Drag car brakes

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9secRR

9secRR

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I’m in the process of putting rear disc brakes on my car. I had rear drum brakes with disc brakes up front.
I’m also redoing all the brake lines. My question is do I need a proportioning valve?

I have read some articles and some say PV is not needed if disc brakes are on all 4 corners. The PV is only needed if it’s a disc up front and rear drum.
 
Most race car brake systems have a adjustable proportioning valve in the rear brake line. Might depend on the master cylinder and where it is mounted. Might need residual valves also if the master cylinder is mounted low
 
Lots of variables to consider, and no single "applies to all" answer. What master cylinder are you using? If it has them, are the residual valves still in place? Piston bore front and rear, master cyl. bore?
If it were me, I would plumb in an adjustable proportioning valve regardless- for $35 you can fine tune your system and save yourself the hassle of having to add it later anyways, and cutting into your nice new lines.
Adjustable Proportioning Valve (doctordiff.com)
 
If you can calculate the braking force you need on each wheel you can use calipers that have the correct size pistons to make the braking equal all the way around.

Not an easy or cheap undertaking

Or you add an adjustable proportioning valve and do some fine tuning to get the fronts to lock up just before the rears.

What you dont want is the rears to lock up first if you can help it. That's how the front end becomes the back end in a hurry!
 
I would put an adjustable one in the line to the rear so you can really zero in on how you want it to stop.
 
Curious...why the swap from rear drum to disc on a drag car? Drums have less drag than disc's.

My avatar had 4-wheel discs but they were on it when I bought it as a roller.
 
I built a 9” and want to upgrade brakes and lines at the same time

AD7A8859-27B8-4D2E-939F-F08AE0087272.jpeg
 
I have stock 73 disc in the front and willwood 4 piston's in the rear. I started off with an adjustable PV plumbed into the rear line and kept giving it more rear brake to the point it was eventually all the way open. Under heavy breaking on a 1/4 pass it just felt out of control in the front. When I originally bought my master cylinder from dr.diff (15/16 bore) he said that if I was running skinny tires in front I should have it on the front line. Switched it to the front and played with it and he was 100% correct. Car stops great now and doesn't feel out of control.

Someone told me after that you really need to calculate the bore size of the pistons in the caliper. Just because I have 8 small ones in the rear the 2 big ones in the front will give more braking so you the piston size. I don't know if that's correct but it seems like it was the situation I was in.
 
I'm running Strange brakes on the front and Wilwood on the rears. I tried a proportining valve on the rear, then in the front, ended up removing it. The car has a 15/16" master cylinder on it and will lock the fronts if you really stand on them. If the msater cylinder ever goes bad it'll be replaced with a 7/8".
 
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Thanks for the replies guys. I will try without a PV and if I need one, I’ll install one.
 
Safe bet is to run 4-wheel discs, smaller diameter MC, and a PV to the rear wheels. All my fast cars have been set up that way....and have been great. MC diamater, makes a big difference. Smaller has seemed better for myself.
 
Update on brakes,
I made 3 hits yesterday and the brakes worked great.
Thanks for the replies
 
RustyRatRod said:
I would put an adjustable one in the line to the rear so you can really zero in on how you want it to stop.
Click to expand...
Yes, both lines adjustable valve
Im setting my Dart up for some road racing will have adjustable valves AND brake pressure gauges front and rear. It appears to me that each track will require different braking. So you can tune the barking for that particular track
 
Brooks James said:
Yes, both lines adjustable valve
Im setting my Dart up for some road racing will have adjustable valves AND brake pressure gauges front and rear. It appears to me that each track will require different braking. So you can tune the barking for that particular track
Click to expand...
Yes, I want my brakes to bark loudly. lol
 
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