I have put a new booster and dual master cylinder on my 66 convertible. Also installed new front disc brakes. Previously was drum.
I have also install a 67 proportion valve as the rear brake was locking up. And this still happens, do I have to change the size of the rear wheel cylinder on my drum brakes? (or what size the rear should be?)or is there a trick that I am not aware of?

 

if you have a residual pressure valvemake sure it's the right way round.
You should be able to reduce the flow using the proportioning valve.

If you don't have a residual pressure valve you might need to get one.

 

If it is a 67 rproportioning valve, it isn't adjustable. OP: you can oreder an adjustable one that goes in the brake line that goes to the rear brakes. You can get them just about anywhere such as Jegs or summit.

 

will he need a residual pressure valve as well or does that only apply to rear discs?

 

That would only apply if the master cylinder was mounted below the wheel cylinder (like in a 32 Ford coupe), so no, he doesn't need one of those. That is to prevent the fluid from going back to the master cylinder which would cause it to overflow. In any case, a pressure valve wont cause the brakes to lock up. If it were turned the wrong way they would hardly work at all, so that isn't an option. The rear brakes are getting too much pressure, hence why they are locking up, so he needs an adjustable proportioning valve to limit the amount of pressure that is delivered to the rear drums.

 

Tyler, the residual pressure valve isn't just to keep fluid from flowing back to the m/c, it's there to keep a constant ~10psi on the rear w/c's to keep the shoes close to the drums, eliminating excess pedal travel. I've got one on my CSRP setup and it works fine.

He may need to back off the shoe adjustment slightly. I'd try that before trying to plumb in an adjustable prop valve.

 

Well, I learned something new today! Glad to know that!