Brake levers go to the bar after sitting

[AbnormalWrench]

I don't know if anyone else has noticed this, but it is weird. I didn't have this problem until the rainy season started. I get done riding. I park the bike and everything is working fine. 5 days later I go to load the bike to head out to the ORV park, and my brakes act like they have air in the system. The levers go to the bar when trying to stop. I eventually figured out that if I grab the calipers and push against the sliders so the pistons get pushed most of the way in, then pump up the brakes, the problem either mostly goes away, or completely goes away. But then the next time I let the bike sit, it does it again.

I have been lazy about pressure washing the bike lately, which might be related. Never had a bike that would do that though