Disconnecting the bottom hose will drain the radiator, but there will still be a lot of coolant left in the engine. It's worth starting with the expansion bottle; if it's the glass type, it's easy to remove and clean out. There might well be a layer of reddish mud at the bottom.
- It'd probably be worth removing the radiator to flush it properly (in both directions).
- Once the top hose is disconnected from the pump, you can reverse flush the head and block by putting a hose into the pump outlet. Yes, there
is a drain plug at the back of the block on Cléon (Sierra) engines, but it's wise not to touch it - it's difficult to get to, and easily broken. What should be a routine service process could turn into a disaster!
- You can flush the heater matrix once you've disconnected the heater hoses from the pump and turned the heater valve to 'on'. I remember you saying that you were thinking of bypassing the heater system, but it'd be a good idea to fix the problem (the valve/tap? Its gasket? The matrix?) - if your car overheats in traffic, a working heater could save you a lot of trouble!
- Once everything is back together, make sure you bleed the system thoroughly.
- Don't be surprised if the coolant in the expansion bottle becomes dirty again; flushing and engine heat will dislodge more dirt. It's time-consuming, but you can improve things by emptying and cleaning the expansion bottle every so often (when the system is cold), and topping it up with coolant. If you pour the dirty coolant into plastic bottles, wait a few days until the dirt settles and filter it a couple of times it can be reused.
Good luck!
I came across this:
http://www.manualesdemecanica.com/images/files/discussion/370/44abc560658b505993934634c04f42d7.pdf
It's from 1981, but might have some useful information in it.
This is great:
