need help on rear end swap brakes dont work right..

i have a 1986 mustang gt 5.0 it had drums on the rear 8.8 .i put a 1994 mustang rear with disk also has a 8.8  . i also put a svo master cylinder & booster the car doesn`t want to break when i really press hard on them  why????????  

Did you swap the proportioning valve ?  Different valves for drum or disc brakes.  Calipers need more fluid to actuate than a wheel cylinder.  They probably looked o.k. when you bled them but it's not enough volume under pressure.  

WELL I GUTTED THE PROPORTIONING valve and added a adjustable valve and bled the system and it got a lot better but sometimes there is no breake pedal it goes all down sometimes  

It sounds like you still have air in the system or a bad part.  If you're confident with your bleeding, I would start disconnecting and plugging the calipers one at a time until the problem goes away to isolate the faulty part.  You could even diconnect the lines from the master cylinder to see how it acts by itself.  Don't forget...old flexible rubber brake hoses like to expand under pressure, taking away fluid volume that should be used to actuate the caliper.  Get a friend to pump the brakes and look at all the hoses to see if any of them are "ballooning" under pressure.  

well i had also replace the two front hoses and the calipers. (replace with stock calipers) and bled the system and bled it and bled it took awhile to get all the air out but got it done. something is wrong.  ANY MORE INFO!!!!!!!!!!!!!!!!!  

If you got all of the air out, then i would think your problem is solved, no?  

If everything on the front is new (not to say I haven't seen a bad new part) and the bleeding is good then I'd say you have a bad caliper on the back or a bad master cylinder.  But I would still disconnect each corner one at a time to isolate the problem.  There could be a seal or o-ring on its way out that could be causing blockage and/or internal leakage.  I've got a lot of experience with hydraulic systems and sometimes the only way to find an intermittent problem is to find a way to make it fault repetativly or by isolating the fault from the rest of the system to where the system works good without it in the loop.  It's a lot of work but a whole lot cheaper than throwing parts at it and not knowing what the problem was.  

thanks wind-e i`ll try finding the problem because almost everthing has been replace maybe wrong combo mix.