Express brake problem

Hello, I have a 2004 Chevy Express AWD van with 30k on it. The problem is that the brake pedal loses pressure from time to time. I had the problem for about 6 months before getting a full rotor and brake job which I thought would correct the problem. I brought the van back to the garage and he said he checked the master and all the lines and can't find anything leaking and no explanation for the loss of pressure. He said to check with Chevy and see if they have a recall or anything. Has anyone heard of this ???? Thanks

G or H and what size?

Any fluid lose out of master cylinder?
Is this a pedal seep down at stop light or with a light pressure on brake pedal?
How far down does it go?
Power brakes?
Any brake lights on?

For any recalls or TSB's on brake problems have your dealer run your vin no.