Is the difference between the 72 power Drum proportion valve and power disc proportion valve that great? I have a complete set of front 71 Skylark Disc Brakes that I think look ready to go to bolt on. I currently have 4 wheel power drums. I bought the front disc MC from Autozone with the larger reservoir for the front- $35. Do you think I will be alright with the current proportion valve?
Jason, front-to-rear bias is different between drum/drum and disc/drum cars. In some cases the master cylinder can take care of the correct volume requirements, but the valve still has a role as to what percentage the front vs rear will do the work. You should always find a donor proportioning valve from the same setup you're going to run, or purchase an aftermarket adjustable valve that will require some tuning to make the system happy. Devon
You can always give it a try to see how the car behaves. You'll need an open stretch of road that gives you enough time to get speed & lock up the brakes. Ideally you want just a bit of front bias, meaning the front tires lock up before the rear. Having rear bias is pretty dangerous...if you feel the rear brakes locking before the fronts, you'll have to change the setup. Devon
Jason- On a stock drum-drum setup, there is no proportioning valve. What you see there is mainly just a pressure operated switch to turn on a brake malfunction indicator if there is pressure in half of the brake system but not in the other (indicating a major leak). The drum-drum system uses a constant proportion of pressure from front to back by using different diameter wheel cylinders (larger in front gives higher front braking force relative to back wheels). If you do not use a proportioning valve on disc-drum, your rear wheels are likely to lock up when you brake very severely and the ass end of your car will slide out to the side, and then in front of you. This happens because the weight of the car shifts forward on to the front wheels when you brake hard, and off of the rear wheels. Less weight on the rear tires allows them to skid easier. To prevent this, the prop. valve sends less pressure back there in a hard braking situation. It does not reduce pressure in front, just back. If you do not find a stock prop. valve you can try going without it, as someone suggested. See if the rear wheels lock up too easily on hard braking. If they do, use an adjustable prop. valve and set the degree of proportioning by trial and error. Brake hard a few times and keep reducing the pressure to the back until rear wheels do not skid. (I have some experience designing brake systems.) Jim
Thanks for the help all, a couple questions: 1. Would a stock disc proportion valve fit right into the Drum set up? 2. Would you recommend any of the ones in Summit as a direct fit? I have read some of the prior posts on this topic, but left a little confused, os all this is helping a lot.
As far as direct replacement goes, I can't say. Plan on having to do some plumbing, including double-flaring the tubing where necessary. Devon
I would not expect either to be a direct fit without you fabricating the tubing you need. The Summit valve SUM-G3910 looks like it should work for your application but I have no personal experience with it. You were right to get a disc-drum master cylinder when you convert to front discs. If you use a drum-drum master cylinder, the residual pressure valve on the front line will keep pressure on the front disc pads after you release the pedal. Not good. Most people don't know that there is a little valve in there to keep some pressure in the lines on drum brakes. Not used with discs. When you use the correct disc-drum master cylinder and the stock brake lines down from it, the stock proportioning valve should connect up. Like Devon says, you should plan on making your own brake lines. Auto parts stores here do "loan-a-tool" of the double flare tool, sell tubing and tube nuts. Cut tubing with hack saw, not with rotary tubing cutter. If you are too uncertain here, get a shop to do it for you. Having your brakes not work properly for you at a critical time would be a bad thing. Jim
Well, I have done plenty of brakes and brake lines a couple of times, but never had to double flare. I guess there always has to be a first time. I have enough mechanic friends though I should be able to make it work. I think I will see how it goes without the proportion valve and then work from there. Jason
I just talked to the guys at inline. This guy told me I could leave the Distribution block on, and in stead add a second valve on the rear brake line after the distribution valve. He supplied the correct adapters and valve. So I think I will just splice that in with double flares, and work on the adjustment from there. Thanks for all the help. Pretty cool the level of help we have here on the board.
BTW, The part number of the valve I bought from Inline Tube was ADJP-01 for future reference. It came to $60 with shipping.
Will do. Since I am moving in a few weeks across country, I think I will hold on the project till I arrive since I need to drive the car and don't want anything to hold the trip up.
Definitely can't use a disc/drum MC on a drum/drum car. I wouldn't suggest a disc/disc MC either. Since discs require more fluid volume, you'd probably run into one helluva touchy pedal. Devon
I just receive complete front disc brake kit for my GS California '68. Is proportion valves are for manual adjust ?
I'm not sure I understand your question completely...is your new front disc brake kit all original from 1968? If so, does it already include the original 1968 proportioning valve also? If not, you'll need a stock replacement or use an adjustable one. Devon
