My '68 had factory front discs with 4 piston calipers and 2 piece rotors. These parts are expensive to repair/replace so I installed later model single piston calipers and one piece rotors. I couldn't get the air bled out of the front lines so I took it to a shop to be power bled, but I still don't have much pedal. The '67 and '68's with factory discs have a metering valve for the front and a distribution valve, and no proportioning valve.

Has anyone done this conversion? Should I replace both the original valves with a prop. valve? Do I need a later model master cylinder? I'm beginning to think I should have kept the original set-up.

You might take a look to see if the master cylinders are the same for the twin piston and the single piston calipers.
Now you said the rotors are two piece are you saying that the bearing hub and rotor are separate?
Napa shows two master cylinders one for disc/drum brakes and one for 4w drum so that doesn't help much,sorry.

Do you have the bleeders at the top? The calipers will fit either side but if they are wrong the bleeders are at the bottom and you can't get all the air out even power bleeding.

You need a different master cylinder. By switching the calipers you have changed the piston surface area and its relation with the masters cylinders bore/stroke capacity. This can get very confusing, but its simple hydraulic principles. I have spent countless hours researching and talking to the tech guys at wilwood to get my system right. I put 4 piston calipers on the rear (originally drums), and changed the fronts to the "big single piston" G- body (and M/C from 89 Caprice Wagon) and could never get a solid pedal.

OK, I'll get a single caliper master cylinder, the one in the elky is probably 40 years old! Do you think I can get by with the metering valve? I guess I'll wait and see how the new MS works out. If I go to a prop valve, then i have to re-do the lines. Thanks

The bleeders are on top, at least I did that right!

Yeah, the hub is riveted to the rotor, these are antiques, very expensive to replace. First generation discs that were used on the early 'vettes. GM hadn't perfected the system yet.

I'm assuming you have drums on the rear? The metering valve may work fine, since you will still have a disc/drum combination. I dont know about the older (68) elkys, but the later ones use a combination valve. It combines a residual pressure valve, and a pressure valve. If the system is engineered correctly ( front/rear/MC ), theres no need for a metering valve. The valves are usually specific to the application (different settings for each model in a car line). I finally settled on an adjustable prop valve and nothing else. If you have rear drums, you need to also use a residual pressure valve on the rear circuit. This holds 10lbs of pressure on the circuit to keep the drums pads "pre-loaded", so when you push on the pedal, you dont waste M/C travel overcoming the springs before the pads engage the drum. The disks dont have any "spring takeup" as they ride very close to the disk.

Hope I haven't confused you.

Heat,

This all makes sense. Yes, the rears are the original drums. How do you know when you have the prop valve adjusted correctly? Just by the way the pedal feels?

Adjusting the prop valve is the fun part. To really get it right, you have to do some "real world" testing. Keep in mind the fronts do about 75-80% of the total braking due mostly to weight transfer when braking. The objective is to have the rears contribute the maximum amount of braking WITHOUT locking to rear wheels. If the rears lock up you are likely going to lose control.

So find a vacant parking lot or lonely stretch of road. Adjust the prop valve at about the middle (count the turns of the knob).

Start slow (10-15 mph) and brake hard in a straight line. If the rears dont lock up, adjust the knob to allow more pressure (or proportioning) and repeat. Increase the speed and braking force until you can get them to lock up. Then, back it off slightly and your good.

I am able to lock up all four wheels at the same time. With 12 discs on all four corners it will stop fast enough to jerk the fillings out of your teeth. Thats about as good as it gets short of having ABS.

PS. Set it while your elky is in the condition you normally drive it in ( eg. spare tire, toolbox, 1/2 tank gas, etc, 350 lb girlfriend, etc)

Heat,

So you must have a block to split the front lines, and the prop valve in the line to the rear. Sounds like a simple set-up.

I have seen a combo distribution valve and rear adjustable prop valve at Summit. I guess that would work too. Do you agree?

There are also fixed prop valves available, but I don't see how one valve would work for all applications, based on what you have told me.

Thanks for the girlfriend tip, I'll load her up when testing!

Bruce