It could be - the plastic impellers are prone to either breaking up or becoming loose on the pump shafts. Use a pattern pump with a metal impeller as a replacement.

If this were the problem though, I'd expect to see some overheating at heavy engine load - although not at light load as the PD is so efficient it needs no cooling under these conditions.

I have found that the only way to properly vent these cars is to remove the fixing screws for the coolant header tank (leave all the pipes attached) and raise it above the level of the vent hole in the heater pipe at the bulkhead. This usually does the trick.

659.