Brakes

[Malk]

Had an issue this weekend after initial braking on a firm pedal it goes very soft pretty quickly and nearly to floor then brakes more or less release!

Is this master cylinder, air or another common issue?

[christopher storey]

Certainly not a common issue, and certainly not one you can ignore. Clearly , there is a lack of pressure in the system somewhere, but whether it is just that air has somehow got in, or whether it is a seal failure is harder to determine. It is worth seeing if it pumps up easily and repeatedly without losing fluid, in which case air entry is more likely ( but still requires explanation ) .However, the one thing one can say with confidence is that if you have had a seal failure, fluid will be leaving the system and you will find that the level in one or both reservoirs has dropped . If there has been a loss of fluid, the first check I would make is on the pipes from reservoirs to a. master and b.servo cylinders . These pipes are notorious for becoming osmotic and allowing fluid to pass through their walls . If a reservoir empties completely then air will enter the system . Secondly, check each of the bleed nipples for security . Then check each union for security . Next, from inside the car peel back the rubber boot on the master pushrod and see if it is wet with fluid inside. If yes, you have a problem there . If none of these reveal anything, then I would suspect there is a failure in the master cylinder shuttle which is, I'm afraid, a longer job

[abowie]

Might be air, might be a leak.
Are you losing fluid from either reservoir?
Both the master cylinder and the servo cylinder have a large seal at the back where the piston goes in. If this fails fluid will leak out the back. With the servo this fluid goes into the big vacuum tank and so you can't see it.
With the master itself it leaks out into the footwell. Look into the footwell behind the brake pedal; can you see leaking fluid or jelly like residue?