Ok so the 67 Turnpike Cruiser has factory front disc brakes. New metal and rubber lines installed, new correct master cylinder and booster, new rebuilt 4 piston calipers and pads and a replacement adjustable proportioning valve. All lines run as per original. System bled with DOT3 fluid. Here is the problem.... When you first get in and start out the brake pedal goes about 1/3 the way down, brakes are fine. After you drive a few miles the brake pedal barely needs to be pressed and the brakes are right there! Ok the warmer it is outside and the longer the drive the brake pedal starts to drop further to the floor before brakes are had. And one time after a long highway drive at speed when I approached a toll booth I was barely able to stop because the pedal was on the floor. So I backed off my speed and slowly the brakes came back to a half pedal. So I am thinking this maybe heat related? Any thoughts? Cooler weather drives does not give the same symptoms.... I was thinking of changing the fluid to a higher temperature one. I checked and brakes are not dragging.

 

Sounds like you have a brake line routed too close to an exhaust pipe, causing the brake fluid to boil.
Why an adjustable prop valve on an OEM (presumably four piston) system? These cars didn't come with a prop valve from the factory, only the metering valve.

 

Are they not the same thing? Factory has the valve right under the master/booster assembly. And as said the lines were mounted as original but yes was thinking the same thing with brake line routing...

 

Absolutely not. The metering valve and prop valve are completely different units with completely different functions. Unfortunately, even vendors who should know better (are you listening, Inline Tube?) get this wrong and incorrectly call the metering valve a "proportioning valve".

The metering valve simply delays the application of the FRONT disc brakes for a fraction of a second to allow the pistons in the rear drum brakes to move enough to take up any slack in the linkage and shoes.
The proportioning valve proportions (get it?) the force of the REAR drum brakes to prevent premature rear brake lockup and potential loss of vehicle control. If the rear brake components are sized correctly (ie, wheel cylinder area and brake shoe area), a prop valve is not necessarily needed. This is what GM did on the 67-70 disc brake cars - the rear wheel cylinders for factory disc cars have a different bore than the ones for all-drum cars. The result was that no prop valve was required. As a trivia note, the early Toronados actually used a proportioning valve in the rear brakes even with four wheel drum systems due to the heavy front weight bias.

If you've installed that adjustable prop valve in place of the metering valve in the front brake circuit, that's a bad thing. A prop valve ALWAYS goes in the rear brake circuit. Get a stock metering valve back in the front brakes. This could be part of your problem.

 

No I did not listen to them I am capable of going down the wrong road without any help!

[/QUOTE]
If you've installed that adjustable prop valve in place of the metering valve in the front brake circuit, that's a bad thing. A prop valve ALWAYS goes in the rear brake circuit. Get a stock metering valve back in the front brakes. This could be part of your problem. [/QUOTE]
Yes I think I actually did that I replaced this


With This on the front brake line.

 

Yeah, you didn't want to do that.

 

Thanks for the info, will get things back to normal next time I want to fool with brake fluid and metal lines!

 

This is one of the better writeups on the subject.

https://www.getdiscbrakes.com/knowle...configurations

 

Thanks, good reading and I didn't even get a headache!

 

Yeah I have 1 66 Starfire that one day I will upgrade to a disk brake system pulled from a 67 Dynamic police car. I have all the parts except for the distribution block and proportioning valve (I have the original metering valve. Trying to decide if I should have the metering valve rebuilt, and buy an adjustable proportioning valve (as in your picture) and leave out the distribution block (not sure there was one in 1967) or if I should just buy a more modern combination valve/distribution block.

 

And before someone asks, I will point out that the 1967 full size cars with disc brakes DID use a prop valve in addition to the metering valve. The A-body cars did not. This is what the full size prop valve looked like. It was bolted to the frame near the rear axle.

 

And to thoroughly document this to death here is a picture from the 67 shop manual of the location of what they called at the time the balancing valve.

All those parts are probably completely lost to the dustbin of history. Including that distribution valve.

 

And they weren't wrong. The purpose of a prop valve is to balance the front-to-rear brake force to avoid premature wheel lockup. If I might be allowed a small digression, most cars with ABS don't even have prop valves anymore. The service manual for my truck calls it a "virtual proportioning function" or something like that. Basically the ABS computer pulses the brakes as needed to mimic the brake balancing function of a mechanical prop valve.

 

Great info.

Slack in the linkage and shoes. This would include the pressure from the return springs as well?

Isn't all that together called residual pressure?

 

Nope. Residual pressure valve is different and is built into the outlet port of the master cylinder. The 2 psi residual pressure valve used with drum brakes isn't designed to hold against the springs and shoes.

 

So a residual pressure valve maintains 2 psi to overcome spring pressure, but a metering valve delays the application of any psi to overcome slack in the springs and shoes? In a metering vale application without a residual pressure valve, what then overcomes the spring pressure?

I'd have never thought slack in the springs and shoes would produce the requirement for a metering valve. Though, if in the same closed hydraulic system you had disc brakes at one end which are not subject to such a variable, then it seems logical.

 

Slight side topic: In taking a 67 Cutlass from 4 wheel drum to the following scenarios, is what I put right?

1. 4 wheel drum 67 Cutlass to (68-72 front disc setup) + stock rear drums.......metering valve and proportioning valve.

2. 4 wheel drum cutlass 67 to 67-68 front disc and proper "disc compatible bore" cylinders in rear drums.......metering valve.


Also, what's a 4 wheel disc Corvette have? Just a distribution block?

 

Correct. I have no idea how a Corvette is plumbed, but typically you would still want some sort of prop valve to balance the front-to-back braking force. You don't need a metering valve with four wheel disc. Note that the aftermarket combo valves are all GM-style that have both the metering valve and the prop valve function.

 

Question on #2 . What is a proper “disc compatible bore” wheel cylinder for this type of application?
As near as I can tell, GM a-body rear wheel cylinders fit disc/drum through 75 g body ?
I have 67 man. 4 drum converted to factory front disc, with metering valve and C3 corvette master cylinder. Soft pedal on first hit but seems to be better/solid with all subsequent steps.
I was thinking I needed residual pressure valve on rear circuit as the vette was 4 wheel disc ? I may convert to 4 wheel later.

 

Better brush up on your research skills. The 1967-70 factory disc brake cars do not use a proportioning valve. To get the proper front-to-rear brake bias, Olds used SMALLER rear wheel cylinders on the disc brake cars. Four wheel drum cars used rear wheel cylinders with a 7/8" bore. Disc brake cars used rear wheel cylinders with a 13/16" bore. This information is in Chapter 5 of your Chassis Service Manual. Don't be surprised if parts catalogs get this wrong in their listings.

 

Hi Joe, you seem very knowledgeable on this topic. I have read the thread a few times and have some questions maybe you can assist me with. I converted my front brakes to disc on my '67 Cutlass. I bought a disc brake master cylinder and the proportioning valve pictured. Do I need a metering valve also and do I need to change out the rear wheel cylinders?

Thanks in advance!

 

The brass combo valve pictured includes the metering valve, differential pressure switch, and proportioning valve all in one housing, so that's all you need. The "hold off valve" in this diagram is the metering valve and performs the same function as the external valve used in the 1967-70 cars from the factory. Note that your aftermarket combo valve is calibrated for the original rear wheel cylinders used on the four wheel drum cars, not the smaller cylinders used on the factory disc brake cars.

 

Ok, thanks, that's kind of what I thought that it was an "all in one" valve.

So I am good to go keeping the original wheel cylinders then?

​​​​​​​Like ZERO instructions came with the conversion kit and it's my first conversion so I am still learning things.

Much appreciated!

 

Yep.