Today I replaced the brake fluid, using a vizibleed tool, all went well and the job was done within half an hour or so. However, when taking the car for a test run to make sure all was fine, I noticed that when the brake pedal when under continuous pressure slowly sinks to the floor and springs back up again. The brakes seem to be working as expected and were responsive when doing an emergency stop to check things over.

So rather than air being in the system, am I correct in saying the master brake cylinder is worn and needs replacing? No ABS lights or any others are displaying either. 

 

Thanks for any help.  

Check that there is no leak of brake fluid. If there's a leak then it could be a pipe which goes to the front or rear brakes. If there's no leak then it could be the master cylinder.

On my fiesta after taking it to MOT it failed as for the what you said where the pedal would go all the way in. After I noticed a wet patch and that fluid was leaking from a pipe which takes the brake fluid to the rear drums. Fixed that and solved the problem. 

if t did not do that before but does it now i would first check the bleed screws. is this the 2015 fiesta shown in your details ? can't rule out master cylinder but does not sound likely for a car of that age.

3 minutes ago, isetta said:

if t did not do that before but does it now i would first check the bleed screws. is this the 2015 fiesta shown in your details ? can't rule out master cylinder but does not sound likely for a car of that age.

It isa 2015 fiesta, by checking the bleed screws do you mean making sure they're tight?