Hope someone can help me figure this one out:

The rear washer on my 1990 245 will only squirt for about 5-10 seconds then stops. Sometimes it will work again after 15-20 mins sometimes it won't until the next day. Pump is running fine when this happens. Swapped it with the one for the front washers: no change. Replaced the little anti-drainback gizmo in the line back by the tailgate: no change. --Removed this device-- tried it: no change. Removed the nozzle for the rear window: you guessed it: no change. Let things rest a while, whole scenario repeats itself. Can't think of anything else to try except run an entire new hose. HELP!