LinksIMHO, a 1500 lb car with 120(?) HP in this day and age...it's over-engineered a bit to put double boosters in the circuit, aside from the piping complexity. Like...I'm not going to autocross, not doing hot laps at Jersey Motorsport...I'm going down to Dunkies for my morning caffeine fix, driving the SO to Second Beach in Newport, or carving some corners in the backroads of Bristol County MA. So...the unworking boosters, assorted vacuum lines, and other items are coming out. That's a given; while waiting for the various bits and pieces of interior stuff to come back...figure it's time to sort the brake system out (along with overhauling/cleaning up the pedal assembly, replacing the original clutch and accelerator cables, etc.) before assuming the Lotus Position and cleaning up the 'back of the dash' from 50 years of unconcerned mucking about back there. And, to be honest...the S2 got away just fine with an unboosted system, and it's within a couple hundred pounds of its later sibling.
So...what to do?
As a starting point (the rear cylinders were replaced recently, along with pads, so, assuming the job was done correctly, going to leave sleeping dogs lie after inspecting)
After this?
1. Open the taps and drain the system, disposing of the existing fluid.
2. Disconnect and remove the vacuum units, patching the holes in the fiberglass, and repainting the side wall on that side (truck bed liner paint, minus the grit which I filter out is my preferred finish for the engine room and wheel arches).
3. Remove the flex hoses from the front and rear. They are original...and will be replaced with armored hose replacements.
4. Rebuild front calipers with stainless pistons (have a set left over from my own stock), paint calipers, reload with fresh pads.
5. I have new disks I was going to put on 693R but never got a round tuit...so, might as well use them here.
6. Remove the MC. It's a girling unit, looks like the original...but who knows? Mucking through the extensive records I have, don't see any indication that it was replaced along the line, but, pretty sure not every bit was recorded in the POs OCD record keeping.
Now...it's all in bits...where to go from here?
There are a lot of thoughts about which direction to travel; let's face it, some of the parts, while made by Girling, were made to Lotus' specs and are not current production. There are docs on the manuals site (either Dan's or my mirror) that go into gory detail about decisions: Nissan/Tilton/Spitfire/you name it. Some bits are still available, some not. It seems at this point it will be pretty much narrowed down to either a Tilton or the Spit master...(in my mind...).
Removing the boosters means that the bore size of the master and travel will need to be somewhat different. In the S2, for instance, the .70 bore took care of the fronts, and the stepped-down bore (don't recall offhand the size) at the rear of the cylinder took care of the rears. Fair enough. The replacements today have a constant bore in various sizes...Tilton, being a universal fittment, is available in multiple bores: 5/8, .70, .75, etc. Generally, the larger the bore, the less travel to fully engage...the smaller the bore, the more travel. Basic fluid dynamics, right? Coming up with a compromise between feel, travel of the pedal, and force applied becomes a question to answer.
Aside from the physics of fluid travel, pipe bending and terminating, etc., and accounting for the removal of the boosters (which modulated the feel of the brakes and balanced the force needed), I'm in the range of thinking that moving from an assisted to a non-assisted system is pointing towards ensuring that the front-rear balance is maintained properly...which means that
1. the fluid force applied to the rear will have to be moderated by a proportioning valve to make up for the larger MC bore feeding them and
2. using the same bore size as the unboosted cars used for the fronts will provide both the right travel as well as fluid force for the calipers (which are the same on the TC as were on the S2).
Am I off here? For the record, I'm basing this on reading Aaron Hines' work done in the early aughts, as well as Dave Maugham's document on his changes...and some of the Lotus West material (thank goodness for the archive, right?).
Where I'm leaning towards (yeah, using a heavy iron speed shop...) is:
1. Tilton 74-700U master cylinder (https://www.jegs.com/i/Tilton/454/74-700U/10002/-1) with reservoir remote mounted up on front trunk bulkhead and front line direct connected to fluid T on front of frame, removing the back-and-forth piping to the removed booster. That means the PDWV goes away. Simplifying the piping means fewer points of failure, which the valve was meant to monitor. (I realize this may result in some comments, so be it...I'm open to reasoned arguments to keep or not)
2. Jegs Proportioning Valve model 63020 (https://www.jegs.com/i/JEGS/555/63020/10002/-1) in place of the rear booster (in essence)
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