temporary loss of brakes!!!!

[al 350i]

the cerbera seems to occasionally loose its brakes, one minute they are fine then the next minute the pedal will go to the floor with little or no braking force. if the car is left for a few minutes (with engine off) the bakes will return and the pedal rock hard but will go again after about 10mins. the pedal doesn't seem to creep and there doesn't seem to be any loss of brake fluid. my only conclusion is that there is a problem with the master cylinder and its allowing the fluid to creep past but could it be the servo at fault?
i have almost removed the pedal box (not as easy as the workshop manual leads you to think it is ) and will examine both servo and master cylinder but would rather only have to replace one rather than both.
has anyone had this problem before or any ideas?

[Brummmie]

al 350i said:

the cerbera seems to occasionally loose its brakes, one minute they are fine then the next minute the pedal will go to the floor with little or no braking force. if the car is left for a few minutes (with engine off) the bakes will return and the pedal rock hard but will go again after about 10mins. the pedal doesn't seem to creep and there doesn't seem to be any loss of brake fluid. my only conclusion is that there is a problem with the master cylinder and its allowing the fluid to creep past but could it be the servo at fault?
i have almost removed the pedal box (not as easy as the workshop manual leads you to think it is ) and will examine both servo and master cylinder but would rather only have to replace one rather than both.
has anyone had this problem before or any ideas?

Loss of servo would cause a hard pedal, with rubbish braking forces. If there is no loss of fluid, my money is on the master cylinder.

[ridds]

Yep, bet they work when you press the pedal hard but not when you press it slowly.

Classic master cylinder seal failure.

[al 350i]

wasn't quite sure what the simptoms of a servo gone was, so i think your right sounds like the cylinder. am i right in thinking its off a ford fiesta xr2, if so should i buy one from ford or tvr?

[ro_butler]

al 350i said:

wasn't quite sure what the simptoms of a servo gone was

If you want to replicate this then drive along, switch off the engine and then try to brake. pretty scary.
Obviously don't try this unless the road is straight and deserted as you also lose power steering.