Join Date: Apr 2014
Location: So Cal, for now
I know of a couple of possible causes... the most common is water having gotten into the brake system. Brake fluid is hydroscopic, which means it will draw water to itself. Brake fluid can not be compressed, water can and over time, even with the brake system fully sealed, enough water will be absorbed that the fluid can't function properly. The result can be a spongy brake pedal.
Out of curiosity, have your brakes been overheated and faded recently, like on a long, downhill run or multiple hard stops in succession? Hot fluid will draw water into itself more quickly.
I had a similar situation several years ago with my motorhome... pedal went nearly to the floor and was "spongy" afterwards.
In my case, it turned out a brake caliper had hung up, which in turn wouldn't allow the pads to retract completely from the rotor. Eventually, heat built up and "boiled" the fluid in that caliper and brake line.
Thankfully, the fix was simple... I flushed all the fluid from the entire system, refilled it and bled it well. The brakes lost their "mushiness" and have operated perfectly since then.
The first thing I'd try is flushing and refilling the brake system
BTW, I did have the pins on which the caliper rides cleaned up... the motorhome had been in storage and a little rust had formed on the pins, causing the calipers to hang.
Best of luck...