Chevy & GMC Vans

Hey
It's getting cold and my heating system which has always been flawless is letting me down. I have heat driving on the highway and nothing but cold air on the side streets. I pulled the hoses off the core and ran water through it both directions. Trimmed the ends of all the vacuum lines I could find. Loosened upper radiator hose looking for air pockets. Nothing helps. Did notice what appears to be less than impressive flow after pulling hose off the core with the engine running. Can it be the water pump failing. Van never runs hot. Always around the 200 mark
Thanks

My best guess is that the impeller on the water pump is wearing out which would necessitate a new water pump.

Does the heat work when you're parked and rev the engine? Does the temp climb over 200 when you do that?

My temp gauge has been reading normal. Temp will climb to 210 and drop 40 when thermostat opens. I get no heat whatsoever no matter how long I rev it or drive it on the back roads. Was on I -95 today and was laughing at the absurdity of it. I also suspect worn impellers that move just enough water to keep the motor cool with these cooler ambient temps. Damn it!
She's been so good for so long I really can't bitch-much

Update.
Dropped the van off at my brothers shop, ProTek Auto in Niantic Ct. He was skeptical about needing a pump so he went right for the vacuum system. There were numerous issues-surprise surprise. With full vacuum, some lines were collapsing and others were leaking causing mixed set of issues. When on the highway and pushing on the pedal the drop in vacuum would allow the collapsed line to open enough to permit a portion of the system to quasi-function. Anyway, thanks for the thoughts and input. Posting a pic of what success looks like
PS. God bless people that have what it takes to do this crap for a living!

I'm glad I was wrong.

Vacuum tubes are a much cheaper fix.

Roll on!

Interesting. I never did like those vacuum powered heater controls very much.
The vans that GM built before 1990 didnt use vacuum lines in the heating controls.
They used the old push-pull cable system to work the heating until 1989.
I've had 2 1987 vans, I like those older gen 1 shorty vans the very most !

Automotive engineers are wonderful at taking something that works just fine and improving it to the point of failure. Pretty soon they should be blessing us with a brand new vehicle where nothing works