Not always so easy.for the love of god, just change your pump!
Pish easy.
I've just spent three hours changing a 15-50 on a Pulsacoil 2000 - ****ing woulnd't drain, pump valves were useless, no access to flats to hold against to get the pump valves off, heat exchanger had to come out to take pump and valves out as an assembly, ****ing thing wouldn't fill, ****ing ****.