Author Topic: 68 Brake issue. Input needed.  (Read 5627 times)

cwyllie

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68 Brake issue. Input needed.
« on: May 18, 2022, 02:12:22 AM »
Hey everyone.
I am having an issue with my brakes and at this point I am not sure which direction to go. Here's my situation.
Original Front disk power brakes. 396/TH400.
Caliper was leaking when I got the car so one of the first things to tackle were the brakes. Had calipers rebuilt (all new pistons and seals) and also had to replace the metering valve as it was leaking from the boot. Bench bled the Master and put it back together. Bled all four corners twice. Test drove around the block and everything seemed fine other than pulling to the right.
Took the car to work about a week later. Drove it to lunch and after sitting in the drive thru for 7-8 mins, went to leave and approaching the street.... HARD brake pedal with indicator light on. Pulled it into the parking lot next door and could not get anything better out of the pedal so had it towed home. Pulling off the flatbed and on the ground, good pedal for just a pump or two then HARD pedal with light. Next day I pulled the check valve from booster and whoosh. Still holding vac. Did the suck blow on check and it's good. Bled all four again and took for a spin. Good brakes for the fist 3-5 minutes then.... HARD pedal with indicator light..
So. Vac/booster issue or HYD issue??? Could it be the diverter valve is tripped? Could it be a bad booster even if it holds vac?
Any advice would be appreciated as I really NEED to get this car out and enjoy it with the Wife.
Thanks. Craig

william

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Re: 68 Brake issue. Input needed.
« Reply #1 on: May 19, 2022, 08:38:52 PM »
Here's a check list from the 1970 Delco manual.

Learning more and more about less and less...

GMAD_Van Nuys

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Re: 68 Brake issue. Input needed.
« Reply #2 on: May 19, 2022, 09:54:08 PM »
I had the power brake booster fail in my 1979 Trans Am and had problems similar to what you are experiencing.  I would also recommend changing your brake hoses as one on the car collapsed internally and locked the front caliper.

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #3 on: May 20, 2022, 12:45:40 AM »
GMAD.
Front hoses are new. When your booster was shot and pedal hard was braking minimal? And did your brake light come on? I suspected booster but everything I have read says that if brake light comes on it is a hydraulic issue so that is throwing me off my suspicion.

GMAD_Van Nuys

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Re: 68 Brake issue. Input needed.
« Reply #4 on: May 20, 2022, 01:36:00 AM »
cwyllie - The "brake light" did come on and as I live on a hill, I doubt if I could have stopped the Trans Am as I had a problem even pressing the brake pedal to stop the car on my driveway.  The brake booster was original and I also replaced the master cylinder when I got a rebuilt booster, which was somewhat tough to find as the Trans Am has 4-wheel disc brakes. - Mark

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #5 on: May 23, 2022, 12:36:17 AM »
I have a booster ordered but still questioning what is going on. Took the car for a spin again and brakes work fine until the engine warms up. Then, back to hard pedal with brake light on. Obviously the booster works when cold. It also is holding vacuum until the next day. Just weird.  :o

David K

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Re: 68 Brake issue. Input needed.
« Reply #6 on: May 23, 2022, 03:30:33 AM »
If you warm the engine without driving the car, does  it happen immediately or after actually driving a certain distance or brake applications?

Gars68Tux

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Re: 68 Brake issue. Input needed.
« Reply #7 on: May 23, 2022, 07:47:25 AM »
I have a booster ordered but still questioning what is going on. Took the car for a spin again and brakes work fine until the engine warms up. Then, back to hard pedal with brake light on. Obviously the booster works when cold. It also is holding vacuum until the next day. Just weird.  :o

Sorry I don't have much to offer in diagnosing the problem. But beings I know how original your car is, I highly recommend saving the booster and rebuilding if necessary. The correct police will spot a repop before the hood is completely raised... ;)

Ya sure it's not the vacuum hose to the manifold? Maybe it collapses when it gets warm.
Garth

68 RS L30 AA 749 Fred Gibb Chevrolet

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #8 on: May 28, 2022, 09:18:25 PM »
DavidK
I have not warmed completely before driving. It is the same situation every time I test it. Booster is fine, brakes fine then a couple miles up the road it acts up again. Once it starts, I will get intermittent ďgoodĒ braking on the way home.  Meaning, no hard pedal, no brake light, and working brakes.
Just pulled the proportioning valve off and it is centered so I think I can reduce that one.
After sitting for a week, I pulled the check valve and booster is still holding vacuum.
Could it be trapped air?? Iíve bled the crap out of them but have head that power bleeding is the only way to get it all out.

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #9 on: May 28, 2022, 09:20:45 PM »
Gars
I donít think so. Itís original and hard as a rock.  :)
No cracks. I checked.

GMAD_Van Nuys

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Re: 68 Brake issue. Input needed.
« Reply #10 on: May 28, 2022, 09:44:28 PM »
cwyllie - The power brake booster is designed to retain enough vacuum to stop the vehicle a couple of times, even if the engine isn't running, for obvious safety reasons.  If you have air in the brake system, it would be evident when you first tried to stop your Camaro.  I still believe that your power brake booster needs to be replaced.  Is the booster original?  When my Mini-Vac didn't have enough vacuum to bleed the front one piston calipers on my 1979 Trans Am, I bled the brakes by using the brake pedal, which ended up damaging my power brake booster. - Mark

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #11 on: May 29, 2022, 03:58:15 AM »
Mark.
Thank you very much for the info.
I think the booster is original but hard to say. No markings and not plated.
I have bled the system several time with the pedal. How would that damage the booster?
Considering a power bleeder for next go round.
Craig.

Gars68Tux

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Re: 68 Brake issue. Input needed.
« Reply #12 on: May 29, 2022, 06:02:44 AM »
The booster, if original, should have a stamped application and date code...http://camaros.org/suspen.shtml#Brakecomp
Garth

68 RS L30 AA 749 Fred Gibb Chevrolet

GMAD_Van Nuys

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Re: 68 Brake issue. Input needed.
« Reply #13 on: May 29, 2022, 03:01:01 PM »
When my Mini-Vac didn't generate enough vacuum to bleed the front calipers, I should have gotten another power bleeder.  I might have not damaged the power brake booster if I hadn't depressed the brake pedal to the floor.  They have one-way bleed valves that you can install on the calipers to make it easier to bleed the system.  - Mark

Gars68Tux

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Re: 68 Brake issue. Input needed.
« Reply #14 on: May 29, 2022, 05:23:41 PM »
Just a thought, did you guys remember to push the button on the back of the metering valve http://camaros.org/brakevalve.shtml#mv when bleeding the brakes ??
Garth

68 RS L30 AA 749 Fred Gibb Chevrolet

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...

David K

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Re: 68 Brake issue. Input needed.
« Reply #30 on: June 11, 2022, 10:10:01 PM »
I didnít see where you mentioned what engine youíre have, and you mentioned low vacuum.

Tinkerr

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Re: 68 Brake issue. Input needed.
« Reply #31 on: June 11, 2022, 10:37:09 PM »
Do you have the proportioning valve on the drivers side of the sub frame. I had a issue with mine, could get a little fluid to the rear, but no full flow. If you have the valve(and you should)crack the line on the down stream side of the valve to check for fluid, if no fluid then crack the up stream side to check for fluid. I could remove the down stream line and only get drips on mine.

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #32 on: June 12, 2022, 02:16:17 PM »
David K. L34 396.
Tinker. That is exactly what I did while troubleshooting yesterday and was happy to see good flow on the backside of the distribution valve. I then removed the line at the right rear wheel and no flow at all. Now, I am in the process of changing the rear axle hose. The fitting on the front of the hose is seized. I hit it with heat, pb blaster and cannot get it to loosen. I am using a flare wrench and I can feel it is about to strip the nut. I am hoping that if I can get the hose replaced and blow out the rear lines to the wheels I may be in good shape.

Sauron327

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Re: 68 Brake issue. Input needed.
« Reply #33 on: June 12, 2022, 03:11:17 PM »
Collapsed rear flex hose is common. When I do brakes on any of my cars I change them all. If you use heat, cut the line in half first, unless you got one end free. If not the heat will make the line pop and wake you up. It really helps others to use correct terminology when referring to brake components. http://www.camaros.org/brakevalve.shtml

Tinkerr

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Re: 68 Brake issue. Input needed.
« Reply #34 on: June 12, 2022, 07:07:06 PM »
Are you referring to picture #1 distribution block under master cylinder or #2 proportioning valve on the side of sub frame.

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #35 on: June 13, 2022, 12:44:42 AM »
i apologize for my misuse of terms. I replaced the MC, booster, distribution block and metering valve. Getting plenty of flow through the prop valve on the subframe. Replaced rear rubber hose today and getting good flow to all four corners.
took it out for a drive and again, same issue. Great pedal, and breaking for the first couple of miles. Then hard pedal, brake light and very little braking power.

Tinkerr

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Re: 68 Brake issue. Input needed.
« Reply #36 on: June 14, 2022, 09:56:02 PM »
Have you been tried replicating the issue at home on jack stands. Make sure brakes are releasing and applying equally. Just grasping at straws.

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #37 on: June 15, 2022, 04:24:54 PM »
Tinkerr.
Yes I have. After installing new MC and new distribution block, and rear flex hose.  I took the car out and once again I had good working brakes until the car was up to temp then it went to hell. Brake pedal feels like it hits a wall near the bottom, brake light comes on, and very little braking.
Next day I put it on stands to experiment. Started car and had good booster, good pedal, no light. Checked vacuum at 15in idle (cold). Pushed pedal about 30 times while warming up and everything was good. As soon as car was to temp, the problem came back. However when I pushed the pedal and hit the wall of resistance about halfway down and the light comes on, if I help pressure on the pedal it would drop another inch and light would go off. I could repeat this over and over.
Somehow there is a pressure issue in the system but cannot determine where the issue is.

Tinkerr

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Re: 68 Brake issue. Input needed.
« Reply #38 on: June 15, 2022, 07:48:08 PM »
Again grasping here. Why does engine heat effect it? I think Iíd replace the booster hose. Use a generic 3/8 hose for testing in driveway only, see if results change. I had my proportioning valve rebuilt the rebuilder  put the seals in upside down, cause a issue for me. So rebuilt isnít always correct. Donít know if it could cause your issue are calipers or the correct sides, bleeders up?

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #39 on: June 16, 2022, 03:00:15 PM »
Booster, hose, and check valve have all been replaced. Calipers are correct sides.
Thinking maybe change out the pressure reg valve on sub frame. That is the only part that has not been replaced on the system.

Sauron327

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Re: 68 Brake issue. Input needed.
« Reply #40 on: June 16, 2022, 08:10:02 PM »
Why not rebuild it?
Does anyone actually work on their own cars anymore from A to Z?

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #41 on: June 17, 2022, 01:54:51 AM »
 Sauron327.
What do you think I am doing? Been chasing an issue and looking for answers so I can fix it.

cwyllie

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Re: 68 Brake issue. Input needed.
« Reply #42 on: June 20, 2022, 01:14:58 AM »
ISSUE RESOLVED 
I replaced both wheel cylinder's at the recommendation of a very helpful CGR member. I also replaced the metering valve again just because it was suspect.
Today, upon completion and bleeding, I have brakes!
Personally, I think it was the wheel cylinders. They seemed to have more travel than the old ones and generally work better. Could probably put all my old parts back on and return the new ones... but I won't. Will try my hand at rebuilding some valves this winter when time allows. Overall, this was a great learning experience with my new car.
Thanks for all of the input from everyone.
Big shout out to my new Camaro buddy.  :)

Craig.

 

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