Guys - 67 with stock drums all the way around - I have the two piece front to rear line with the proprotion valve on the frame rail under drivers seat.

Confused. Had a soft pedal to the floor and a leak in my right rear wheeel cylinder. Replaced cyclinder - went to bleed and had no pressure in the line. Helper pumped the brake pedal and I checked if I had pressure in the other side rear and had none there as well. This other side is a relatively new wheel cylinder and I know has been functioning. What could have reacted to my loss of pressure when the wheel cyl leaked and essentially shut down my rear lines? Could that proportion valve under the drivers seat have done this? Master cylinder react in some way...or did my master cylinder suddenly die?

Any thoughts on how to proceed here greatly appreciated....thanks to all my firends here....JD

It could be a few things I would first check ifyou have presure at the master to the rear lines. Have a freind put preasure on the pedal then crack loose the rear lineat the master. Note : cover everthing up to protect paint well! If you have psi there move further down the line near the valve. Could you have run out or very low on fluid with your leak? It could be the over the axle flexable ruber line aswell, Change it anyway if its original.

Good thoughts...but I forgot one cructial piece of information leading to my confusion. Although I have no pressure in the rear..I have hard pedal! The softness of the pedal that was present when the rear wheel cyl leaked has firmed up....with a strong pedal. I wouldn't think it would be an issue of loosing too much fluid when the original leak started, if I now have hard pedal??? It's as if I have become blocked somewhere from the proportioning vlalve up to the master cylinder.

First off, I'm wondering what a proportioning valve is doing on a drum brake car?....This valve is only for disc/drum cars.....There should only be a distribution block, located under the master cylinder, on a drum brake car!.....Assuming that all brake lines and hoses are not bent or kinked, I'd probably be looking at a defective master cylinder and that's assuming that the brake pedal pushrod is properly adjusted!

Ron, '67 A/C drum brake cars could have the prop valve. Mine does, although A/C is long gone.

Ron, '67 A/C drum brake cars could have the prop valve. Mine does, although A/C is long gone.


OK Robert, I wasn't aware he had A/C!....Will you forgive me??

:yes:Sure

Of course I don't know if he has A/C either.

Thanks!!:thumbsup:

I have had many problems trying to bleed rear brakes if fronts had pressure. I think I would first try to gravity bleed it. The other possibility is the proportioning valve might be stuck, if indeed that is a prop valve located only in the rear line. I always thought a proportioning vale worked with the pressure from both lines and when it got too far out of balance it shut off one side.

Jeff

Guys..Yes I have a 67 AC car...and the valve is part of this setup as it is on 68 and later higher performance setups....if I recall GM engineers evaluated the valve on 67 cars that had AC due to the slightly heavier weight....

I am just wondering if this valve could have failed/clogged....
I guess I'll break the lines loose from the valve tonight and see if I get flow...

Jeff...I guess that is the heart of the matter...I don't know how this valve really works internally either, or if ican actually **** down one side. If so, maybe I should bleed the fronts some and the valve will open up...?? grasping at straws here....

Jeff....Gravity bleed? Sorry, not quite sure what a gravity bleed is...??

Gravity bleed is when you crack open a bleeder valve and let gravity take over in the bleeding process but you need to make sure to check the master so that it doesn't run dry of brake fluid!

Your valve has a low pressure warning switch. Most likely when your rear cylinder "leaked" it shut off the rear circuit. You'll need to re-center the valve. You can take it apart to do this or open a front and a rear at the same time and bleed, the valve should re-center(equalize pressure). Did the light on the dash come on?

OK..interesting. I know there is a switch at the junction box that is right under the master cylinder..is this what you are talking about? The valve I was referring to is under the drivers seat located on the subframe rail. Does the junction box have the ability to modify pressure....?......I thought it was just a passive junction box.

That's called a distribution block which is located under the master and no, it has nothing to do with controlling pressure to the front and rear brakes......This block also has the brake warning switch built into it.

As some others have said, it sounds like the valve has shifted sending all the fluid to the front. It's a built in safety feature so you still have brakes when a line blows. Bleed the front and it should reset the valve. Then bleed the rear.

Your valve has a low pressure warning switch. Most likely when your rear cylinder "leaked" it shut off the rear circuit. You'll need to re-center the valve. You can take it apart to do this or open a front and a rear at the same time and bleed, the valve should re-center(equalize pressure). Did the light on the dash come on?

Ford's had that system early on, but GM never used a switch that blocks fluid flow. The Camaro warning switch is spring loaded to self-center and turn off the brake warning light once you lift your foot off the brake pedal. It can not turn off the fluid flow.

I would check for a shot master cylinder, and check for a blockage in the rear line. A chunk of rubber from a rotting brake hose, or a piece of seal material is all it takes.
David

I have a dumb question. How old are the rear flexible hoses on the car. I had a set of aftermarket disc brakes installed on my 68 drum brake car. The mechanic could not bleed the rear brakes. He relplaced the rear flex lines and viola it worked. It seems that the rear brakes had not been working for some time and and the linings were like new. I was only using 2 front drums. No wonder it was a brute to stop.
He found the rears were clogged solid.

I didn't think the distribution block could moderate flow (Dave thanks for clearing that up). Still, there is the proprotion valve under the drivers seat which I do believe can moderate flow, mostly intended for disc brake cars but also shows up on 67 AC all drum cars. But even if working properly I don't know if it would shut down the rear. I think more likely is the suggestion made by many of blockage, my rear flex lines are very old so they may be the culprit.

A lot of people have sugggested even with no obstructions in the rear line I could have a firm pedal (and an inability to bleed the rears) just by pressure in the front lines. I worked rear brakes only once before and don't remember this, but I am not very experienced. The mster cyl has separate chambers for front/rear and I don't know how isolated they are. Dave, if you are still out there thoughts on this idea?

A very common problem we hear on this forum is the rear brakes won't bleed. Often it's a lack of full MC piston return. If the piston can't return fully, the reservoir replenishment port won't be uncovered and no fluid enters the rear MC bore. You can verify this by having someone step on the brakes and crack open the rear bleeder on the master cyl. If you have no bleeder screw, the line can be backed off and fluid should appear. Be careful, it can easily squirt all over!
Another sign is when you step on the brakes, fluid should briefly squirt up into the reservoir out of the replenishment port.
Many times the master cylinder rear pushrod hole is the wrong depth to match the booster pushrod there are two lengths of rod,two depths of MC's. Pulling the MC away from the booster a bit with lines connected should allow you to feel if the pushrod coming out of the booster is too long, extra resistance will be felt.

Fluid can be pushed past the seals from rear chamber to the front MC chamber.
The prop valve on the lower frame rail has a removeable plug, inside is a plunger and seal, perhaps a spring, I can't remember. I think I'd verify that the line is no blocked by blowing compressed air through it or cracking open the MC bleeder or line and testing that the MC is pumping before messing with the valve. Try and narrow down what area is not functioning, line or MC.
David

Here's my page on it: http://www.pozziracing.com/brakes.htm#Booster

I had the same problem. You have to bleed the master cylinder. it's simple; just like bleeding the calipers. loosen the lines while someone works the pedal. use a big rag.

Guys - Thanks for all the good input - found the problem over the weekend. First disconnected line after proportioning valve on frame rail - had a good gravity bleed. Reconnected, then broke in just before rear flexi line - good gravity bleed. Reconnected, with still nothing bleeding at either bleeder. So, replaced from flexi line on back (flexi line and both 3/16 hard lines that run along the rear axle) and had a good gravity bleed at both bleeders. Wanted to share results of this blake bleeder tool I bought from advance auto parts. One-man brake bleeder kit with a little bottle, some plastic tubing, and adapters to fit the bleeder. Kit was 7 bucks and worked well for me so I thought it was worth a mention. Anyway problem solved and back on the road with good rear brakes. Thanks again to all...:beers: JD.