Do you have 100% shoe contact on both shoes? (Front and back on both sides) If so your powermaster is not giving full pressure to the cylinders . You mentioned line lock ,if it’s plumed before the proportioning valve it can cause a imbalance to the rear wheel cylinder. I’m a little fuzzy on the exact dynamics of what goes on but the brake light will come on when you mash the breaks and activate the line lock. Front brakes do play a part in this too .They add resist to the foreword motion too .do the front tires look tike there rolling. Take a video.
I like my Powermaster brakes,most people go vacuum or hydro boost, I have not tried to rebuild one ,may have to someday.My pedal is a bit touchy with the S10 cylinders with light pedal pressure , but drivable. Must be supper carful in the rain.And harder than Chinese arithmetic when pushed hard not mushy.
I use the cheapest Raybestos brake shoe. I buy two sets and use the primaries, again a little more surface area don’t think it’s the secret ether.When I pull the street tires off the drums slide off, no drama plenty of clearance knock them on the ground to get the dust off and bolt the slicks on.
