Help Calculating time to recharge air tanks

[nuclearnerd]

This one is making me feel dumb:

I'm trying to optimize the performance of our (rookie team) pneumatics system. One basic parameter is how long it takes the KOP compressor to charge the tanks from the working pressure (60 psig) to the storage pressure (120 psig). I've tried estimating this a couple of different ways, but they all go pretty much like this:

1) Assuming temperature stays constant (not a fair assumption maybe):

P1 * V1 = P2 * V1 (from the ideal gas law)

So ending with a storage volume of 35 in^3 at 70 psig (84psia), to get to start with a larger volume at 60 psig (74 psia):

V1 = P2/P1 * V1
V1 = 84 psia / 74 psi * 35 in^3
V1 = 39.73 in^3

2) and since we started with 35 in^3 of 60 psi air, we need to add:
deltaV = 39.73 - 35 = 4.73 in^3

3) Looking up the performance values for the ViaAir 090c (http://www.viaircorp.com/90C.html#tabs-2), we get a flow rate at 60 psig of
Vdot60 = 0.53 cfm = 15.26 in^3/s

4) So the time to bring the tank from 60 to 70 psig is:
t60to70 = deltaV/Vdot60
t60to70 = 4.73 in^3 / 15.26 in^3/s = 0.3s

5) Repeating this for each 10 psig interval between 60 and 120, I get a total recharge time of about 1.7s per 35 in^3 tank.

Here's my problem - we're seeing much longer recharge times on our robot. With three tanks at above 60 psig, the compressor runs for over 20 s before turning off

Have I made a mistake in my arithmetic or assumptions? Should I have paid more attention in fluids class? Does ~2s per tank to recharge from 60 to 120 psi sound reasonable in other teams' experience?

(note -I get pretty much the same answer if I just use the viaair flowrate at 60 psig and do the calculation in one interval between 60 and 120 psig)