doing drum to disc swap. Can I use the drum proportioning valve? or do I need to use the one I got from donor car?
If you're going from four wheel drum to four wheel disc, you shouldn't need anything. If front wheel disc rear drum, you will need the matching prop valve or an aftermarket adjustable one. Devon
Ive been thinking about this up grade also. What will happen if you use the stock drum valve with front disc?
If you have 4 wheel drum brakes on the car then there is no proportioning valve, just the distribution block. Like Devon stated, you will need to add either a factory drum/disc or aftermarket adjustable proportioning valve but you can retain the original distribution block.
With an arrangement like that, during a hard stop the rear axle is unloaded and may lock before the front which can become more than a handful. http://sports.racer.net/brake_bias.htm Devon
DaWildCat is right. You will end up with a hard pedal and will stop fine under normal conditions. A panic stop is realy that. The rear locks up and your are chasing it.:eek2: Not that I would know ou:. I now running http://www.jegs.com/i/Wilwood/950/260-8419/10002/-1?parentProductId=744516 You can tune the balance with a turn of a knob. Now they just need to come up with something to get rid of the panic stops!
Great. Thanks for clearification. Going from 4 drum to front disc rear drum. I will use the valve I got from the parts car
drum/drum porp valve will be about 50/50 distro..no the best for a disc/drum setup but works well for alot of people drum/drum valve will also work well according to people with a disc/ disc but dis/cdrum valve on disc/disc setup sucks...brake shudder from lack of pressure...brakes hanging up! lol oh yea fixed dis/cdrum is like 70/30 not sure what a disc/disc is....60/40?
I dont want to start a fight , but. I have been told by some that. The proportioning is done by the master cylinder and the other brake components all tied together.The master has bigger chambers front and rear, for disc/drum. the same for drum/drum and disc/disc. valving and piston size of actaul master cylinders are different for all these combos. the line sizes. i.e. 3/16 line in front, 1/4 to the rear. Sizes of caliper pistons, sizes of wheel cylinder pistons. All this determines the amount of presure, and where it goes. The Proportioning valve is more of a safety. If pressure is lost in any port of the valve, an internal piston slams ( slides) that port shut so pressure is sent to other lines to still allow car to stop. I'm not saying this is fact. but. makes sense to me. Heres a thought. why is there 2 front lines coming out of valve? no need to proportion left and right front separately. Another thought. Anyone ever have a wheel cylinder or brake line blow. repair the problem, then have trouble getting things to bleed. Or maybe never bleed till you replace the valve. Cause it slammed to the other ports and stuck. I have. Someone out there has the answer. Please share. I really want to know. I have had this discussion a million times with freinds. Thanks for reading Darrell
all the aftermarket kits I have seen seem to use the same master cylinder...wheather it be there own house made or the 60s corvette style..only difference in the kits are the porportioning valve.
I went from 4 wheel power drums to manual Wilwood front discs and rear drums. Kept the original drum distribution block in place. Zero problems at all and my car sees a fair of amount of track time too.
The factory proportioning valve for rear disc systems only comes into play after a specific line pressure is reached.....basically during panic stops. It will reduce the line pressure to the rear brakes and prevent them from locking up. Another component of the 71-up 'combination valve' is a 'hold-off' valve....this will cause the rear brakes to apply slightly before the front brake do....it's purpose is to keep the car straight when brakes are first applied. '70 and earlier GM disc systems didn't even use a proportioning valve! ...just a hold-off valve. Master cylinders for drum systems have a check valve built in. It'll keep a little pressure in the lines to overcome the brake shoe return springs. That check valve is omitted on disc portions. One of the best reference sources I've seen on GM brake systems is the Buick Chassis Service manuals. Several are online at www.teambuick.com. here's the '71 brake section: http://www.teambuick.com/reference/library/71_chassis/files/50-b.pdf Note that the hold-off valve should be manually depressed during brake bleeding. :TU:
