Heat Question

[kd3yt]

2001 Blazer Trailblazer 65000 miles.
Checked the radiator level yesterday and it was down a little bit (Put about a cup of coolant in). The puke tank was where I normally keep it - above the hot mark. This is the first it has ever been low. I check the puke tank any time I am under the hood and open the radiator cap at least once a month.
When I turn the heat on, it is strange. I have heat if I put is straight on defroster or on defrostor and floor. If I put the control to blow air out of the dash vents then it is cold!!!
Also today I checked the radiator level again and it was down about half a cup.
Does this sound like a thermostat problem? Might I be having problems with the blend door? I put a stant cap on at least a year ago. Also when looking in the radiator when it was low, it looks clean and shiny as a whistle.
All help greatly appreciated!!

[berniescout]

change the radiator cap

[s10blazerman4x4]

Stant would be good choice.

[s10blazerman4x4]

lol didnt see that. You could check the thermostat too.There not that expensive to buy.

[DaveLance]

Get your system pressure tested to find out where it is leaking from. Do all of your vents work normally? Any hissing sounds behind the dash?

[kd3yt]

Update::

I have put a new stant cap on. Also today I put a new thermostat in.
Both hoses going to heater core are hot. When I move the temp control all
the way to the right on the read mark I have great heat in all positions. If I lower the temp - I still have heat in defrost and defrost/floor position, but other positions are cool.
Would this possibly be air in the heater core?? Question: to flush the heater core, I remove the two hoses and spray water first in one direction and then the other. Should I cover the distributor and other stuff directly below hose connections before I spray??

Any and all help and suggestions greatly appreciated!!!

[DaveLance]

It certainly wouldn't hurt. Did you ever find out where you were losing coolant?

[kd3yt]

No I did not. After the first two times it was low, it hasn't been
low since. Thought maybe the old cap was bad.