Hello,
I've removed all the a/c from the 350, I've just retro fit an aftermarket heater/demister under the dash. I've connected up the heater hoses, from the control valve to the heater & the hose back to the pump. My old control valve has no vacuum hose connected. I have an in-line on/off valve in the
feed hose that I can cut off flow to the core when I wish. Seem to have restricted flow though, is it because of no vacuum on the control valve?

regards, cus

 

It depends on the year and model (which you did not tell us). For the A-body cars, 1972-earlier have normally closed valves that require vacuum to open. 1973-up have normally open valves that require vacuum to close.

For the full size cars, 1970-older have normally closed, 1971-up have normally open.

Of course, if someone installed the wrong valve, all bets are off.

 

Thanks Joe,
Car is a 1973 Delta 88, valve looks pretty original

 

The original valve for a 1973 D88 is normally open and should present no restriction without vacuum applied. Are both hoses to the heater warm when the engine is warmed up?

 

I investigated more last night, flushed the heater core, hooked up to the manifold,
all was flowing OK, was blocked at the water pump, had calcified a bit in the base of the tube, cleaned out & presto, has been bunged off for quite a few years I think.
Thanks for your help