Thre are two different rear brake cylinders listed for the 1987 S-10 pickup. Of the two sizes, you want the ones with the larger internal bore (I am pretty sure it is 7/8" instead of 13/16").
When you converted to vacuum brakes you had to run a vacuum line over to the vacuum reservior. The little plastic thingie that connects to the hose and 'snaps' into the opening of the vacuum reservior IS your check valve. You prolly need to A) bleed the brakes and B) not use them so much when coming up to the line. Another, more difficult remedy, is to run a vacuum canister from Jeg's or Summit. But you still gotta have the lines well bled.