Sounds like you are suffering from an air lock, you can change the washers and taps as often as you like but if you fail to drain down properly and then refill the system without purging the air out properly then this is what happens. This is a typical example of why plumbing should be left to the experts and not DIY'ers. You could try to force the air out by pulling out the washing machine and taking off the hot hose from the machine and the pipe valve, then take off the cold hose from the machine and attach it to the hot water pipe valve. open the hot valve then open the cold valve only for a short while though; this should if you are lucky force out any air left in the pipes. If not then perform a full system drain down (properly) and refill the system without causing any air locks.