Can you jailbreak an iphone with a computer when its disabled to break the passcode?

Ps my ios is somewhere between 7.2.1 and 8.0.0

You need to be able to unlock the device in order to jailbreak. The jailbreak tool wouldn't even start the jailbreak until the iPhone has the passcode disabled as it needs to boot up and launch the jailbreak app to start the jailbreak process. If it's disabled then you need to enter DFU mode and then do a restore using iTunes which will remove everything. You can't just bypass the passcode like that.