Most likely not the prop valve, but if the low pressure switch was triggered, you could have just the front brakes working and not the rear. This may be your issue.
I don't know your tire or brake setup, but if you have skinnier and/or shorter tires on the front than the back, your front brakes will lock up easier as you have less contact with the road with a skinnier and/or shorter tire than a wider and/or taller tire.
A smaller bore master cylinder would create more line pressure. Stock size is 24mm bore diameter. A smaller 7/8" bore size was also available and came equipped on early manual brake cars. You put a manual brake master cylinder on a vacuum boosted car, you will have super sensitive brakes. Same effect will happen with upsizing to a larger booster or upgrading a booster from single to dual diaphragm.
Also, going with a caliper with a larger piston area over stock will also make your brake more sensitive as the clamping forces will increase with more piston area. Going from a stock single piston, to a dual piston Blazer setup will make your brakes more sensitive as the piston area went up.
Pads could also be a factor as the front pads could be more aggressive than the rears causing the brakes to lock up easier.
Bottom line it could be multiple things.
What is your brake setup front and rear?
What pads are you using?
What has been changed out in your brake system?