The trigger voltage on many older flash units far exceeds the 5 volts that is considered the safe limit on todays digtital cameras. If you manage to get it to work you may "fry" some circuits in the camera. Not worth it.
Many older flash units had up to 250v. trigger voltage, I have a pile of old Vivitar 283's I won't even try for that reason.
My advice: shop for a SunPak "dedicated" to modern Canon's. That's what I did until I switched to Micro Four Thirds gear.