Brake Proportioning
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Fleet Mechanic
Joined: Nov 2000
Posts: 1,713
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From: In my House
Brake Proportioning
....So I'm wondering how the proportioing works when you have 4wabs. I know there is a little fitting on the side of the master cylinder for the rear lines, but it seems like my rear brakes are barely working. I did all new lines last year (rear disc brakes) and I have bleed the snot out of the system after putting in a used HCU to try and cure my fading pedal issue, but it still seems like the rear brakes are not working as "much" as they are supposed to. After driving the truck, you can get out and feel the rear hub area and it is hot, But I wouldnt; dare pull a trailer with the truck...thats how terrible it is.....I brought it to dealership about a year ago and they said its fine....Which it is not...any advice?
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Joined: Apr 2004
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If the rear discs are hot after a stop, something is happening. Are the rear pads wearing? Discs nice and shiney?
What happens if you stand on the brakes?
What happens if you are going downhill empty and hit bumps or sand etc with the brakes on?
Your ABS should kick in for at least the rear wheels, which would signal again something is going on there.
This is a bit longish, but give some insight on these valves etc. I think your abs system would react only to the wheels locking and not have anything to do with brake pressure otherwise.
Proportioning Valves
What happens if you stand on the brakes?
What happens if you are going downhill empty and hit bumps or sand etc with the brakes on?
Your ABS should kick in for at least the rear wheels, which would signal again something is going on there.
This is a bit longish, but give some insight on these valves etc. I think your abs system would react only to the wheels locking and not have anything to do with brake pressure otherwise.
Proportioning Valves
Logistics Pro
Joined: Jan 2001
Posts: 3,740
Likes: 37
From: Central NYS
When you installed the used HCU did you power up the ABS pump and then cycle the ABS solenoid for the rear line to force any air out of the ABS chamber and into the rear brake line? And THEN re-bleed the rear lines?
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
A way to see what the system is doing is do a safe slid on a loose dirt or gravel surface then look at the front and rear slide patches made.
Depending on the ABS and proportioning that works, on the result you see.
Remember the basics or what is being accomplished.
Upon braking, weight is lifted off the rear and transferred to the front.
This calls for reduced fluid pressure on the rear brakes to prevent rear lockup and loss of control.
The intent is to end up with near equal friction braking on both front and rear.
The ABS is normally so fast acting that it finds that balance near instantly.
Providing there is no fault.
Also, the operation was designed using a set of original conditions for disc pad friction, tire traction, diameter and vehicle weight.
Change any one of these or mix pads type and tire types too far and ABS gets challenged trying to find an instant bias.
good luck.
Depending on the ABS and proportioning that works, on the result you see.
Remember the basics or what is being accomplished.
Upon braking, weight is lifted off the rear and transferred to the front.
This calls for reduced fluid pressure on the rear brakes to prevent rear lockup and loss of control.
The intent is to end up with near equal friction braking on both front and rear.
The ABS is normally so fast acting that it finds that balance near instantly.
Providing there is no fault.
Also, the operation was designed using a set of original conditions for disc pad friction, tire traction, diameter and vehicle weight.
Change any one of these or mix pads type and tire types too far and ABS gets challenged trying to find an instant bias.
good luck.
Logistics Pro
Joined: Jan 2001
Posts: 3,740
Likes: 37
From: Central NYS
1. Use a scan tool with bi-directional control to command the ABS pump on AND the solenoid(s) on.
2. Do a safe slide test on a slippery surface (see bluegrasses' post above) to force a wheel to slide. The ABS module will then activate the solenoid for that wheel.
Thing to remember is that if there was air in the HCU chamber(s) it will now be out in the regular brake line area and can now be bleed with a regular brake bleeding.
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