Recently my brakes have become mushy and the pedal travel is more than usual. I pulled all 4 wheels off, no leaks found, no bulging brake hoses and brake pads are almost like new. The fluid res. level is sitting about 1/3 below the max mark. Could the recent real temps. have anything to do with it. This is my second winter with this car. I don't recall having this problem last winter. Also as a related question. I am going to add brake fluid, but I don't know what is in the car now, DOT 3 or 4. If it is DOT 4 and I add DOT 3, would that be ok. I need to take care of this ASAP. It'd bad enough braking in snowy weather, nevermind with mushy brakes.
Thanks in advance.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member