Suzuki Jimny Brake Pedal Sinks To The Floor, help!

Pretty recently I've noticed the brake pedal sinking to the floor and the majority of the time it feels like I haven't got any brakes, first of all ive checked the fluid level in the master cylinder and it's ok I've inspected all brake lines and callipers and have no leaks, I've also bled the brakes to see if there was any air that maybe in the system.
I've read elsewhere that it is most likely down to the seals that have gone in the master cylinder and that buying a reconditioned or 2nd hand master cylinder is pointless because you are never guaranteed they work so I've now got the choice of paying £150 for a new master cylinder or have ago at replacing the seals in the master cylinder myself with only the smallest bit of mechanical knowledge and the seals cost £18.00.
Has anyone else had any experience in replacing the seals themselves and does it always work?
Also any places I can get a new master cylinder for under £100?
The car is a Suzuki jimny 1.3 reg: W366 EWU

Check that the pads move freely on the sliders and that you don't have a seized calliper.

Had the same issue.
www.bigjimny.com/index.php/forum?view=topic&catid=6&id=57024

I have a used master cylinder if you decide to go that route

Does it pump up with repeated swift presses and then stay up until you let go of the pedal? If so it could be a front wheelbearing collapsed, and the disc pushes the pads back as the wheel rocks.

If the hand brake goes almost vertical and the pedal to the floor then your rear brakes need adjusting to bring the shoes back out. I had this at the weekend after changing the rear shoes as I had taken apart the adjusters to clean them.