Bleeding Brakes on a 1967 GTO.

Hello All,

I have recently purchased a 1967 GTO that has front Disc Brakes. I am having troubles bleeding the front brakes.

According to everything i am reading, i have to "hold open the metering valve" that is found underneath the master cylnder. My question is:

1) How do i know it is open, and when it is open how to i keep it open.

Also i do know know if it is working correctly. This was installed before i got the car but the brakes were never used being that the car was in pieces for over 6 months and brakes were never filled or bled.

please help

email is jscheidet@yahoo.com

I don't know anything about holding open the metering valve. That really doesn't make sense to me. I've got a 67 with discs on the front, and you bleed the brakes just like you do any other car. Front is a piece of cake compared to the back. Don't bother with the Mighty Vac tool, it doesn't pull enough. Just get a friend to pump the brake, and you bleed it at the wheel. Better yet, get some Russel speed bleeders on the calipers and forget the friend. The thing you have to remember, your system was probably bone dry so it'll take more time to run brake fluid through all the lines. The best and easiest way is with a Snap-on pressurized system you attach to the master cylinder, pump it up to about 15 psi and just go around bleeding each wheel. in 4 minutes, you're done.

Mike,

This is what is making me nervous. I had a friend pump the brake as i was at the wheels. I did the backs first and they took awhile, which i am aware of because of them being bone dry.

But now at the fronts i must have opened that bleeder 20 times and i heard nothing.

So i decided to go step by step ( this is after reading about the metering valve). So i went to the proportioning valve, that is the next step from the metering valve, and i check the outline to the back and there was fluid. When i checked the outline to the Fronts it was still dry. This is what is confusing me.

Do you think the Pressurized system will work? The funny thing is that i have bleed more brakes then i can count and i cannot understand why i am getting nothing out of the fronts...not even the slightest hissssing.

let me know
Jim

The metering valve that you are referring to acts as sort of a dam, in the event that the front brakes lose pressure, you will still have rear brakes & not lose all fluid. If I recall, it is incorporated into the end of the combination/perfortioning valve.

If fluid is lost out the front, the valve is supposed to shift in the passage to prevent fluid loss from the rear brakes (and visa versa, in the event of fluid/pressure lost in the rear).

Anyway, this has to be depressed while bleeding in order for fluid to get passed it and "recenter" it, so to speak. I am not sure what they used back then, but in the mid 90's when 4WABS was introduced, you had to use a special tool to depress a simular valve on the brake module. It sort of looks like a schraeder valve on a tire valve. General motors lists J-35856 as the tool for the mid 90's apps, that may work on yours.

Otherwise, you can probably get a 3rd hand involved to hold that valve in, while someone pumps the brakes and you do the bleeder work. Once you get a bit of pressure, the third guy can go back to holding your beer.

Good luck!

If your goat is a 67 and has the standard brake set up with disc brakes in the front, it has a dual bowl, dual plunger master cylinder and one metering or pressure compensation valve mounted on the frame on the driver's side almost directly under the master cylinder. If the plunger is screwed up, you need to replace the valve, or at least take it apart and try to clean it. This proportioning valve is what applies more pressure to the front than the rear brakes since the fronts do about 65% of the braking. If you don't get any hissing at the bleeder in the front calipers, your master cylinder may not be primed or your proportioning valve is not working properly. Incidentally, I believe the proportioning valve is designed to favor the front brakes in fail safe mode, so I'm leaning towards the master cylinder not putting anything out. Pressurizing the system with the Snap-on brake pump eliminates that guess work, at a considerable expense however. Aren't brakes fun ? I don't what the other guy was talking about with 4 wheel ABS, the 67 never knew of that system. Maybe a BMW or Porsche, but sure as hell not a goat. The fluid of the master cylinder from either separate bowl do not mix in the proportioning valve. The piston actuates a spring loaded plunger which works like a relief valve. Losing brake fluid in the front has no effect on the loss of fluid in the back, its a pressure thing.

Mike - If you read through my post thoroughly, you will see that I was making reference to a tool that is currently used for ABS that may be able to be used on his GTO. I was not implying that a GTO had ABS.

First step, pull the master cylinder and bench bleed it. Make sure both the front and back outputs are flowing. Then bleed the brakes. If you don't get fluid then you know there is a problem downstream. Maybe the proportioning valve or the metering block is corroded. I just replaced a complete system from the master cyl to the rear drums. It took a long time to get fluid to the bleeders, Russel bleeders by the way.



The Goat Doctor

Tony Walton hit the nail on the head, follow his advice on bleeding your brakes and you shouldn't have any problems

James,

When you step on the brake pedal the metering valve allows approx 3-30 psi pressure to the front wheels until the pressure at the rear wheels reaches a certain point (approx 120psi). After that it allows full pressure. If you are bleeding by having someone pump the pedal they will push hard enough to open the valve and allow fluid through. If you gravity bleed then the valve will allow fluid through because there will be no pressure (which is less than ~3 psi). If you use a pressure bleeder then it is important to know at what pressure the pressure bleeder operates. If it fall into the 3-30 psi range (most are about 20 psi) then it will be necessary to hold the metering valve open or fluid will not get by it. Most of the GM valves have to be pushed in to hold the valve open. Those that don't fit into that category will have to be held out during the bleeding. You should be able to tell by looking at the end of the pin on the metering valve. If it has a slightly enlarged head then it is usually the "pull out" type. If the pin is straight, it will usually be the "push in" type.

Bob Ferdenzi
Master Power Brakes

James,

When you step on the brake pedal the metering valve allows approx 3-30 psi pressure to the front wheels until the pressure at the rear wheels reaches a certain point (approx 120psi). After that it allows full pressure. If you are bleeding by having someone pump the pedal they will push hard enough to open the valve and allow fluid through. If you gravity bleed then the valve will allow fluid through because there will be no pressure (which is less than ~3 psi). If you use a pressure bleeder then it is important to know at what pressure the pressure bleeder operates. If it falls into the 3-30 psi range (most are about 20 psi) then it will be necessary to hold the metering valve open or fluid will not get by it. Most of the GM valves have to be pushed in to hold the valve open. Those that don't fit into that category will have to be held out during the bleeding. You should be able to tell by looking at the end of the pin on the metering valve. If it has a slightly enlarged head then it is usually the "pull out" type. If the pin is straight, it will usually be the "push in" type.

Bob Ferdenzi
Master Power Brakes