The Stovebolt Site Forums The Shop Area Driveline proportioning valve location

hello , i have a 1939 chev truck 1.50 ton , i have updated the rear end from a 1990 Apollo motor home with disc brakes . the front brakes are still the original drum brakes , i have replaced the single master cylinder with a dual output master cylinder . many people state that a proportioning valve needs to be installed in the brake lines . with the drums up front and disc in the rear where is the correct placement for the proportioning valve on my system ? all articles i have read are for disc brakes up front and drum brakes in the rear and mine is the opposite . thank you 1pete1

Hy 1pete1, it doesn't actually mater where in the braking system plumbing you install the proportioning valve, what matters in your case is how you augment the proportioning valve. In a braking system with front disc/rear drum most of the braking effort is in the front brake system, disc brakes require more pressure, and if that increased pressure was allowed to be sent to the rear drum brakes those brakes would lock up causing a dangerous skid situation. To be honest with you I am not sure how your brakes will perform, I think that maybe an adjustable proportioning valve installed in the front brake circuit and adjusted to give the best braking effort without constantly locking up the front wheels may be the best way to set up your unique system. I am sure others will be along with suggestions as well, good luck with getting it to work appropriately.

You'll need to put the proportioning valve so that it reduces the effect of the rear disks. If you don't do that, the rears will tend to lock up. You'll need to read up on the install instructions for the proportioning valve as to which way it goes in.

On edit: Just read 3B's reply. I'll ask my co-worker who is doing the same thing on his 63 'Vette. I may have it backward.

I agree with the second post, you will need to drive the think to see how it acts. I would bet the fronts will lock before the rears if a standard master cylinder was plumbed like all other trucks. It takes a lot of force to stop those big disks, and with equal force going to the fronts, I can see the drums locking up.
You should consult one of the aftermarket brake suppliers for their input, but I bet I know what they will say, add disk to the front! The adjustable proportioning valves simply limit the pressure coming out of them, so reducing the front pressure maybe the only way to keep things under control. Plumbing the master cylinder backwards should let the rears activate first, and if the truck is heavy enough, it could be all that is needed.

I haven't had a chance to talk with my co-worker yet, but I remember him saying he talked to Wilwood about it and they explained how to do it. They understood it was unusual but he said they were very helpful.

Wilwood Engineering, Inc
4700 Calle Bolero
Camarillo, CA 93012
Phone: (805) 388-1188
Fax: (805) 388-4938

OK, I talked with Ernie my co-worker and he confirmed what I had thought. Wilwood told him that even though disks take more pressure to operate than drums, they are more effective. They told him that it was unusual (disk rear and drum front), but that others had done it and they sold him a dual circuit master cylinder and proportioning valve where both the front and rear lines from the master go into the valve, but that the front valve circuit was just a pass-thru (basically a junction block) and that the adjustment occurred on the rear brake circuit to keep the rears from locking up. They recommended it be adjusted with a pressure gauge, then a cautious test drive and further adjustments to get the brakes balanced.Oh his setup, once he upgrades the fronts to disks, they told him he could take the valve out of the rear circuit, but should at a minimum readjust it to restore proper braking balance.

Hope that helps. Again he said that Wilwood was very helpful when he talked with them and he had not yet bought anything from them at the time, so you might give them a call to confirm. With some particulars on your vehicle, they could probably give you some good recommendations.

Good Luck