# '69 GTO Booster/Master cylinder problems



## Turley (Aug 23, 2012)

This is probably an extremely simple question, but I must preface this with the comment that this is my first restoration project. I recently bought a 1969 GTO, in poor but drivable condition, and have started to slowly restore it. In the begining, the brakes were extremely hard. I assumed it was the brake booster, but went ahead and replaced both the master cylinder and brake booster. I have now bled the brakes several different times. I have done this correctly, from back to front, and starting with the furthest tire away. There doesn't seem to be any air, as I have a smooth flow of brake fluid and not the sputtering that I had in the begining. When I start the car the brakes turn to mush, straight to the floor. Any ideas? The front brakes are disc and the back are drums. I haven't replaced or changed anything else with the brakes or brake system.


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## Instg8ter (Sep 28, 2010)

power brakes? Drum or disk?...is your vacuum line hooked to the booster?, do you have a big cam in the car, if so you may not be making enough vacuum at idle.


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## harlan41 (Aug 19, 2012)

Make sure brake calipers on the correct side, or you will never get the air out. Bleeder screws are to be on the top of caliper, when installed. If master cylinder is not level when bleeding, jack up the rear of the car to level it , to bleed out.


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## Turley (Aug 23, 2012)




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## BearGFR (Aug 25, 2008)

Hey Turley, welcome to the club.

When you replaced the master cylinder, did you bench bleed it before you installed it? That's very important (and different from the regular on-car brake line bleeding process) - and could be the source of your problem if you didn't do it.

There are several references that show how to do it -- here's one:





Bear


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## Turley (Aug 23, 2012)

The front are disks and the backs are drums, power obviously and yes there is a vacum line coming off the booster.


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## Turley (Aug 23, 2012)

The breaks were bench bled but it was my first atempt at it. I'll take another shot at it this weekend. The master cylinder definitely isn't level either, I'll jack up the back end and see if that helps.


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## Turley (Aug 23, 2012)

There is a unit attached to the booster in the first photo up above. There is a line from the rear brake reservoir of the master cylinder to it, what is this?


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## BearGFR (Aug 25, 2008)

Turley said:


> There is a unit attached to the booster in the first photo up above. There is a line from the rear brake reservoir of the master cylinder to it, what is this?


Ah.. yes that's the disc brake 'hold off' valve. Its job is to prevent the rear brakes from "seeing" pressure until the front brake system pressure has reached a certain threshold. Discs require more pressure to operate than drums do - so without the valve you'd have an imbalance and a tendency for the rears to lock up on you. Some folks call these "proportioning valves" - but I'm pretty sure the ones in 69 worked slightly differently than a true proportioning valve does. I've always seen them referred to as 'hold off valves' in the 69 manuals.

You did jog my memory though... on the back side of it there should be a plunger (may be covered by a rubber cap). Sometimes to get a proper bleed of the system accomplished you have to push and hold that plunger in, like with the flat side of a screwdriver or something.

Bear


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## Turley (Aug 23, 2012)

The video was helpful, I don't think I spent enough time bench bleeding. I'm gonna pull off the master cylinder and bench bleed again. Then I'll bleed the lines again and see where I am. If that doesnt work I 'll play with that pluger. I feel like I might be on the right track now, these breaks were starting to piss me off, haha. I really liked the car because it needed a ton of work but was still drivable. It's been sitting for a couple weeks because of the break situation. Maybe this'll get me back on the road. Next I'm re-wiring everything, I got the full kit in the other day.


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## ALKYGTO (Mar 29, 2010)

Is your brake light on the dash lit? If so the distribution valve may have shifted. It needs to be centered to properly bleed the system. I've run into this, especially on my Riviera, where I need to "reverse" bleed the system. 

You could also try to "gravity" bleed them. Start farthest from the MC, brake the bleeder loose, go drink two beers, tighten the line and repeat for the rest. :cheers

And if it does'nt work, you won't really care :lol:.


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## Turley (Aug 23, 2012)

Nice, haha, that seems like a win/win


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## Turley (Aug 23, 2012)

I guess you just crack them open a little?


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## Instg8ter (Sep 28, 2010)

post any problems here Turley as you can see you will get a myriad of answers and suggestions from guys that have "been there, done that..." and usually one of us will stumble on the right answer....post some pics of the projects looks sweet in your avatar....:cheers


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