My defroster selection will only blow for a couple of moments, perhaps 10 seconds, then the air flow diverts to the floor as the heater. The fan works well, the AC selection works well. Is there a vacuum accumulator or diaphram somewhere under the dash or a weak seal somewhere?

Sounds like a vacuum problem for sure. I'd lean towards the actual selector(the lever that changes the modes), in the center console, as the culprit. It is basically a vacuum "router" that determines where the vac. supply goes. They have a tendency to dry out over time, and start leaking a little vac. The "default" mode is Defrost, so something is telling the mode door to switch to heat. Prolly a leak in the selector switch.

I'm late in a reply, I've been working on wheels, brakes, and replacing the one size fits all (but not mine) replaced after market cable for the hot/cold selection. I've inspected the router, a vinyl or urethane device it seems. It looks to be in good condition, but I definitely here an air noise up in the dash area, seems to be behind the radio, when I move the selector from defrost to heat to vent. In AC mode the sound disappears. I was wondering if there is an accumulator or vacuum actuator somewhere I can't see.

terntes

I've heard that noise!  There is one of those cute little vacuum actuators hidden between the heater box and the firewall.  You only have to remove most of the heater to get to it.  They cost about $50.00 plus ten hours of your time.  But you might be lucky and find it is easier to get to.

Larry

Terrence, The vacuum actuator behind the radio is going to be REALLY difficult to get to and I hope you dont have to go that route.  Before you do, check the vacuum hose that feeds the heater / defroster selector.  This hose goes through the firewall about  in the center.  It gets its vacuum at a "T" connection near the headlight vacuum line one way valve. near the brake booster.  Check the vacuum connections.  I had a very similar problem and found that I had cracked  the 25 year old plastic "T" whle replacing the one way valve to my healight system.  I hope its as simple as that.

Crossfire,

I was hoping that would be an easy fix.. I lucked out on mine but had already located the vacuum actuator and was trying to figure out how to get to it when I found my vacuum leak under the hood... good luck getting to it and let us know how difficult it is.