Brake pedal firm then soft.

[Rutty]

My brake pedal is firm when pressed but after about 15 seconds the pedal slowly goes to the floor. No brake fluid is lost. I seem to remember this most likely to be the master cylinder seals. I am i right?

Thanks 

Mark

 

[glang]

If theres no fluid leaks (check its not coming out of the end of the MC and running down inside somewhere) then its almost certainly going to be the internal seal in the MC. You can sometimes confirm this by carefully watching the fluid level in the reservoir to see if it rises as the pedal goes to the floor because the seal is leaking and the fluid is being pumped back into the reservoir....

[Rutty]

I have checked for leaks and the fluid level is staying the same so its not leaking.  I will get a seal kit in the morning. Thanks

[glang]

does the level rise as the pedal goes to the floor? Fitting a seal kit isnt always successful and I went for a new MC - for about a tenner I got a 3/4" bore ebay item which is bigger than standard although with a small reservoir and now have great  pedal resistance....

[Velocita Rosso]

Take m/c cap off, (two people) press pedal and check for air bubbles in the reservoir fluid

[glang]

I dont understand where any air bubbles could come from? Mark says initially the pedal is firm so unlikely to have air in the system......