The PO upgraded the brakes in my '70 Cutlass S to disc using what was apparently a '68 or '69 442 donor. I don't know what parts he changed and what's original here, but the brakes worked for the most part when I drove the car home. They lost pedal real quick which I blame on the master cylinder and will be replacing that before driving again.

The old master cylinder has the bleeders on it but also has this round component bolted up with it. What is that round thing?

Does the proportioning valve look correct for disc front/drum rear, or can you tell externally?

 

The round thing under the master cylinder is part of the proportioning valve system for the front discs, used from 68(?) up through 70.
In 71 it was incorporated into the combination valve attached to the frame rail.

From what I remember, the valve on the frame was the same for both front disc and front drum cars. The 68-70 front disc setup had the additional valve below the master cylinder.

 

Actually, the valve under the M/C is the Metering Valve, which is not a proportioning valve. Note that it is plumbed into the FRONT brake circuit. A prop valve goes in the rear brake circuit. These cars did not use a prop valve but actually balanced the front/back brake bias through careful selection of wheel cylinder diameter and brake lining size. The item on the frame is NOT a proportioning valve, it is only a distribution block. There is no proportioning function in it and it is the exact same part used on four wheel drum cars.

The parts in your photo are correct for a 1967-1970 factory disc brake installation. Note that 67-68 discs used the fixed four piston calipers, the 69-up cars used the single piston sliding calipers.

 

Thanks for clarifying, Joe.


So what does that mean for refreshing parts in my system? I have a new master cylinder, do I need to retain the existing metering valve, replace it or remove it? I'm assuming it needs to remain in the system...do they go bad and how can I tell if it has?

 

Yes, keep that valve.

 

The metering valve is simply a cast iron housing with a piston, o-ring seals, and a spring. The biggest problem is that since brake fluid is hygroscopic, rust can form in the housing, causing the piston to stick. The valve can be rebuilt, but there are no kits - you have to buy o-rings individually from a place like McMaster Carr or MSC or Grainger. Reproductions of the metering valve are sold by the tubing and brake kit vendors. I have no experience with them. Alternately, you can replace the whole mess (metering valve, distro block, etc) with a newer brass combo valve.