Faulty air lines, faulty valves, faulty brake chambers, faulty pop off valves on the air tanks
brake leakage? or vacuum assisted vacuum leak?
The brake pedal has nothing to do with it. If air is in the system then bleed the brakes.
oil leakage
Air in the brake lines
Three (3) psi for single vehicles, four (4) psi for combination vehicles.
Air leakage increases.
Either there 1. IS leakage somewhere and you're not finding it OR 2. your brakes were not properly bled OR 3. your master cyliner is shot. I'm leaning towards 1 or 3.
Air unlike brake fluid can be compressed. When there is air in the brake system it compresses when you apply the brakes. This causes a loss of brake pressure on the brake pads and results in much longer stopping distances. This is dangerous and will cause an accident.
With the foot valve applied, four psi/minute for combination vehicles, and three psi/min for single vehicles.
Due to expands of air in summer air pass through pore causes leakage more.
Worn brake shoes or pads. A leaking brake pipe. Low brake fluid. Air in the brake fluid pipe.
Air being charged to the brake system. When you pull the brake valve, the air in those lines gets purged. When you release the brakes, it has to refill those lines and chambers.