Bee Saint Louis

Got the scarebird intermediate rear disc conversion brackets installed onto my 71 supreme. Car had front disc brakes and rear drums originally. I used the 82 Eldorado calipers, brakes, and rotors. So far everything is bolted up and fitted. I had the axel hubs turned down to fit inside the rotor hats. My question is what should I do next? I took the top off of my master cylinder and loosened bleeder screws on calipers. I got brake fluid to come thru then tightened them and left it as is. (Everything up til my 36-soon-to be 37 years of experience with brakes has been little to none)

Do I need a different master cylinder if I have the disc/drum one on here currently?
Do I need to install a disc/disc proportion valve?
Any help, comments,or suggestions are appreciated. Thanks in advance.

oldcutlass

You should have gotten a parts lists and direction with the kit? I would assume a disc/disc MC is required and parking brake adjustment will effect how the rears work if its not set up first.

joe_padavano

At a minimum you need to remove the residual pressure valve that is in the outlet port of the rear side of your disk/drum M/C. The preferred approach is to swap to the Corvette style M/C that everyone sells, but removing the valve will work so long as you make it a point to check the fluid level relatively frequently. Your disk/drum M/C has the small rear reservoir.

Given that your combo is not a "stock" arrangement, the better choice would be to get an adjustable prop valve and adjust it for best braking by doing test stops.

oddball

Disc/drum prop valves are different than disc/disc.
The drum units have a slight delay built in to let pressure build on the rear - to overcome the springs - before sending pressure to the front. You really don't want that on rear discs.
Then there's the actual proportioning. The common fixed valve - which bolts right in place of the common GM combo block - might work just fine, or might not.

You can get a disc/disc valve pretty cheap. MBM is the actual manufacturer for most of the resellers. I've seen prices vary by $40 for the same stupid thing. Right Stuff had the best prices last time I was looking.

If I was you, I'd put in the common valve first and see how it handles. They're cheap, and it's a quick bolt-on. Any adjustable unit will require some modifications.

If the rears are locking up before the front, then you can add an inline adjustable valve along the main front to rear tube to further reduce the pressure.

If the rears won't lock up, then the common valve isn't letting enough pressure through, so now you're looking at a bigger problem. You'll need either a plain distribution block (with no built-in proportioning) plus an inline valve, or a prop valve with a built-in adjuster. None of the adjustable prop valves fit anything like the stock block, so you have to rework all of the brake lines.