proportioning valve ?
Thread Starter
Instructor
Joined: May 2003
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From: Hilton Va
proportioning valve ?
I have a 69 Vette. I've had brake problems for a long time. I've changed everything but the fluid splitter/proportioning valve or whatever you want to call it. I want to know what can happen to this valve and what makes it go bad. What do they do when they rebuild it and why is it so doggone expensive? I've had it apart, in the past, to check it, but I don't recall there being anything that would be much problem. Is there any other splitter on any other vehicle that would work on this thing? I don't want to do away with the valve as some have done. Any comments?
Melting Slicks
Joined: Feb 2004
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From: largo florida
The only thing the valve does in a 69 is act as a distribution block and has a warning light. It does not proportion and does not block one bank off if something does happen to the brakes.
Instructor
Joined: Jun 2004
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Originally Posted by carhound
I have a 69 Vette. I've had brake problems for a long time. I've changed everything but the fluid splitter/proportioning valve or whatever you want to call it. I want to know what can happen to this valve and what makes it go bad. What do they do when they rebuild it and why is it so doggone expensive? I've had it apart, in the past, to check it, but I don't recall there being anything that would be much problem. Is there any other splitter on any other vehicle that would work on this thing? I don't want to do away with the valve as some have done. Any comments?
Race Director
Joined: Jul 2004
Posts: 12,684
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From: Scottsdale Arizona
Originally Posted by 69monzaredbigblock
What do you mean by brake problems? I have a 69 and use the stock valve,I believe it's called a differential pressure valve. It will redirect fluid if it senses an imbalance between the front and rear brake pressure. It can be tricky to bleed brakes with this valve in the system. I have no problem stopping my big block with stock brakes (no power brakes) now that I have thouroughly bled and recentered the valve.
i don't think that this is totally correct. I believe that the valve only sets off the "Brake" warning light. I don't think that it actually redistributeds the brake fluid.
Melting Slicks
Joined: Feb 2004
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From: largo florida
Not sure why so expensive for a rebuilt one, probably becuase of the tremendously low demand for them. I dont think they go bad very often, and it isnt like many corvettes are missing the part. And if they are, it really wouldnt affect it as long as the lines are attached right.
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Melting Slicks
Joined: Nov 2001
Posts: 2,668
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From: Eastern US XX
Originally Posted by lostpatrolman
Nope, the early distribution blocks only set off the warning light. They do not redirect fluid, or act as proportioning valves.
When EITHER the FRONT or the REAR system looses pressure [due to a leak], the piston in the block moves TOWARD the side that lost pressure. When that happens, TWO things happen - 1. The piston closes a switch that turns on the BRAKE lamp in the dash and 2. The piston BLOCKS the path that lost pressure so you don't pump brake fluid out the leak.
Le Mans Master
Joined: Apr 1999
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From: Raleigh North Carolina
Originally Posted by pws69
What the distribution block ACTUALLY does is the following:
When EITHER the FRONT or the REAR system looses pressure [due to a leak], the piston in the block moves TOWARD the side that lost pressure. When that happens, TWO things happen - 1. The piston closes a switch that turns on the BRAKE lamp in the dash and 2. The piston BLOCKS the path that lost pressure so you don't pump brake fluid out the leak.
When EITHER the FRONT or the REAR system looses pressure [due to a leak], the piston in the block moves TOWARD the side that lost pressure. When that happens, TWO things happen - 1. The piston closes a switch that turns on the BRAKE lamp in the dash and 2. The piston BLOCKS the path that lost pressure so you don't pump brake fluid out the leak.
That's correct.... I did a series of tests on several of these and also disassembled & documented their internal construction. I have digital images. Then, I ran the left front brake line directly back to the master cylinder (with a specially constructed line & M/C cover) and test drove the car. The valve did indeed "slide to one side" and "block the fluid".
The rear brakes stopped the car, the light turned on. But..... it did not do it so well that I would trust my life to it. The car stopped, but I almost hit my garage door.
The "blocking off" action is also what stops you from bleeding the brakes... but... this only happens when the valve is actually working. Mosty often, it is all gummed up on the inside.
The twist: If it's IS working properly, you cannot easily bleed your brakes unless you open both a front & rear bleeder at exactly the same time. Ergo.... most of them have failed long, long ago.
The design changes from year to year, but some of the ones that I dissassembled had o-rings inside that had deteriated. Some of them have no o-rings.
I did not disassemble a 69 version. But I did do a 70.
FYI
Last edited by Tom454; Oct 15, 2004 at 05:03 PM.
Thread Starter
Instructor
Joined: May 2003
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From: Hilton Va
Soooo, would I be better off without one, like has been suggested or not??? I took mine off again today. Took it apart and tried to slide the center section by pushing a small drill bit (blunt end) through the ends and could not get it to move. It looks like it is centered. I blew through both inlets with my mouth and nothing seems to be stopped up. I have had trouble getting a good hard pedal. What would YOU suggest?? A rebuilt one cost about 125.00 with a trade in. If I put a rebuilt one in that works, will I be able to get a good bleed????? 
Melting Slicks
Joined: Feb 2004
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From: largo florida
Originally Posted by pws69
What the distribution block ACTUALLY does is the following:
When EITHER the FRONT or the REAR system looses pressure [due to a leak], the piston in the block moves TOWARD the side that lost pressure. When that happens, TWO things happen - 1. The piston closes a switch that turns on the BRAKE lamp in the dash and 2. The piston BLOCKS the path that lost pressure so you don't pump brake fluid out the leak.
When EITHER the FRONT or the REAR system looses pressure [due to a leak], the piston in the block moves TOWARD the side that lost pressure. When that happens, TWO things happen - 1. The piston closes a switch that turns on the BRAKE lamp in the dash and 2. The piston BLOCKS the path that lost pressure so you don't pump brake fluid out the leak.
Last edited by lostpatrolman; Oct 16, 2004 at 09:55 PM.
Melting Slicks
Joined: Nov 2001
Posts: 2,668
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From: Eastern US XX
Originally Posted by lostpatrolman
NO, the early distribution blocks are distribution blocks. They do not redirect fluids. They didnt start doing that until somewhere in the mid 70s. This issue has been discussed, diagnosed, taken apart, and tested already. The 69 AIM even specifies it as a "switch assembly" Several people have bypassed the block, including norval. Suppsedly gave them all a firmer pedal. No reason not to bypass it.
Perhaps they don't "redirect" per se, but they do BLOCK the path that has lost pressure.
