70 Cutlass: I知 converting from four wheel drum to four wheel disc. Is it necessary to change the proportioning valve. In my think the distribution of fluid stays the same. Is that correct?

 

I'm just going to follow along. I believe I know what I'd do but I'm looking forward to responses.
EDIT: Well, my statement was a rather weak statement. I'd install a combination valve and re-plumb the brakes. Still looking forward to reading responses though.

 

Your four wheel drums don't have a proportioning valve. (Yes, I'm aware that early Toronados did use a prop valve on four wheel drum systems due to the heavy front weight bias on those cars).

The Chinesium combo valves that are included in all aftermarket disc brake kits are one-size-fits-none. Let's once again review brake valve functions. All 1967-up cars with dual circuit brakes have a differential pressure switch that turns on the BRAKE light if you lose pressure in one side. This is NOT a proportioning valve, it's just a distribution block with a switch in it and was federally mandated. It is not functionally required.




There are three types of actual valves in the brake system, residual pressure valves, a metering (hold-off) valve, and the proportioning valve. The residual pressure valves are built into the outlet ports of the factory master cylinder. Disc brakes typically do not require an RPV.




The metering valve is used on front disc/rear drum systems. It's purpose is to slightly delay the application of the front disc brakes to allow time for any slop in the rear drums to be taken up. This is done to prevent the rear brakes from locking up prematurely. The 1967-70 factory disc brake cars used an external metering valve under the master cylinder.





The proportioning valve is designed to reduce pressure to the REAR brakes, again to prevent premature rear wheel lockup under panic braking, which can lead to the back end braking loose and a spin. The 1967-70 factory disc brake cars did NOT use a proportioning valve. (That statement applies to the Cutlass line. The full size cars DID use a prop valve with disc brakes in those years). The front/rear brake bias was controlled by proper selection of the wheel cylinder diameters. The disc brake cars used a SMALLER rear wheel cylinder than did four wheel drum cars. This is a key point that is lost on most people (including the people who sell disc brake kits). You are spending considerable dollars to add rear disc brakes, then you install a proportioning valve that throttles the pressure to those rear brakes until they provide the same stopping power as drums. Your money, your call.

The reality is that brake system design is far more complex than that. What matters is the friction force between the tire and road surface at each of the four wheels. Hard braking causes forward weight transfer, which means that the rears do even less work and are more likely to lock up. Wheels and tires of a different diameter and width than OEM dramatically change the expected braking force at each corner, as will changes in weight distribution (like a larger, heavier engine). Yes, wider, stickier tires will let you use more braking at each wheel. The factory used a wide variety of proportioning valves, each calibrated for a specific model. The one-size-fits-none aftermarket valves are calibrated for who knows what. The right way to approach any braking system changes is to use an adjustable prop valve and drive test the car to set the correct proportioning level.

For completeness, I'll also point out that the factory combined the functions of the differential pressure switch, the metering valve, and the prop valve into a single COMBINATION valve (get it...). This is what all the aftermarket companies sell today.

 

WOW thanks for all of the great information. I never have to ask another brake question again lol

 

Yes +1 thanks . was thinking what parts i should need for dual circuit mastercylinder in my 98 and keep the drums.

 

so to change form manual drum to manual disc all I would need is a disc brake m/c and a combo valve? Car was a power drum car but put a lumpy cam in so had to remove booster.

 

I deleted your post. No need to quote Joe and then ask your question. I'll let Joe answer your question.

BTW Joe, thanks for the info on disc/drum brakes and hydraulic valves.

FWIW, I know other manufacturers use similar technology. My 1986 Ford truck uses a hydraulic valve that limits brake force applied to the rear drums so they don't easily lock-up with the bed unloaded.

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The 1967-70 Cutlii did not use a proportioning valve on disc brake cars. All the prop valve does is restrict the pressure to the rear brakes to prevent premature rear wheel lockup under panic braking. In place of the prop valve, Olds used rear wheel cylinders that were smaller than those for four wheel drum cars, which had the same effect as reducing the pressure to the rear wheels. For an aftermarket conversion, you can do it either way. To avoid changing the rear wheel cylinders, a combo valve is probably easiest. The problems with this solution are 1) the Chinesium aftermarket combo valves have very poor quality resulting in a lot of leaks and 2) the factory used a variety of combo valves that were tuned to specific car weight distribution and tire sizes. The aftermarket one-size-fits-all combo valves use a single valve spring tuning, so it's almost guaranteed to be calibrated wrong for your car. An adjustable prop valve is a better choice if you're comfortable in performing the brake testing needed to adjust it for your car.

 

Joe, correct me if I am wrong, but it was my understanding that the undersized drum brake cylinders stopped in 68 and the following was the setups:

66-67: distribution block and undersized rear drum brake cylinders
68-70: distribution block, separate proportioning valve
71 up: combination valve.

Separate thought: I don't think the distribution block did anything other than turn on the brake light if one side lost pressure. Useful knowledge: master cylinder failures do not cause the brake light to come on, because zero in the front and zero in the back is not a pressure difference, ha ha ha.

 

Sorry, no. There was no prop valve used on the 1967-70 disc brake cars. The metering valve was under the M/C, but no prop valve (and yeah, I realize that even Inline Tube incorrectly calls the metering valve a "proportioning valve"). Full size cars did, and Chevelles did, but not Cutlii. And in my experience the differential pressure switch usually turns on the BRAKE light right after you foot hits the floor and you say "Oh $#!T"

 

This is a really great discussion. Thanks for this info everyone

 

Other than looks is there a difference between the 69' and 70' metering valve?
Dustin