My brakes are acting strangely. When approaching a stop light or stop swign, my truck will swlow down normally as a get down to about 5mph. At about that speed, the brakes lose strength and the truck will continue to roll forward another 10 -20 feet. The brake pedal does not go to the floor and there is no leaks that I can tell...the reservoir is full. I had a shade tree mechanic replace a broken spring on the left rear assembly, but to his surprise it did nothing to solve the issue. Have you heard of this problem before?