I have a 91 RRC that I replaced the driver side brake caliper and hoses on about a three months ago. Bled the system with new fluid and everything seemed fine up until today. I drive a lot of highway miles and when I don't use the brakes for long periods the peddle gets soft and moves about half way down before catching and slowing the truck. After that, the pressure builds back up and the brakes feel normal and firm for a while but then it goes soft again. The accumulator and pressure switch were replaced last year. I have not found any leaks at the lines or calipers and the fluid level is where it should be.
Is my ABS/master cylinder on the way out? The truck has 279,000 miles on it.