My "T" is a little gutless!

[Sunny the Model A]

that's low compression for a stock T, it should be a bit higher around 55 psi. so that means pressure is being lost. now when you did the test did you fully open the throttle to get the full volume of air? if the throttle is closed it won't give the right reading. now if you did and it still only gives 40 psi, I'm suspect of piston ring and cylinder wall wear. that's what is most common on T and A engines. Got any other symptoms from the T that might explain it? since you say you rebuilt the carburetor several times and the coil boxes I'm thinking it's internal but let's not jump to conclusions. so by your description over a certain point it just bogs down. well if it struggles to get 30 mph that tells me it can't pass peak torque which occurs at approx 950 rpm, with stock tires thats about 25 mph so it's consistant with your description