Hi
I had five of these tractors through the years, and I had very good brakes on them all, yet they all came to me with bad brakes.
It is easy to take off the brake cylinders at the rear of the tractor and ensure they are clean and the seal inside is good, some have an O Ring, some have a lip seal, clean all up well, blow out any dirt rust or dust and be careful putting in the piston to the cylinder as you could cut or damage it, use some lubrication on the seal and inside the cylinder, you will soon know if you have done either when you get fluid to go back there.
As you seem to be having trouble getting fluid to leave the master cylinder, ensure there is no leaks in the pipe work, I would fill the feed bottle for the master and keep the pipes off? or loose back at the brake cylinders, some times this works, sometimes nothing will pump if you do this due to the fact the master cylinder will now suck air in rendering it unable to pick up any fluid to push back through the system, try each method.
It might be worth ensuring all the pipes going to the brakes are tested and not leaking, visual check for cracks or holes, and air at lowish pressure to put you mind at rest.
Again, air will be present in the brake pipes but can be bled off,
but air getting in somewhere to the master cylinder will cause it not to be able to fill its cylinder with fluid, I had to find and remove the appropriate pipes on the suction side of a master cylinder on a lorry once and simply put my finger in the inlets to block them and this allowed the cylinder to SUCK fluid into its chamber to start the process.
Hope this helps.
One other thing that is important, when you get the brakes working, adjust them one wheel at a time off the ground, tighten them up until they lock, then back off a couple of turns, I forget exactly how many, but be careful to undo them enough as if you don't they will lock up as they start to heat, carry the appropriate spanner with you until
you are sure they are not catching or locking as you work.
Others will advise you who know more than me, its been a very long time since I worked on an 8011, I really liked them, pity they are not in production.