my 1992 cadillac brougham heat doesnt blow hot enough?

[livelifetothefullest]

i have a 1992 cadillac brougham 350 5.7 v8 and i have the heat temp at 90f and the air comming out feels warmish cool? even when i warm the car up and drive it for a while it just aint hot as it should be what do u think is wrong here? also coolant was topped off to the correct mark and then later on it went down one full mark?
my brother has the same car and when he puts his heat on 90f it comes hot very warm like hot.

 

[DavidL]

Replace the thermostat with a 195 degree one. Also check the proportioning valve on the incoming heater hose.