I have a 1990 LX 5.0 with an SN95 `94 Cobra conversion. I have never had any problems with the brakes functioning properly, only occasionally hearing a light metallic pinging sound from the front calipers that sounded like the padsquickly seating themselves. Nonetheless, my mechanic (a mustang shop) gave the brakes a clean bill of health at that time a few months ago....

The problem is that in the past few days in the cooler fall weather, my pedal is occasionally going soft, one timeto the floor with very little stopping action. Sometimes it is high and firm with good stopping action, sometimes it goes soft. It has been very inconsistent and like I mentioned, only since the colder weather has arrived.

The car is now packed away for the winter so I would like to get to work on a few things, this obviously being one of them. Where should I start looking?

Thanks.

 

I would look at the master cylinder.I think you might have a bad seal on the plunger that's allowing fluid to get past.It's only an intermittent problem right now but it WILL get worse with time and lead to a total failure.
Obviously this is a dangerous condition and I would not drive the car again until this is fixed.