C10 Rear Drum brakes lock-up...

[MarkIV396]

I did a search on the site to see if someone has had a similar problem. My 1970 C-10 (Heavy 1/2) Manual brakes fine 1 or 2 times then both rear wheels grab & cannot get truck to move at a stop. Brakes worked fine up until we got some heavy rains here in KC last week. Also I get a really hard pedal when this happens...
I know it's both back -as touching the wheels, they are hot! Rebuilt Master cylinder installed about 8 mo.s ago & whole system bled. I've had the truck 10 yrs. & the brakes have never been really good.
I know air will expand & cause this to happen if you don't get it out of the system. I would go for wheel cylinder, If it was just 1 wheel doing it - but, it's both.

Any Ideas? I don't have alot of $$$ to throw at this.

Thanks.