What causes that old car weak fan problem?

[Skunksmash]

One thing I have noticed over the years, an old vehicle will usually make a lot of racket, but not move too much air. I am speaking of the climate control system in the 73-87, specifically. I have owned plenty of old cars and trucks through the years, and most have the same problem. You can turn the blower motor up to max, but the air only comes out of the vents at half that, or 3/4 at best. It's like the flow is somehow obstructed. My truck is no different. The blower motor will go real fast, and make some noise, but minimal air comes out. Especially considering that it's on high. Having tore the dash and gauges all apart in a recent project, I was able to make sure that all the lines which the air flows through, are in good order. There seems to be no reason (at least to me) that it shouldn't work just as good as when it was new. So what causes the decline in airflow? Anyone ever took one of these systems all apart and found anything that could be the cause?

[Flamingbig10]

after a wile the gunk that used to seal blower motor to the box and box to the firewall  get old and air leaks from there

[Irish_Alley]

what about leafs and dirt in the vent under the cowl?

[Skunksmash]

Didn't know that could be a problem. I will have to check that. Is it hard to get off?

[Skunksmash]

I guess it's just those two bolts you can see when you raise the hood? Don't want to bend it all up trying to get it off..

[Irish_Alley]

it might be easer to just take the vent panel off from inside the cab on the passengers side from there you should be able to clear anything out. that how i did mine

[Donut]

In my case it was a combination of things.  (BTW no AC on mine)
Seals on the blower box were gone, quarter size rust holes through the bottom of the blower box.  Blower motor slowing down with age, (a little oil and cleaning helped some) squirrel cage with worn blades, mouse nest and 35 years of crud built up around heater core, flapper door not fully functional (crud/rust, getting this working helped a bunch) air passages not lining up anymore (dash pad/defroster in my case) and just a bunch of crud built up in the air passages. a good cleaning helped. 

It didn't take long to figure out one of the PO's had a black dog and lived/worked near a rock crushing plant.

Even the electrical fans in my house slow down with age, it's not unusual for me to get a new fan and have it move twice the air as one a few years old.