Brake proportioning valve checK??
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
Do you mean checking for proper proportioning function? If so, the answer is no. In order to check it, you need pressure to both inlet ports, then a gage on the outlet port. If you want to see if the switch is closed (light on) just check continuity between the switch terminal and the valve body. It is a normally OPEN switch.
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
Originally Posted by BIGJIM13
So what goes bad in a proportioning valve - if you can spray brake cleaner in all the ports and it comes out other ports its clearly not clogged!
Thread Starter
Drifting
Joined: Oct 2003
Posts: 1,352
Likes: 18
From: Dallas Texas
Ok..one more question , I found another proportioning valve in the garage - I open them both up - on the one the little spring inside with the rubber seal stays pushed in - on the other it will not stay pushed in - which is right?
Thanks
Thanks
Pro
Joined: Nov 2000
Posts: 594
Likes: 0
From: Tucson AZ
What year is your C3? I believe that proportioning valves started in 1974 (someone?). Prior to that, they were just a differential pressure switch that turns on the brake light if pressures differ between the front and rear systems. Maybe someone else knows for sure 
Craig
Craig
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Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
Originally Posted by BIGJIM13
Ok..one more question , I found another proportioning valve in the garage - I open them both up - on the one the little spring inside with the rubber seal stays pushed in - on the other it will not stay pushed in - which is right?
Thanks
Thanks
Thread Starter
Drifting
Joined: Oct 2003
Posts: 1,352
Likes: 18
From: Dallas Texas
Thanks - will probably get a new one -
but just so I understand the function.... if the rubber seal moves freely, then it is always pushed up against the end plug, so the main hole is restricted and fluid flows through the two smaller holes! At what point does the rubber seal move to center? at full braking!
but just so I understand the function.... if the rubber seal moves freely, then it is always pushed up against the end plug, so the main hole is restricted and fluid flows through the two smaller holes! At what point does the rubber seal move to center? at full braking!
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
Maybe these pictures will help.
This is the proportioning spool.
This is the insides of the diff piston.
Basically, the rubber piece on the piston sits over the hole in the center of the endplug. During normal braking, the fluid goes through the 2 smaller holes on either side. When the pressure reaches a predetermined point (known as the "knee" point) the spool moves in the direction of the rubber and closes off flow to the brakes. In simplistic terms, the spool opens and closes "proportioning" the pressure to the rear brakes. On C3's, the rear pressure will be 43% less than the front pressure after the 500 psi "knee point" is reached. So if the front pressure is increased by 100 PSI, the rears are increased by only 43 PSI. I hope I didn't just confuse everbody.
Just a little more info. If the rubber disc in the side of the end plug leaks, water gets inside and corrodes the "spool". If the spool can't move, the valve will not proportion. It could also stick in closed position causing no rear brakes (this is rare).
This is the proportioning spool.
This is the insides of the diff piston.
Basically, the rubber piece on the piston sits over the hole in the center of the endplug. During normal braking, the fluid goes through the 2 smaller holes on either side. When the pressure reaches a predetermined point (known as the "knee" point) the spool moves in the direction of the rubber and closes off flow to the brakes. In simplistic terms, the spool opens and closes "proportioning" the pressure to the rear brakes. On C3's, the rear pressure will be 43% less than the front pressure after the 500 psi "knee point" is reached. So if the front pressure is increased by 100 PSI, the rears are increased by only 43 PSI. I hope I didn't just confuse everbody.
Just a little more info. If the rubber disc in the side of the end plug leaks, water gets inside and corrodes the "spool". If the spool can't move, the valve will not proportion. It could also stick in closed position causing no rear brakes (this is rare).
Last edited by Duke94; Jun 7, 2006 at 05:52 PM.
Instructor
Joined: Apr 2006
Posts: 147
Likes: 0
From: New Jersey
Originally Posted by Duke94
Just a little more info. If the rubber disc in the side of the end plug leaks, water gets inside and corrodes the "spool". If the spool can't move, the valve will not proportion. It could also stick in closed position causing no rear brakes (this is rare).
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
Originally Posted by mirrorfinishman
On a 69 vette; Is there any condition where the proportioning valve could cause sustained excess pressure at the rear brakes?
Instructor
Joined: Apr 2006
Posts: 147
Likes: 0
From: New Jersey
Originally Posted by Duke94
Actually, 69's did not have a proportioning valve. Just the warning switch. If your valve is mounted vertical on a bracket off the frame rail just below the M/C, you have the warning switch only. If it's mounted horizontal on the frame rail with 2 bolts, it HAS the proportioning valve. Hope this helps.
1- Is it basically just a 'flow diverter' with a pressure warning switch?
2- Since it is an original part that has never been replaced; could it possibly be faulty? Thus contributing to the rear brake caliper remaining pressurized (and heating up) after running at highway speeds.
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
Originally Posted by mirrorfinishman
The car is a 69 and it is mounted vertical.
1- Is it basically just a 'flow diverter' with a pressure warning switch?
2- Since it is an original part that has never been replaced; could it possibly be faulty? Thus contributing to the rear brake caliper remaining pressurized (and heating up) after running at highway speeds.
1- Is it basically just a 'flow diverter' with a pressure warning switch?
2- Since it is an original part that has never been replaced; could it possibly be faulty? Thus contributing to the rear brake caliper remaining pressurized (and heating up) after running at highway speeds.
2). There's not much to go wrong. 2 rubber o'rings on a brass piston and not much else. Are both rear brake dragging? If only one, then it can't be anything upstream of the rear brake line (the one that goes from the valve to the rear). If it's only one brake, sounds like the caliper may be bad?
Pro
Joined: Aug 2005
Posts: 627
Likes: 1
From: Naperville IL
Is it possible for the distribution block to go bad and cause the front brakes to pull to one side? My 73 just recently started pulling hard to the right when I hit the brakes and as far as I can tell the front brake pistons all move freely and there is no air in the lines. I know there isn't much to go wrong there, but I also noticed the brake light doesn't come on while bleeding, which I'd think should happen if all the brake fluid is going to 1 wheel.
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
Originally Posted by chris73cpe
Is it possible for the distribution block to go bad and cause the front brakes to pull to one side? My 73 just recently started pulling hard to the right when I hit the brakes and as far as I can tell the front brake pistons all move freely and there is no air in the lines. I know there isn't much to go wrong there, but I also noticed the brake light doesn't come on while bleeding, which I'd think should happen if all the brake fluid is going to 1 wheel.