On 12/20/2021 at 10:13 AM, w3wilkes said:

Assumption: early car and a late M/C...

The external difference from early to late is the size of reservoirs that are swapped. What's the internal difference? What would happen if you swapped the late reservoirs to the early reservoir positions and hooked the rear brake line to the front M/C position and the front brake line to the rear M/C position?

Was this such a dumb question that nobody will respond? @Zed Head @Captain Obvious

I don't find it to be a dumb question, but I don't know enough about the internal details of a master cylinder to know if the ports can be used interchangeably

I think that it would work just fine.  The bores are the same for front and back and the stroke is the same.  So performance would be the same.  

They completely redesigned the internals of the MC besides moving the reservoirs.  Might be that they just moved the mass of the larger reservoir and its contents back to where it exerts less leverage on the assembly, for protection against inertial forces and vibration.  That would be a typical engineering "perfection" that might happen in a redesign.  It started in the front because front brakes = front reservoir, but got moved.  There might be better reasons but I can't see them.

On 12/20/2021 at 12:13 PM, w3wilkes said:

What's the internal difference? What would happen if you swapped the late reservoirs to the early reservoir positions and hooked the rear brake line to the front M/C position and the front brake line to the rear M/C position?

Well I'm no brake expert, but I certainly appreciate the callout. With that in mind, here's my read.

I agree with Zed Head that since the bore and stroke is the same, everything would be the pretty much identical when you pressed the pedal DOWN. However, I'm less confident that everything would be OK when you let the pedal back UP.

There are check valves built onto the pistons inside the master cylinder and there are also check valves screwed into the outlet ports of the master where the brake lines exit. I haven't studied any of it in great depth, but there is no guarantee that the characteristics of each pair of check valves are the same.

You need to make sure that when you RELEASE the pedal, you never create enough of a vacuum out at the ends of the system such that you would pull air backwards past the wheel cylinder or caliper seals into the brake system.

Those check valves are all a balancing act between the return forces at the destination ends and the vacuum created inside the master cylinder chambers when you let up on the pedal. And since one chamber is filling large calipers that don't have any return springs while the other chamber is filling small wheel cylinders that DO have return springs, I could be easily convinced that the smart people who designed the master cylinder knew that and designed different check valves for each circuit.

So, what could go wrong? When you let off the pedal, you could suck air in backwards past your corner seals (wheel cylinder or caliper). This could make it hard to bleed the system, or even worse, it could cause spongy brakes if it happens under normal use.

Or it could all work out just fine.

@Captain Obviousbrings up some good points about likely differences between the early and later M/C's. To help visually show the internal differences I cut and pasted scanned cross section images from my 70 and 72 FSM's. I believe there has been discussion on this forum by others that have used the later M/C on early cars and the solution was to modify the brake tubes and not simply swap the reservoirs.

Here is a link to one of the discussions on swapping M/C's

 

Edited December 24, 20213 yr by CanTechZ

To be clear, the original question is hypothetical. I had always read that people would re-work the lines to go to the marked F and R connections. Did they do this just because that's the way it's marked on the M/C?

What made me think of this is the zx distributor swap when we were told that you had to have the E12-80 matchbox on the distributor. The E12-92 actually works exactly like the E12-80 if you only use the top connections. I've been running a E12-92 for years and it works perfectly with just the top connection in use.

EDIT: How many rear disc conversions have been done and the original M/C used? If there's a big difference in the F and R, wouldn't this mess up the rear brakes?

Edited December 24, 20213 yr by w3wilkes

5 hours ago, w3wilkes said:

To be clear, the original question is hypothetical. I had always read that people would re-work the lines to go to the marked F and R connections. Did they do this just because that's the way it's marked on the M/C?

What made me think of this is the zx distributor swap when we were told that you had to have the E12-80 matchbox on the distributor. The E12-92 actually works exactly like the E12-80 if you only use the top connections. I've been running a E12-92 for years and it works perfectly with just the top connection in use.

EDIT: How many rear disc conversions have been done and the original M/C used? If there's a big difference in the F and R, wouldn't this mess up the rear brakes?

Rear brake bias is one of the potential issues with the rear disc brake swap.