Proper Radiator Flush

[TodFitch]

Yes, it is a '35. I haven't had the opportunity to check further since posting (was on a cruise), but intend to this weekend. I suctioned out all the coolant/water, and it has been sitting dry for about 6 weeks. My thoughts were 1) to dry it out to see if new coolant would break it up easier and 2) if there wasn't enough coolant to keep it from freezing (it does happen in Texas occasionally), the frozen water would have somewhere to expand other than cracking block or something.
I do have a copy of the Master Maintenance Manual, but it is not precise in its explanation/steps.
The movement of the needle in the gauge is more of a slow movement to and fro rather than a sharp bounce. I'll be pulling the cluster this weekend to check in back.

The temperature gauge is a sealed unit, there will be nothing to see on the back side of the dash unit.

If it is slowly going up and down, then most likely the temperature in the head where the sensor bulb is located is going up and down in temperature. Some blockage in the cooling system or perhaps the wrong or defective thermostat that is cycling.

[TrueReb]

Yes, it is a '35. I haven't had the opportunity to check further since posting (was on a cruise), but intend to this weekend. I suctioned out all the coolant/water, and it has been sitting dry for about 6 weeks. My thoughts were 1) to dry it out to see if new coolant would break it up easier and 2) if there wasn't enough coolant to keep it from freezing (it does happen in Texas occasionally), the frozen water would have somewhere to expand other than cracking block or something.
I do have a copy of the Master Maintenance Manual, but it is not precise in its explanation/steps.
The movement of the needle in the gauge is more of a slow movement to and fro rather than a sharp bounce. I'll be pulling the cluster this weekend to check in back.

[Lew]

Is this for a '35? If so, Tod is correct that the mech. temp. gauge shouldn't be bouncy. I found my '36 elec. fuel gauge would wander due to a poor ground at the tank sending unit, corrected by a pigtail wire from the chassis to one of the sending unit screws and cleaning the metal surfaces as needed. The oil pressure gauge should ideally not be bouncy either.
There is a section in the Ply.  '34 to '36 Master Maintenance Manual (available in reprint) or on Ebay about flushing the block and radiator
separately.

[TodFitch]

What year car?

It seems unlikely to me that a "mechanical" temperature gauge would bounce around, just not in the nature of that beast.

So I am guessing you have a car new enough to have an electrical temperature gauge. So I would concur with CG Sailor that you should be looking for a bad connection or frayed wire between the dash unit and the sender on the engine.

[CG sailor]

I would flush it as normal. This could be just a residue from several thing. After you flush it and go through a couple heat cycles, recheck it to see if it came back.

For your gauge, check to make sure you electrical system is working properly. Check the ground or fuse. I believe that the power for it runs from the ammeter and that nut may not be tight or rust building around it. when you drain the radiator and before you refill it, pull the sending unit on the block and clean the brass with a green pad. I would put an aftermarket gauge in it untill you get the other repaired (if needed). Some folks might get all hub hub about not being correct but, they wont rebuild your engine if you blow a head gasket. Its not like we can run to the local parts store and pick something up. Safe not sorry.

[Go Fleiter]

is the gauge feeded via a small tube? Probably You find gummy in it too!

[TrueReb]

I took on the task of cleaning my cooling system. I was going to do a normal flush and fill, but saw that the radiator might have issues. There is a brown gummy substance, thick in places, that I can see in the top of the radiator and was wondering if there is something SAFE to use to clean it out before I re-attach the hoses and fill, or should I go ahead and do that then put in some sort of additive?
Another problem is the dash gauges bounce around a bit and I can't get a true reading if the engine is over heating. I'm not going to add an aftermarket gauge, as I'm trying to keep the car authentic. Is there someone that rebuild analog gauges?