# Brake proportioning valve need to be changed for drum to disc?



## Keltz (Feb 24, 2010)

Title says it. Does the proportioning valve need to be changed, or are they the same? Finishing up my brake swap so I can put my rear discs on this weekend. 
Can I use the brake proportioning valve for the rear drums since you can adjust it? Or do I need to purchase one that is specifically for rear discs?


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## OddJobb (Nov 6, 2004)

Some say you are supposed to replace it. I converted to rear disc and kept the origial one for drums. No problems at all.


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## germancarnut51 (Mar 23, 2008)

Personally, I've always replaced the rear proportioning valve when doing a rear disc brake conversion.

Someone else actually took the time to do some research about what the difference was between the two proportioning valves, and I thought he found out that the proportioning valve designed for use with rear disc brakes actually let more pressure be applied to the rear disc brakes than the rear drum brakes would allow.

If that's true, then when you switch from rear drum brakes to rear disc brakes and don't change the proportioning valve, the rear brakes will work, just won't work as well as they were designed to, leaving you with less rear braking than you could have had. The proportioning valve is designed to keep the rear brakes from locking up before the front brakes, and that, it will do.

You just won't get all the braking out of the rear brakes that it they were designed to generate. Kinda defeating one of the reasons that many people convert to rear disc brakes.

You can get a used proportioning valve from a salvage yards for a reasonable amount of money (I had two spares that I sold for less than $50 each), or you can buy a new one for $100-$150.


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## OddJobb (Nov 6, 2004)

germancarnut51 said:


> Someone else actually took the time to do some research about what the difference was between the two proportioning valves, and I thought he found out that the proportioning valve designed for use with rear disc brakes actually let more pressure be applied to the rear disc brakes than the rear drum brakes would allow.


That would be me. 

I can't seem to find that thread however. One of the proportioining valves held more residual pressure to the rears I think. I think it was the one for drums.

But now that I think about it, I got more rear brake bias before I adjusted it. So although it may have held more residual pressure I think you need to 'close' the valve a bit more to keep the brake bias the same.

->Found the thread:

http://forums.vwvortex.com/showthread.php?5088237-rear-disc-conversion-help!!!&highlight=valve

Also, StopTech has some good information on this topic as well. Especially all the factors that will affect brake bias.

http://stoptech.com/technical-support/technical-white-papers/proportioning-valves


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## MusicCityGTI (Feb 19, 2004)

The brake clamping curve is different with the drum valves vs disk valves!!!

At the threshold, the drum valves will lock up the rear disks and bad things will happen unless you change the valves. The last thing you want is to lock up the rear brakes before the fronts. There have been several posts over the years about people tracking their cars and this happening.


I'm not engineer but in summary, the stock prop valve for the drums actually "feel" ok when just tooling around town and with a few quick jabs on the brakes at low speeds. Even if the rear lock up first at 10-15 mph you aren't likely to spin and all will seem ok. When you hit the track or experience an emergent situation at interstate speeds and must stop as fast as possible.... rears can lock and the car will want to spin. It isn't pretty. 

I have rear disks on my Rabbit and reran all new hard-lines with universal prop valves from Summit racing. It took some dialing in but the brakes an my little Rabbit feel great now and more importantly, actually stop the car much faster than before.


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## OddJobb (Nov 6, 2004)

I brake hard on the street and I have had my car on the track (Road Atlanta SCCA HPDE) since the conversion. As long as you adjust the valve you'll be OK. I dialed in about a 50/50 bias using the stock drum proportioning valve.


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