i have a 68 mustang that i am going to do a rear disc setup, do i need to have a proportioning valve on it like a disc/drum setup?

 

no need
balance w/ correct size calipers.

 

I would consider it. Test the brakes without, the fronts should lock before the rears do, for safe brake operation. If the rears lock first, get a PV.

 

Just avoid any "wet" surfaces till you are certain you have good balance.

 

Unless you know for certain that the brake packages are correctly balanced for your car, install an adjustable inline valve to the rear.

 

I used a csrp in front and ford explorer in rear. Plumbed in a pv anyway. Tested on wet car park and adjusted. Next time round I'd do it again

 

PV wastes energy; i wouldn't want one unless req'd to balance front:rear.

if you opt for disc/disc PV make sure it is fully adjustable; some designs will not allow a full bypass setting.

i would try to pick correct calipers so PV not needed.

no...i'm not a PV hater...

 

thanks guys for all the info, i think to be safe i will install a pv i will open it fully and adjust it as need, i am running wilwood 4 piston up front and 2 3/8 single piston in the rear.

 

I have the SSBC billet setup, for the price you can't beat it. Just having the option of adjustability is nice. There are a lot of things to consider that effect f/r weight. I don't believe any out of the box kit will compensate correctly for the differences. Sounds like you have a nice brake setup, so I'd finish it off right.