My guess is the seal wasn't solid. Likely one of three things - the sealing on the pump is faulty or when you hooked it on the valve went in on an angle or you didn't push the pump on far enough down the valve. There was probably enough of a seal on the rear that you managed to pump the tire up - though you probably put in two or three times the number of strokes normally required- but had little seal on the front.
Hint: make sure you use the ring screws that come with most new tubes to hold the valve in place- then once aired take them off.
Last edited by Nbob; 07-10-10 at 01:07 AM.