Author Topic: 68 Brake issue. Input needed.  (Read 5626 times)

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #15 on: May 29, 2022, 07:44:46 PM »
Garth. That button is very difficult to move if at all. I have read many differing opinions on this matter of pushing the button in during bleeding. If I am manually bleeding using the brake pedal, does it need to be pushed in? Or is that only if using a power (pressure) bleeder?
Craig.

Gars68Tux

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Re: 68 Brake issue. Input needed.
« Reply #16 on: May 29, 2022, 08:04:27 PM »
hmm, I was under the impression it should always be pushed in during bleeding. It can be hard to tell if it's releasing just by feel. Fluid should flow out the caliper bleeder freely just by gravity if the valve is released. I wedge a piece of wood in between the button and booster to hold it while bleeding.

I wouldn't be surprised if the valve is stuck. It may've leaked a little and corroded. Probably needs rebuilding. 
Garth

68 RS L30 AA 749 Fred Gibb Chevrolet

GMAD_Van Nuys

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Re: 68 Brake issue. Input needed.
« Reply #17 on: May 29, 2022, 08:41:20 PM »
You will only have to push-in the button on the metering valve if you are using a power bleeder that cannot generate enough vacuum to pull fluid past the valve:

https://www.camaros.net/threads/metering-proportioning-valve-plunger-question.460739/

As you have already replaced the metering valve and haven't a problem with the front discs locking up when braking, I still think your problem is that the power brake booster is bad. - Mark

Gars68Tux

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Re: 68 Brake issue. Input needed.
« Reply #18 on: May 29, 2022, 10:39:08 PM »
Sorry, I didn't catch where the metering valve had already been replaced.

My Chiltons manual for bleeding brakes manually states; "IMPORTANT: on cars with front disc brakes, it will be necessary to hold the metering valve pin during the bleeding procedure...."

When I change fluid every 3-4 yrs (precautionary maintenance), I just let gravity do the work. The front calipers won't bleed this way unless the metering valve button is depressed. So that's an easy way to tell if the valve is open or not...
Garth

68 RS L30 AA 749 Fred Gibb Chevrolet

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #19 on: May 29, 2022, 11:07:18 PM »
Just finished swapping the booster out. Wow. That was fun!  :o
I pulled the switch off the prop valve and it is center.
I ordered a power bleeder but now thinking maybe just do the same brake pedal bleed that I have done three times now. All three times I had good flow to all four corners.
Tomorrow I will bench bleed the master and go from
there. Cmon good brake pedal!

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #20 on: June 02, 2022, 03:00:23 AM »
Update.
So, turns out the "repop" booster did not work out. The mounting surface on the booster that the Master cyl. mounts to has a counter bore (recessed) on the repop. Booster that was removed does not have this recessed bore. When the master is bolted to the new booster, there is a preload situation on the MC piston.
I got a block down the road and had to bring her back. By the time I got to my driveway. The brakes were fully engaged and fighting the 396 all the way.
Put her up on the stands and all four were locked up. Removed the Master mounting nuts and brakes released. Booster came from Camaro Central. Going to call them in the morning to see if they have a correct booster that will actually work. If not, I may have to get the old one rebuilt.
Any suggestions for an aftermarket booster that is closer to factory specs or a source for rebuild (close to Utah) would be helpful. Also, anyone encountered this situation?
Craig.

GMAD_Van Nuys

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Re: 68 Brake issue. Input needed.
« Reply #21 on: June 02, 2022, 03:23:49 AM »
cwyllie - If there is an O'Reilly Auto Parts in your area, they show (at my local store) a rebuilt power brake booster for a 1968 Camaro with front disc brakes and the 396 engine:

https://www.oreillyauto.com/detail/c/brakebest-brakes/brakes/brake-hydraulics/power-brake-booster/69bab478e25a/brakebest-power-brake-booster/obh3/501129/v/a/14021/automotive-car-1968-chevrolet-camaro?pos=3

Gars68Tux

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Re: 68 Brake issue. Input needed.
« Reply #22 on: June 02, 2022, 04:28:48 AM »
Would like to see a photo of the recess? Are you aware of the 2 different pushrod lengths, depending which MC is used? They just pull out of the booster to replace.   
Garth

68 RS L30 AA 749 Fred Gibb Chevrolet

Gars68Tux

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Re: 68 Brake issue. Input needed.
« Reply #23 on: June 02, 2022, 11:02:37 AM »
The 2 common length booster pushrods are 2 3/8" and 4". Also have heard of repro boosters having slightly too long pushrods and folks grinding them down.
Garth

68 RS L30 AA 749 Fred Gibb Chevrolet

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #24 on: June 02, 2022, 02:44:57 PM »
Garth.
I will compare the push rod length and get a photo of both boosters to you via email.

Craig.

Sauron327

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Re: 68 Brake issue. Input needed.
« Reply #25 on: June 02, 2022, 04:52:42 PM »
The gap between the booster pushrod/intermediate rod and MC should be .020. That is why your brakes are locking up. You are exerting pressure on the system. This should be checked prior to bench bleeding and final assembly.

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #26 on: June 02, 2022, 07:20:18 PM »
The new booster push rod is approximately .120" longer than the old booster. This is measuring from where the MC flange mounts to the booster mounting area to top of pin.
Does not seem to just pull out of the booster. If I can get the rod removed I can machine the end to be the proper length.

Sauron327

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Re: 68 Brake issue. Input needed.
« Reply #27 on: June 02, 2022, 08:04:35 PM »
I've done many disc conversions. The last one was on my car and the out of box new MC and booster was plug and play. I did measure the gap. I just put a dab of 3M strip caulk on the end of the rod with some veggie oil on one side so it does not stick. Push the MC on the booster till it seats, gently remove the MC and prick what's left on the rod to determine gap. Brakes worked great. Or you can measure it yourself, or buy one of these fancy tools: https://techtalk.mpbrakes.com/how-to-series/got-a-soft-pedal-dont-forget-the-booster-pin-to-master-cylinder-gap. Some rods don't come out of the booster. The rod can be ground and rounded with your Roloc and finished to a smooth surface if is still on the PB. I'd check the age and quality of your MC. I changed mine because it was a 20 year old MC leaking out the back and filling the PB. It was a reman GM MC. I'm doing a 57 truck now and the brand new MC was junk and leaked out the back.

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #28 on: June 05, 2022, 08:52:35 PM »
Update.
I did end up grinding .150" off the booster push rod in order to relieve pre load on MC and thought I would be Golden. Re installed booster and MC. Same issue. Hard brake pedal that about goes to floor with brake indicator light coming on. There is some intermittent braking, meaning that once in a while, I do get a good pedal without the light and good brakes. As soon as I let up on the pedal and re apply brakes, same thing as before, no brakes.
I also checked the vacuum and was getting around 12" is all. Adjusted the carb to lean out a bit (was running rich) and was able to increase vacuum to 15". Is this enough?? Everything I am reading says you need 17"-18". Could this be the culprit?
At this point I am  looking at rebuilding or replacing the MC and  the proportioning valve. Just seems that this has to be a hydraulic issue due to the dash light coming on when brakes are applied. Metering valve has already been replace do to the old one leaking from the rubber boot.

Thoughts??

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #29 on: June 11, 2022, 07:56:40 PM »
Replaced that MC and proportioning valve. Rears will not bleed at all. Before replacing the for mentioned, rear would bleed, but slow.
Tried pressure bleeding with a wedge on the metering valve button and tried the push and hold pedal method with nothing at rears. Plenty of flow at fronts. Seems like the new Prop valve may be off center. UUUgh...