My old brown mk1 is suffering with a really soft brake pedal. Real soft, as in it goes to the floor and only just slows things up.
 
I pulled all the wheels off and checked calipers and rear wheel cylinders, checked lines and fittings. No leaks anywhere. The reservoir is still full too, and there is no leaks from the end of the master cylinder either.
 
Does that point to the master cylinder being stuffed? Or should I try bleeding the system first before assuming a bung master cylinder?

Could try bleeding the system & looking for air bubbles but if it has deteriorated all of a sudden  and there's no external leaks the cause would be seals in the master cylinder.
If you have a second person, you can check by stomping on the pedal hard while the second person checks the fluid surface in the reservoir for indications of  fluid flow back.

Thanks SFE. I'll drag the wife or one of the kids down to the shed and get them to do some stomping to see what is happening.

Agreed, same thing happened to my old 'Tina a couple of years back, turned out to be just fluid, phew...
Happy stomping Drew...

How old is the fluid in your system? If its been in for yonks will not be as effective. If your going to do a bit of " bleeding " completely bleed the entire system and replace with new fluid. Len

Hey Len. The fluid is very old, probably 10+ years. So definitely time to replace that.
 
I gave it a bit of a bleed yesterday, from the rear axle bleeder, a bit of fluid came out then there was nothing for a bit. Not sure if that is air in the line or that the master is just not pumping. Will check the fronts and see if they pump out today.

If you can get a  helper and have some catch cans for 2 wheels, open the bleed valves in pairs, front and rear and pump the whole contents of the master cyl through a couple of times to ensure all the old fluid is purged and hopefully all the air.  Does a Mk 1 have a proportioning valve for the front and rear circuit like the Mk2? If the rubbers are perished maybe it is getting stuck? Len