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4 wheel disc and proportioning valve

dirtdrdave

Jr. Member
Joined
Mar 28, 2008
Messages
61
Hi All,
I previously installed a Tom's power front disc brake kit including proportioning valve for disc/drum. I recently added BC Bronco's rear disc brake kit. I am tempted to leave the current proportioning valve in place, but I think that may decrease the effectiveness of the rear discs. Is this correct? Should I be using an adjustable proportioning valve? Thanks, Dave
 

G's Baja Bronco

Bronco Guru
Joined
Aug 7, 2010
Messages
1,362
Loc.
Sunny SO CAL
I used the stock 99-04 mustang prop valve since I got the HB from one and it came with it. so far it seems to send power appropiately. I own an 01 stang and it also has 4 wheel disc (gt, with cobra frnt calipers).

have you tried it with the existing Pvalve?
 

spap

Contributor
Bronco Guru
Joined
Jan 2, 2010
Messages
2,457
When I had my 76 with front disk and added rear disk from a Lincoln I left the stock disk/from prop valve in. You might loose a little power to the rear, but you could slam on the brakes and it made it hard to lock up the rear tires when you braking, I thought that was a pretty good think. Try it first in panic stops and see how you like it, you might be surprised.
 

svobronco

Contributor
Full Member
Joined
May 31, 2011
Messages
309
I would highly recommend using an adjustable prop valve,I've been using one for years, they're cheap and they work really well. I just looked at mine and it's labeled "Direct Connection"....how old is that??? Made by Kelsey-Hayes IIRC
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,833
Brake bias requirements are highly variable. There are way more factors then "I have disks" or "use part x from a vehicle with 4 wheel disk brakes". Such factors (but not all) are; bore size (mostly calipers), rotor size, pad material, weight bias (hard top or not, what engine, spare, bed packed with spare parts or not, how heavy are the bumpers), center of gravity (taller truck will swing more weight to the front axle under braking), that is also dependent on wheelbase (long wheelbase won't shift as much as a short wheelbase), and even suspension tuning. All this stuff goes into how the tire is loaded against the road, thus how much traction it has, and that dictates how much braking it can have, which is controlled by the size of the brakes, size of the calipers, pad material and ultimately the pressure in the brake system that is applying the whole mess.

Really the only way to know if it works is to go out and drive it and see what happens. If it is well balanced, you are done. If not then there are a lot of ways to get the system more into balance. You can play with pad material, center of gravity, add a heavy bumper (or take one off the front), and ultimately play with a proportioning valve.

A proportioning valve is not a good way to take a lot of brake action out.
Scenero;
lets say a good brake system has 1500 PSI at all 4 wheels for good braking. The driver can modulate that pressure as needed.
If the rear brakes are crippled to 900 PSI and the fronts still get 1500 for it to stop well all appears good, right? Wrong. What happens when the driver presses a little too hard and locks them up. Lets say 1600+ front and 1000 rear. OK the driver has to back off the brakes to unlock the wheels. Once a wheel locks you have to go below the threshold pressure for it to release (you can search out static and dynamic friction from your physics book for more details). So the fronts need to back off to say 1200 PSI and the rears to 700 for the wheels to release. The problem with a prop valve is to lower the regulated pressure you have to go below the inlet pressure. Brake fluid is in a closed system. you can't vent the rear pressure back to the master cylinder until the supply is lower then the output. So the driver will have to lift off the brakes to no more then 600 PSI to get the rear brakes to unlock, which is less then half the pressure for good front braking. Too much rear brake with an aggressive proportioning valve is a bad fix. I only like to see a proportionng valve used for very fine tuning.
 
OP
OP
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dirtdrdave

Jr. Member
Joined
Mar 28, 2008
Messages
61
OK, so i drove it with the disc/drum proportioning valve. The rear discs do lock up with hard braking. Does this mean the proportioning valve is still giving too much pressure to the rear? I assume this means I should get an adjustable valve? Last stupid question, I rebled the fronts as well and now it pulls to the left. It didn't do this before, so I assume the calipers and pads are not the issue. I guess I caused this with the bleeding? Thanks, Dave
 

xcntrk

Bronco Guru
Joined
Jan 12, 2012
Messages
2,473
Loc.
NOVA
I believe it's the same prop valve you get with either the front disc kit from TBP or the front/rear full disc kit.

I too had this full front setup when I added my rear disc kit. The rear bias was a little strong at first (rear could lock up under heavy braking). But I've since added some weight to the rig (cage and other fabrication) and it's gotten better. I'm going to wait until I get my full bumpers on (heavy) and reassess if it still needs an adjustable prop.

Maybe you're too light?
 
OP
OP
D

dirtdrdave

Jr. Member
Joined
Mar 28, 2008
Messages
61
Xcntrk,
Maybe you're right. I was going to upgrade the cage to a full family cage and the rear bumper is off the car, as well as the spare. Maybe that would be the difference.
Thanks, Dave
 
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