Almost a third of GMB tied pubco members claim their debts are over £50,000 while the vast majority (73%) said their annual income is below £10,000.
An independent survey, conducted by researchers associated with the University of Durham, revealed that the average rent paid by GMB members is £38,794 while 93% said their turnover was below £500,000 with almost a fifth (19%) estimating annual turnover is between £201,000-£250,000.
The GMB said 3,000 had responded to its survey, which it will forward to the Office of Fair Trading.
Only 1.9% said their personal income, after rent and costs, was above £41,000.
Unsurprisingly, the vast majority (87%) said they believed their rent is unfair, that changes need to be made to the way in which rent is calculated (83%) and that trading levels were misrepresented by the pubco at the outset of taking on the lease (87%).
Also, the survey estimated that tied tenants were charged 60% more for an 11 gallon keg of Carlsberg than the free trade and 64% more for Carling. This resulted in over a 10% increase in the price of a pint to customers.
The union is still threatening a ballot on possible industrial action by its members, although no date has yet been set. Any industrial action is likely to start with a switch off of all Brulines beer flow monitoring equipment.
In response, British Beer and Pub Association chief executive Brigid Simmonds told the BBC: "The pub companies and breweries that run and own pubs are making very clear changes, making changes to their codes of practice, which implements the new industry code, which will make the system more transparent and make sure that people have the right training and therefore people who run pubs, who really understand the contract they are going into. That's hugely important."