Brake warning light 2005 Burban?

[Ccrider1]

After driving about forty miles brakes started feeling a bit soft.As i drove into my driveway the brake warning light came on. I suspected a brake fluid leak but reservoir was full and could not find any obvious fluid leaks. Havent had a good look at all wheels/brakes.After a few minutes ,I pumped the peddle a few times and brake warning light turned off. Sounds like a master cylinder possibly?
Anyone know of any other possability? Truck has about 135k miles and all brakes done within a few hundred miles.

 

[zora]

Would look at each wheel to check for beginning of leak otherwise master cylinder is suspect.

 

[Ccrider1]

Would look at each wheel to check for beginning of leak otherwise master cylinder is suspect.

Exactly my plan.thanks for confirmation.will update.