Master cylinder??

I've got a 1990 clipper cabriolet. I've had a wee problem for a while now with the braking. Brakes work ok, but when I'm sitting at a junction, after around 10-15 seconds, the pedal ends up going right down to the floor. I've changed brake fluid, brake cylinders & shoes all to no avail. There doesn't seem to be any leaks & the fluid level holds ok. Was just watching an old wheeler dealers episode with the GTi & they had a similar problem, so they replaced the master cylinder. I was going to try this, so a few questions. I'm no mechanic, but looking at what WD did, it looked just like a few bolts, is it that easy. Also, cars at work just now so haven't had a chance to check, but I've been looking at master cylinders & some have 6 ports some have 4, any idea what one I need?
Cheers

I had a failed mc..bought replacement from autofactor..they just checked type required via reg.. but double check the stroke when fitting and diameter. Mine was 6 port but came with 2 blanks to fit. Then just need to bleed brakes after fitting  

Sounds like your master cylinder is shot, I had the same problem when I put my scirocco on the road after restoration.
Spme master cylinders have 6 ports as 2 ports are taken up by the brake light switches and the other 4 are the brake lines. Post up a picture of yours and should be able to tell which one you need. I guess yours will be the 4 ports as it should have the brake light switch on the brake pedal.
It can also be a pain to remove the bottle on the top without breaking the ports which push into the cylinder and it can be tricky to get the air out of the system with out a pressure bleeder, I had to bleed the master cyinder first from the brake hose then move on the brakes.
If you are not confident in doing the job I would get a garage or someone knows what they are doing as brakes are a important part of the car and you don't want them not to work!