Brake Problems

[Rhino]

I haven't worked much with brake systems before, but here is my problem...When sitting at a stop light the pedel softens the truck starts rolling an you have to push the pedal nearly to the floor, if you let off and press down again it catches like normal....this dosn't happen every time, but it would be a bad thing if this happened when i was trying to stop while moving, any one have any ideas of where i should start looking for the problem..



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