First off, welcome to the forum...
Second, I have an 87 RENIX (get used to that term) and have replaced the pressure bottle...
Easy way to burp the system is place vehicle rear wheels slightly higher than the front by drving up onto a ramp or blocks.
Next, with the engine cold (IE Overnight cold), remove the bottle cap and fill the system until the level of coolant is at the top of the post inside the bottle.
Leave the cap off and run the engine until warm with the heater on FULL HOT, fan on low or high (doesn't matter, so long as the heater fan is on)
Add fluid as neccessary to keep the level at the top of the post.
Once the thermostat is fully open (upper radiator hose bing HOT, as well as the air coming from the radiator fan (about a half hour), shut it down, let it cool and recheck the level on the post, if at the top, re-install cap securely.
that's it, the entire burp sequence....
As for the flush, I flush mine by running water through the system until clear (engine not running, just use a garden hose on a low pressure setting) water comes out the lower radiator hose.....
You can run the engine for SHORT (30 seconds to a minute) periods to force clear water through the block and heater core to help flush those out....( you will need to temporarily reconnect the lower hose so the water has a chance to circulate, then drain it again)
Once it runs clear, than let the system drain throughly (there are drain plugs in the block) and refill with 50/50 antifreeze/water mix. Proceed to burp the system using the step outlined at the begging of the post....