Do MP3/4 Copal Shutters function differently ?

I am building a 3 shutter camera and have been buying MP3 and MP4 copals for this purpose. The last one I bought seems to work perfectly except that when you release the shutter it opens to about 90% and then you must relax the cable release in order for it to complete it's operation. I thought that this might be a defect but a friend said that perhaps this shutter was used in conjuction with another shutter or some form of lighting etc... that required a lag thus the hesitation was designed into it from the start.

Can anyone give me a run down on the shutters available from MP3 and MP4 Polaroid cameras?

Thanks in advance.

Hmm. I have two ex-MP-4 Copal 1s, one ex-CU5 Copal 1. All work normally. If the cable release you used with it works properly on your other shutters, your last one probably needs service.