[superbirds]

I think Jon said it earlier, I'd try to connect a pressure gauge in the back to see if the pressure changes when you press the pedal. I planned to fix mine with a rebuild kit, I found the replacement before rebuilding it, plus my old unit did not look great to me. The seal/bellows had a couple holes in it. you can pull that unit off and pull the cover off where the air line connects.

Before I fixed my throttle I drove the Newell 200 miles from Indy to Jenison MI. When I pressed on the pedal it just wouldn't go. Even pressing the pedal down to the floor it took a while for the rpm's to increase. Driving home was an adventure - I got stuck on the side of the road a few times because of the lack of throttle function.
I thought it had to be something in the front pressure unit until after I replaced the rear unit. I couldn't find the front unit to buy so I guess I got lucky. I think the back unit was cheaper and easy to find on Ebay. Throttle works perfect now, which is awesome from where I was at first.
Your's sounds the same to me.
Mine is a 1975 and the old unit looked original to me so I just replaced it as part of my testing process and to prevent future issues.
Good luck - slow an steady always works for me.
Hang in there. Just my experience, hope it helps.

nice pictures - prices seem high tough - http://www.coachconversioncentral.com/airthrottle.htm