Brake Metering/Proportioning valve plunger question!

[DAVEN1256]

Mine is a '68 with power front disc brakes.

I had my Metering Valve (or Proportioning Valve depending on what you call it) rebuilt by Brakeboosters.com a couple of years ago. I'm just now getting ready to fill and bleed my restored brake system for the first time.

I know that when you are bleeding the brakes that the plunger on the back of the Metering Valve has to be pushed in in order for fluid to flow to the front brakes. I've read where you can have someone helping you push it in or put a clamp on it.

My question is.....just how much force does it take to push that plunger in??? I tried doing it by hand and it wouldn't move......I put the valve in a vice and gently turned it and the plunger still won't budge. I had wood between the valve and the vise jaws to protect the valve and it started crushing the wood instead of moving the plunger!...….That amount of pressure not moving the plunger hardly jives with someone being able to push it in by hand.

I'm not sure how to precede here. Do these things stick? I'm afraid of putting any more pressure on it for fear of breaking something.....Any thoughts are appreciated?

Thanks...….Dave

[jvb6648]

That plunger seats against the hex end of the valve. When fluid from the master cylinder enters the valve it has to overcome spring pressure to unseat it and allow the fluid to flow. so the plunger will move out. By pushing it in too hard you could damage the rubber or neoprene seat. The other valve is for the brake warning light an has a shuttle inside that needs to stay centered when bleeding.

[Mike S]

 I don't touched that plunger when bleeding the brakes and never had an issue getting a high & firm peddle.

Mike

[Mike S]

 Oh, and one other thing, from an old Chilton's the directions mentioned to hold the valve in, not press it in. In other words, don't let the valve shaft push outwards. Heck, it even mentioned holding it in with a piece of tape! 

Mike

[68camaroz28]

I don't touched that plunger when bleeding the brakes and never had an issue getting a high & firm peddle.

Mike

I'm with you Mike as we have never moved or fooled with the plunger to bleed brakes.

[68camaroz28]

Dave, not a whole lot with those rascals and kits can be purchased if needed. Check this out!
https://www.musclecarresearch.com/gm-3905525-autopsy

[olddragracer]

that valve holds off pressure to the front brakes on disc brake cars so that the rear brakes can apply lightly before the front calipers apply. The rear drum brakes have a residual check valve in the master cylinder that holds something under 10 PSI on the rear system at all times. When bleeding the rear brakes that valve may need to be held or kept in its seated position to get fluid to the rear wheels. Bleeding is always done at the wheel furthest from the master cylinder first.[ rear]  Once you get some pressure at the pedal you usually do not need to need to hold the valve. Holding the valve helps get fluid to the rear brakes first. I hope my explanation is OK and correct. 

[Emo1956]

Dave , I have a 68 with 4 piston calipers and I had to push it in too in order to bleed the brakes. I used a tire iron not the type that you change your tires with but 1 you would use in a tire shop. It was a straight bar about 30-36 in long with a flat spot on the end. I would push it between the valve and the booster. Not hard just a little pressure and then bleed the brakes. After bleeding remove the bar . I wish I had a pic of it .   Emo

[Emo1956]

I found 1

[DAVEN1256]

Thanks for all the replies.

I know what the function of the metering valve is and why it's necessary. The one and only thing I was trying to find out from this post is.....is there something wrong with my metering valve?

Both the '68 Chevrolet Chassis Service Manual and Chilton's state that when bleeding a front disc brake system, the plunger on the back of the valve must be depressed! The don't say " might have to" or "if necessary." The Chassis Service Manual says "must be held" and Chilton's says "it will be necessary to hold."

They both state that once the plunger is depressed (opening the valve inside), it can be held down by hand, with a clamp, or even with tape. If something spring loaded can be held down with tape, I would think I should be able to push it in with finger with little resistance. But my plunger will not budge, even cranking on it in a vise

My metering valve was rebuilt by a professional two years ago so I wouldn't think it could be rust frozen inside.

So that's all I really want to know...….is there is something wrong with my valve?

Thanks..…….Dave

[Mike S]

 As Jim mentioned, it moves outward. I have a spare valve and opened it yesterday to examine and see it can only move outward, not in.
It is spring (very thick spring too) loaded.
I believe the directions stating to hold it in means to hold it such that it can not push outwards.
I recommend to try bleeding first and if there is an issue, then put duct tape across it from pushing out (thereby holding it in).

Mike

[olddragracer]

I believe post #10 is correct. I just read the 1967 chassis shop manual and it says [valve in open position] and I believe that should read [valve in closed position].  The plunger comes out as the valve opens. The manual is almost always correct but I think this time it may be wrong.  Remember the manual is referring to bleeding with a pressure bleeder that is maintaining 40PSI on the system and that may push the plunger out. It may be necessary to hold the plunger in bleed the rear brakes first.

[DAVEN1256]

I am certainly not disputing what anyone is saying, I'm just trying to get this straight in my head and so far I haven't been able to do it. I going to explain in my mind how I think the metering valve works and if I've got this all screwed up,  please point out to me where I'm going wrong.

1) The valve holds off pressure reaching the front brakes until the pressure in the valve reaches 30 to 40 lbs.

2) I would assume there is internal valve of some sort inside the metering valve held close by a spring preventing the fluid from flowing past it. When the fluid pressure reaches the 30 to 40 lbs, that's enough  pressure to to overcome the spring tension and the internal valve opens letting the fluid through. (Similar to the way a radiator thermostat opens when the coolant gets hot enough).

3) When you are bleeding your brakes manually (without pressure), you don't develop the required 30 to 40 lbs necessary to open the internal valve so the plunger exists so you can open the internal valve manually.

Right or wrong so far ? Probably wrong!

This is the part I'm having a hard time getting my head around. If the fluid pressure opens the internal valve pushing the plunger out, why would you want to push the plunger back in while you are bleeding like all the books say to? Wouldn't that be closing the valve stopping the fluid flow which is what you don't want to do?

Just trying to get it all figured out......Dave

[jvb6648]

You are right up to 3. When you pump the brake pedal you will make the pressure needed to unseat the valve and push any air through to the wheel cylinders. If you have a power bleeder that won't make 30 to 50 psi you will need to PULL the plunger to open the valve. No reason to hold plunger or try to push it in, you'll be shutting off the flow of fluid. The '68 GM service manual does not even mention doing anything with that valve. Just bleed the system. '68 manual stated the 30 to 50 psi.

[Kelley W King]

I too have never done anything with the valve to bleed the brakes.