My old drum brakes held great with the parking brake. If I were to have pushed the brake to the floor, without flooring the Trac it would not move. That is a fact. If they don't hold right, they arent working correctly. Pull the drum and check everything out. There is no way I could drive 20 MPH with my parking brake on.
Nelson, Disk brakes have a better track record for stoping then drums do. Light duty chassis like the F-53, F-250 SD, F-350 SD, F-450, F-550, and the LCF all have 4 wheel disk brakes. Interesting enough, they also have a solid rear axle.
Anything larger then that uses air brakes and they are a whole different animal.
An IRS is nothing more then a FWD setup on the back of a vehicle. We know how well FWD vehicles handle the abuse of towing and heavy weight. With an IRS, you have a harder time adding leafs if you need to haul more weight simply because it doesn't have leaf springs.
Tom