When you take the chassis apart, you'll have full access to the inner workings.
The fitting I'm speaking of on the side of the pump leading to the heater.
You'll find a length of copper tubing running from the pump to the heater, and it has two threaded pressure fittings on it. You'll need to remove that copper tubing to be able to unscrew the fitting off the pump (well...it's easiest to do it that way).
I recommend unbolting the small steel plate on top of the heater (has 2 wires connected to it), as it covers up the pressure fitting that connects the copper tubing to the heater.
Then, unthread the other end of the tubing, and you'll be able to pull the tubing loose from the fitting on the pump (takes a little wiggling).
Then, just unscrew the fitting from the pump, and clean it out.
Do all of this slowly, as there is a component in the pump on a spring - it shouldn't jump out when you take the fitting off (it never has on mine), but perhaps yours is put together a little differently.
Just put everything back together as you took apart, and give it a shot.
At least, that's worked for me twice now.