Z1OwnersClub GB

Why would I loose equal pressure on all
4 cylinders. it blow oil out tru the
breeder pipe & hasn't ran since it's a gpz 1100 (83)

Could be a number of causes depending on the circumstances at the time of the pressure loss.
Likely suspects if the bike was being ridden are partial heat seizure leading to ring and bore damage or bent valves due to camchain tensioner failure etc.

if it happened after a long layoff, could be the rings are gummed up and not gripping the bores well , with compression loss and a lot of blow-by, causing crankcase compression and hence oil blowing out the breather