i have a 92 s-10 body on a chevy blazer frame. im using the s10 master cylinder. the front brakes work great but i cant get the back ones to work very good if you jack the back up it will slow the tires down but wont stop.is it possible the s10 master cylinder wont work.

something is not right, my buddy just put a full size truck prop. valve on his s10 with full width axles with the s10 booster and m.cyl. and it works great.

With a tandem master cylinder you will get the same pressure to both front and rear.

Are you still using the S10 combination valve? If so that is your problem.

im using the 79 chevy brake lines hooked wright to the s10 master cylinder

Then the problem must be with the 79 chevy brakes.

Have you bled them completely?

and checked the rear brake cylinders?

i replaced both wheel cylinders. i will try to bleed them again thanks