My 1986 XJSC brake pedal travels excessively on very light pressure, even if engine is off.  Pedal is firm on braking with greater pressure.  Have replaced master cylinder seals.  Any thoughts as to what else it might be (and where to source a master cylinder, if that's the answer!)

Thanks

Thanks.  I'll check those ideas out!

No loss of fluid though....

wouldn't a fault at a slave give me uneven braking, rather than a very soft pedal?

No loss of fluid also points to master cylinder. Although generally yes, that gives uneven braking. I believe this is due to a split braking system (which your car undoubtedly has) having two pistons and two outlets and so effectively two master cylinders in one. So fluid leaking past the seal of one piston but not the other means that you'd still have braking on one circuit (usually one front and the diagonally opposite rear caliper/slave cylinder).

I have put new seals in a master cylinder before but then found that the bore is worn and letting fluid past the seals regardless of how good they are. For me this has only been on a single circuit system so it led to a consistent loss of braking all round, not uneven. However if a twin circuit master cylinder's bore was worn all the way along it could in theory cause the symptoms you describe as fluid could get past both pistons.

I would say it was worth taking the cylinder off and checking the bore from front to back for any increase in diameter which would indicate wear.

Great - thanks.  I've been looking for a master cylinder online and from breakers, but haven't found one.  I'm also concerned that using a used component will possibly not solve the problem, since I imagine it's just wear and tear......