I've got 2 retread tyres on my trailer which was made i the 70's, is that bad
Yep.
Rubber ages and major failures are more prevalent in older tyres. People tended to blame retread failures on the retreading process and to some extent the type of work the original carcass might have been put to.
From the RACQ website.
"Can a tyre be too old?
Car tyres can deteriorate with age. A tyre that’s been sitting in the spare wheel well for years, even though it’s never been used and has plenty of tread could be ready for retirement.
Heat, sunlight, ozone and other environmental conditions can all cause deterioration of the rubber compounds. In service this can lead to tyre failures.
Some industry experts suggest that worn out or not, a tyre may have passed its useful life after about five years, however this is not recognized in law.
A production date code is usually moulded into the tyre sidewall. Consult a reputable dealer for further advice on interpreting this code."