Just recently I started having problems with my brakes. While driving, eveything seams normal, the pedal has a good feel, the brakes are responsive and firm and really no problems at all. However, when sitting at a red light for a while, the pedal will slowly drop eventually all the way to the floor and the truck will start to roll forward. I have to quickly pump the pedal and then I instantly have a firm pedal and brakes again. I have manual brakes, NOTHING power. Today, I installed Russel speed bleeders and bled all 4 brakes till the fluid was perfectly clear and obviously no bubbles at all. The bleeders worked great, but the job was still a PITA because the bleeders worked so good and FAST that the master cylinder needed filling every couple of minutes. I was convinced that the thorough bleeding would solve the problem but unfortunately it didn't, the exact same conditions as before. I inspected all the lines and didn't see any leaks. I really am at my wits end here on this one.

Anybody have any ideas?