my road going f78 has rather spongy brakes. its always passed the vehicle test no problem, so it was never enough to worry me until recently when I bought an f73 as a spares jeep and found out just how quickly they're really meant to stop! I've bled them several times, most recently when changing rotors and pads a few months ago.. I started back left, back right, front left, front right, twice around the system with no bubbles after the first round.

there's a small amount of brake fluid leaking where the master cylinder meets the vacuum assist body-she needs a top up roughly every four months. so I assume the gasket there has gone but I imagine there's a leaky seal somewhere internally in the master cylinder, too.
I'm thinking of swapping over to the one from my spares jeep, but I've never changed a master cylinder and as such a vital safety component I'm a little worried about screwing it up.

can anyone here reassure me about:

a) any difference there might be to the brake systems between a 97reg f78 and a 00 reg f73
b) bleeding the master cylinder itself? I don't see a bleed nipple on it...
c) finding two new gaskets, or can they be reused,(which I doubt) or can they be made?

totally unrelated question about the starting system. had to wire in a push button to the heater plug relay because I literally can't find the heater timer unit to diagnose the system. only have a LHD f70-75-77 manual and I'm in Ireland so mine is RHD. I can't find it after maybe an hour of scrabbling under the dash. where is the damn thing and what does it look like!?

any help, warnings or commiserations on anything gladly accepted.