So I took photos of the relevant pages with info you were looking for.
I first looked in the Service Manual, Datsun, Model 510 Series - Chassis and body, and found no reference what so ever to the fact that a510 even has a tandem brake system. It did cover both disc and drum front brakes - strike one!
Next I figured I go through my collection of Datsun Service Bulletins, I have one introducing the 510 series in Aug 1967, it has a nice photo of a single circuit system; with an inlay of a front disc brake... strike two.
OK, so I figure, well the 1969 model update should have it; even though most US bound 68's got tandem circuit brakes (I know not all of them, but most did); all Canadian 510's got the single circuit brakes for the 1968 model year, but both Canada and the US got tandem brakes in 1969, so there must be an update for the 1969 model year, right? Well yes, there is mention of the tandem brake cylinder and a nice exploded view on one, but no info other than this... strike three.
I figure I'm done, but wait - what about the Datsun Parts Catalog...
I got this page, and thought I’d run out of options Bert...
Then I flip over the page and I have this - pay dirt!
I found the diagram you asked for Bert; Item 8.01, P/N 46100-26100 and it's officially called a "Assy Brake Indicator Switch"
You guys are bringing up some interesting info. Out of all the years I have been playing with 510's, I had no idea that there was a restriction built into the “T” at the front of the transmission tunnel - thanks Jeff.
As far as the hydraulic action and how different M/C piston size, front caliper piston size and rear drum piston size all add up, I have to think about how to explain it in fluid dynamics and pressure. However, as I understand it:
A 1” M/C cylinder bore with 100 lbs of pedal pressure traveling 1” distance in the bore will exert 100 lbs of line pressure (assuming a pedal ratio of 1:1 – which it isn’t, I know, but were are only talking theory here). This 100 lbs line pressure acting on a 4” front caliper bore will net 400 lbs of force but only travel .250” of distance, and inversely the same M/C will exert 50 lbs pressure on a ½” bore but travel twice the distance of the M/C (2” travel at the rear ½” cylinder. Someone correct me if I’m wrong here.
So increasing the front caliper piston size (or the square area of all your pistons added together for multiple piston calipers) will decrease the amount of piston travel, however increase the pressure available.
At the same time an equal amount of fluid is acting on the smaller rear wheel cylinder engaging the rear brakes faster than the system was intended for the given braking scenario (because in dislexidimes car it has a tandem M/C, and equal amount of fluid at equal pressures is being delivered to both circuits – brake switch indicator issue aside). Remember that a drum brake is self engaging and requires very little pressure to actuate its design.
You are not slowing down because the front brakes (where 75% of the braking work takes place) have not yet engaged, so you will push harder to try to slow the car down. Now you have a situation where the rear line pressure is way higher than a stock brake system would have normally had for this same given situation (because the front calipers have not engaged due to lack of piston travel), more pressure builds at the rear = more force on the rear cylinders = IMPENDING DOOM!
“What we have here is a failure to communicate” … or a failure in design because the engineered design of the 510 brake system as a whole has been screwed with, and it’s two individual circuits are no longer balanced.
I hope I got this right – it took a five tries….(and likely confused the hell out of few guys that were reading between edits)
The fix here is obviously balancing the front and rear M/C sizes if a tandem system is used. And since this is near impossible given the few choices we have.
The next best thing is a balance bar between two single M/C’s to balance the tandem circuit. This means lots of fabrication, but hey this is how rear race cars are built and it’s the best option. This balance bar gives a linear adjustment of both circuits, allow you to mechanically adjust the amount of pedal pressure each circuit is given.
The next option to fix the problem is an adjustable proportioning valve, which limits the pressure to a single circuit that it is plumed into. This is the easier route most people take. An adjustable proportioning valve is not linear, but it does seem to work for most applications.
I’ve driven badly biases brake systems – it’s scary. Anyone out there running 280ZX front brakes and stock 510 rears, you have a badly front biased system that will not allow the rears to do anything at all.
Also this discussion has nothing to do with pad materials. Want to really screw up your bias – put a set of metal masters in the front and go driving! Cold brakes, rainy night – you’re toast!
Anyways, too many of my thoughts on the subject.