Completely empty the reservoir fill with hot ,not boiling water and run through the system.
Perhaps a bit of vinegar to this?
OP, yes, I believe it should have a retention valve at its output. However, this would only delay the initial spraying and then would be normal. Are you sure there are no leaks in the supply tubing? A small leak in the tubing further toward the pump would allow the fluid to drain from the tubing when not in use and delay spraying and the stream may appear normal. A larger leak would delay spraying and result in less volume.
Mine cracked here and I repaired it;
I have the replacement tubing, but I did this repair 7 years ago and is still holding. I am waiting on installing the new tubing until needed. You may need to remove the access covers under the bonnet/hood to see if it is wet in there.
If you do not see improvements with cleaning the nozzles and no leaks are discovered, then you may need a new washer pump.
