Driving today, pedal feels as usual except for every now and then when goes soft almost to the floor. Car still stops, and if I left off and re-engage it feels normal again, but will recur frequently.

Most likely master cylinder, right? Brake booster and vacuum issues should have the opposite affect if I'm not mistaken.

What brake fluid do you guys use?

 

My xj6 was doing the same thing until it failed completely. I replaced the MC and all is well.

 

I had a stuck piston on a rear caliper that heated up the fluid, and got this symptom every now and then as the fluid boiled, (I didn't know this at the time). Finally on a long holiday trip the pedal went to the floor on a long downhill stretch with a bend at the bottom. pumping the pedal got the brake back, but I then pulled into a garage with smoke billowing out from the rear wheel arches. A mechanic managed to free it off, and we had our holiday, but I had to top up the brake fluid every couple of days. On return, got the car to a Jaguar specialist, and the calipers were replaced with rebuilt ones including new handbrake calipers. The rear calipers, being inboard, are the ones that get neglected.