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Old 01-10-2011, 08:46 PM   #1
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Question BRAKE ISSUE, please help!

So I brought the Jeep in to a mechanic today to check the brakes, needed new rotors and pads in the front. The problem, however, is that whoever owned it before me put in an aftermarket proportionality valve for rock crawling? I put a question mark there because they can't get the new rotors and pads on I guess because something with this locks them in place, I think that's how it worked, and I know nothing about this re-routing of brake lines and alternate setup as of now. Anyway, the mechanic's a buddy of mine and he wrote up a description of the problem for me so I could ask you guys on here (we both figured you guys and girls would know a little bit more on the subject than we would). So here's what he wrote:

"Front calipers are holding pressure. The vehicle drives and brakes fine (seems to) so the calipers themselves are o.k.. Aftermarket proportionality valve was installed to control only the rear brakes after the factory combination valve. Upon replacing pads and rotors, calipers held pressure. I'm wondering why??"

Any help would be greatly appreciated guys, thanks in advance!


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Old 01-10-2011, 08:51 PM   #2
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hmmm... proportion valve plumbed in AFTER the factory valve. Cant imagine how that would help. Crack a line at the MC...retract the calipers and go. See if the brakes lock in the front when you go back together.

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Old 01-10-2011, 09:32 PM   #3
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they did say something about doing a test with a crack in the line. They suggested just re-routing the lines back to the factory proportion valve and getting rid of the aftermarket one altogether. Either that or replacing the aftermarket one, but they don't know if its faulty, let alone how it works in this system. I wish I had the original work orders of the stuff done to the Jeep--I bought it from a dealership and the previous owner put a lot of aftermarket stuff into it. Thanks for the reply though, I'll tell them what you said tomorrow when I bring it back.
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Old 01-10-2011, 09:44 PM   #4
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For the time spent troubleshooting, it might be worth just re-routing a couple new lines instead. Then, pull the parts off and sell 'em on eBay
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Old 01-12-2011, 09:34 AM   #5
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Thanks for the input guys. They did the test with the crack in the line and it ended up being the brake booster...which I'll admit I've never heard of. As for the aftermarket proportion valve, I guess that stays for now, but I don't know what good it's doing by the sound of it...
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Old 01-12-2011, 12:55 PM   #6
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Actually what's called a "proportioning valve" isn't what it implies.
It's more aptly called a "hold-off valve." Some carmakers call it by the right name.

The purpose - when you first apply the brakes the valve prevents the first 2 or 3 pounds of pressure from reaching the disc pistons, but the rear cylinders get the pressure.

Before any pressure is applied, the disc pads are already in position, but the rear shoes are pulled back away from the drums. That first 2 or 3 pounds of pressure moves the shoes into place so both front and rear will start braking at the same time. Without it, the front applies first, causing nose dive and unsafe braking.

Using 2 valves in series could create just the opposite effect, the rears would brake before the fronts, sending you spinning in slippery conditions.

The so called prop valves are merely a spring loaded pop-off valve - on some that spring pressure is adjustable.

You cannot reduce static hydraulic pressure with a valve or orifice.
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Old 01-13-2011, 08:09 PM   #7
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Actually what's called a "proportioning valve" isn't what it implies.
It's more aptly called a "hold-off valve." Some carmakers call it by the right name.

The purpose - when you first apply the brakes the valve prevents the first 2 or 3 pounds of pressure from reaching the disc pistons, but the rear cylinders get the pressure.

Before any pressure is applied, the disc pads are already in position, but the rear shoes are pulled back away from the drums. That first 2 or 3 pounds of pressure moves the shoes into place so both front and rear will start braking at the same time. Without it, the front applies first, causing nose dive and unsafe braking.

Using 2 valves in series could create just the opposite effect, the rears would brake before the fronts, sending you spinning in slippery conditions.

The so called prop valves are merely a spring loaded pop-off valve - on some that spring pressure is adjustable.

You cannot reduce static hydraulic pressure with a valve or orifice.
RRich,

The mechanics left the aftermarket hold-off valve on. I asked if this would cause any harm by leaving it connected, and they assured me it would not. Any thoughts?
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Old 01-13-2011, 08:17 PM   #8
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Originally Posted by Bluemonster View Post
RRich,

The mechanics left the aftermarket hold-off valve on. I asked if this would cause any harm by leaving it connected, and they assured me it would not. Any thoughts?
what is this hold-off valve anyway? is that you can press a button and holds the brakes or what?
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Old 01-13-2011, 08:41 PM   #9
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Blue - have another look at my post #6.
If the if both valves hold off the pressure for 5 lbs each, then in series like that it would take 10 lbs of pressure before the front discs worked.
I don't know if that would be noticeable or not. It may tend to cause the rear to work first before the fronts in a weird squirrely fashion on slippery surfaces, like ice. You may find even a slight touch to the brake pedal causes you to spin. Dunno, try it!

If it's still there and you've finished paying the mechanic, see how you like the way it stops.

Try this - run about 30 MPH on a flat gravel or dusty/slippery surface. Turn like you are going around a bend - not mild, not sharp. Apply the brakes while still turning. If the rear locks up too soon I'd remove it. If it stops to your satisfaction let it be.



"""""what is this hold-off valve anyway? """""
Jason - reread post #6 too - it explains it.

"""""is that you can press a button and holds the brakes or what? """""
They do make such a thing, but it's not called a hold-off or proportioning valve. I think it's called a Hydro-Loc. You press the brake pedal, then activate the Hydro-Loc. It keeps the pressure in the brake lines until you depress the pedal again.

It's like a parking brake except it locks all 4 wheels instead of just 2. Handy on steep loose hills. Not legal to use as a parking brake for the street.
I have one on one of my rigs, I've never used it.

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