My car was originally undersealed and there's wet sealing gunk everywhere. It's impossible to see clearly in the area where the leak is evident.
I think I must have flexed the steel line when replacing the rear hose and induced a crack or pinhole in the 50 year old rusty line - anyway the leak is definitely coming from some inaccessible point in the steel line.
You make sensible points, but I think I'm stuck with replacing the lines.
Finally got all my brake lines and hoses replaced - and so far no leaks.
The original lines were installed before the body was fitted so without removing the body, replacement was never going to be quite as easy - but I've encountered worse jobs on the C3 (sawing off the trailing arms for instance). The line from the master cylinder to the rear brakes runs along inner face of the drivers side chassis rail and while I would normally raise the car on the QuickJack to work underneath, this is one job where the access is easier with the car up on jack stands. I bought a full kit of replacement lines, clips, blocks and hoses in original steel rather than stainless.
The big plus was they were available off the shelf, which stainless were not and although the new lines are preformed, some manipulation to get them fitted was going to be necessary and mild steel is a lot easier to work with than stainless; also my original lines have lasted 50+ years and I'd expect the replacements to do the same. Either way, I won't be around to find out.
I disconnected the lines from the calipers and pumped out fluid before tackling the replacement. I resisted the temptation to cut the old lines out in pieces, since if they didn't come out in one piece, the new ones were unlikely to go in. The long front to rear line was the trickiest to remove and fit; as it runs along the chassis rail it passes through a small gap where a cross member joins the rail and it's then cranked up at both ends, at the front to connect to the proportioning valve and at the back to the LH block and hose connection. With the old line out and the new ready for fitting ....
I located the leak that started all this....
I fitted a new rear block to the line before installing as access for connection is very limited when in place. The rear crossover install (with new clips) was straightforward followed by new hoses and hard lines to the calipers. To fit the front crossover the two fan cowl support brackets had to be removed......
..... to allow the line to pass behind them, then new front hoses with copper washers ( I reused the originals after annealing) were fitted.
The last connections were made inside the engine bay at the master cylinder and the proportioning valve (it's not actually a proportioning valve insofar as it doesn't differentiate pressure to the front and rear brakes. It's a shuttle valve which in the case of loss of pressure to one half of the system shuts that part off ). I adjusted the hard lines and trial fitted those to the master cylinder to ensure that quick connection could be made after I had filled and bled the cylinder. It's very important to ensure that the lines are properly aligned with their connection points since it's very easy to cross thread the unions if they're not. The connection to the bottom of the proportioning valve - connecting the line arrowed below....
...is practically impossible to break or torque up without a crowsfoot spanner on long extension bar - fortunately I had one.....
Incidentally, the unions on the new lines were correctly threaded but some of the hex's were smaller than the originals, which had me fooled for a while.
I cut up and reformed the old hard lines from the master cylinder to the valve to make bleed tubes and fitted these to the cylinder before refilling with DOT 3 fluid and pressing the pedal to force out the air.
With the cylinder bled I removed the bleeders and reconnected the system lines....
Last job was to expel air from the whole system, which fortunately was done easily, although rather slowly, by a gravity bleed at the calipers....
With a nice hard pedal achieved and after a final check of all connections for leaks, I'm back on the road! Hooray!