Brakes Work Intermittently
 

1968 Camaro - 396 with Vacuum Brake Booster

I was having issues with brakes going out when driving and having very little stopping power.
I installed a new master cylinder and brake booster, I bled the brakes a few times.

Now the brakes will sometimes be better than ever; the brake pedal will be all the way back and extremely tight with great stopping power.
Then they will intermittently go soft and have very little stopping power, but if I'm driving a while, they may come back and start working great again.

*I'm thinking the brake lines may be getting hot and boiling the fluid? My next "fix" is to look at how close they are to the headers...
*My 2nd theory...whenever they get tight, the pedal want to come all the way back. If it's not adjusted correctly there will be constant pressure on the pedal thus causing the brakes to heat up, again boiling the fluid(like I'm all ways pushing the brake pedal down)

how much vacuum do you have at idle??

Was the master cylinder bench bled with the proper procedure? How old are the soft hoses to the rear axle and calipers? Is there any chance that the brake fluid got contaminated? You said you bled the brakes, are you getting clean fluid? When you replaced the master cylinder did you replace the filter/elbow going to the master cylinder.

You may want to try and drive the car without the power booster hooked up and see if it still have the variation.

With a car not running on a flat surface you should be able to push it by hand. If you can't then the brakes are grabbing.

@SoCal67 I don't have the car with me right now, but I will put a gauge on it at Idle and maybe also see what it is at 2500 RPM whenever I get back to the garage.

@Gorn The master cylinder was bench bled per directions provided. I don't know how old the soft hoses are (I've never changed them). None of the fluid is old, but some is reused (I ran it through a coffee filter). I don't think I have a filter/elbow going to the master cylinder, but everything after the end of the brake line is new. It did come with a new proportioning valve that I installed.
- I'll try to drive it without the booster, maybe that way I'll know what it feels like when the booster goes out vs the brakes just not working correctly.
- I'll also try to push it on a flat surface, but I think it will roll fine. The brakes only get tight when it's running and I can tell they start grabbing because it won't even move when it's in drive.

I appreciate the comments, I'll have to give these ideas a whirl, but I won't be able to for about a week unfortunately.

The soft hoses can cause calipers and wheel cylinders. I change mine every 10 to 15 years. The hose will come apart in the inside, then the only thing that can get thru is high pressure. Depending on how bad the hose will depend how brakes are I have seen it from just making brake pad wear a little to fast to locking up the wheel.

Your brake fluid idea with the coffee filter is a little miss directed. The problem with old brake fluid is water in the fluid. If you took your car to a track the only thing they will test is the moisture content of the brake fluid, Most car have that have not had their brake fluid changed in the last 5 years will most likely fail. The moisture will turn to steam when brakes get hot and act just like air.

Sure sounds like you could use new hoses and a flush of new fluid.

OP

I mentioned vacuum check because if low at idle and when you are on the gas the vacuum drops so if not enough braking will suffer as it is vacuum assisted (brake booster). Motors with big cams have this issue so a separate vacuum canister or hydraboost is the solution for that kind of thing

Do you have drom or disc brakes?

if your brake hardware is not working right (drums) than the shoes can stick on the drums and not release. On disc if the calipers are not sliding right on the pins that can cause brake drag

If your soft lines are...old, replace.They rot from inside out. do the rear one also

Yes, this issue is not fixed yet. Here are some more facts that I think should narrow it down:

• I've bled the brakes multiple time with all new fluid, no success.
• There is always vibration when applying brakes
• Front: Disk Rear: Drum
• I jacked the front up and started engine. The tires free spin, the brakes work when pressed, and the tires free spin right after release.
• Per usual, as soon as I start the car the brakes work well
• After about 5 minutes, the brakes are very good, zero slack in pedal
• After 15 minutes of driving, I drove home and jacked the front end up. The tires were locked to where I could not spin them by hand
  • I unplugged the vacuum hose, no change. I shut the car off, no change, still locked up
• After 5 minutes they started freeing up to be able to spin by hand
• After I could spin them by hand again, I started the car, hit the brakes, released them, then checked the tires. I could still spin them by hand still.
• I went on a drive again, the brake pedal started getting weak and getting pretty close to the floor board after about 40 minutes of driving.

My new theory: Although a new master cylinder didn't fix this, maybe the new one was faulty?
My rotors definitely feel warped, but I don't think that is the origin of this issue. None the less I am about to buy some new slotted/drilled rotors.
New Caliper are about $250 each, so I'm hesitant to just start buying more new parts.

I would not get rotors until this is sorted out. One issue at a time, adding new variables could complement things. You can get a brake pressure gage and test pressure ay each wheel and at the booster. At lease then you would know its a issue coming out of the master cylinder or an issue at the wheels. Also on of those none touch thermometers might help to track boiling fluid. . .

You did replace all the soft lines in the car? The one at the rear axle seems like it always get missed.

Thanks for the reply Gorn! The pressure gage would help narrow it down, but that also gives me the idea to crack lines loose in different locations to see if the tires free up. I know this would get air in the line, but it may be worth it.

No, I have not replaced any soft lines. When the car was cold, I applied brakes and let off and the tires would immediately spin again. Do you think heat could cause the bad soft lines to close up?

Both front tires lock up after driving, so it's either both soft hoses, both calipers, or the master cylinder.

Thanks Again!

Is it the heat causing the issue or the issue causing the heat? When a hose fails in this matter the main complaint is the wheel getting hot they locking up. Once it cools down everything goes back to normal. You are right both hoses failing is rare, I have only seen it when a car is stored for extended time. I replaced one hose in a Vet for the wheel locking up and a week later the other side failed in the same way and 3 weeks later the rear line started leaking. Luckily I had wrote on the first work order all hoses should be replaced. That was a 1968 that sat for about 10 years and I did it in 1987. Like I said 10 to 15 years they should be replaced to avoid getting little pcs of rubber in your system. I did all of mine a couple of years ago as preventive maintenance.

BTW why are you calipers so expensive? Have you shopped around? Rebuilding them is so cheap and easy. It cost me $10 per caliper. RockAuto had the rebuild kits for my 93 on clearance for $2.

Had same problem
 

Sounds like the same problem I had, chased it around like you. It's going to sound crazy, the little plastic vacuum valve on brake booster I replaced with an Acdelco valve. Sorry don't have part number. No more problems after I changed it. Make sure you use a Acdelco

@Pesscomp, I don't believe this is vacuum related as when I unplugged the vacuum or even shut the engine off, the tires were still locked up. I also loosened the bolts between the master cylinder and the booster and put a spacer in there to remove the possibility of incorrect rod adjustment.

This weekend I started the car and let it warm up without driving it and the issue still occurred. I cracked the brake line loose after the master cylinder, brake fluid squirted out and the tires freed up. I ordered ANOTHER new master cylinder yesterday, hopefully I can get it installed this weekend. Stay tuned.
*I also bought new slotted and drilled rotors with pads, but I'm not installing them until later. I want to know exactly what the culprit is.

When I removed my master cylinder to replace it, I noticed a rubber grommet(maybe a dust protector?) was pushed into the MC and was preventing the piston from returning to home position. I went ahead and replaced the master cylinder anyway and did not install any kind of grommet between the booster and MC. I drove the car for 30 minutes, the brakes seemed to work great, right when I pulling in my drive they seemed to be getting weak. Unfortunately, I won't be able to test it any further for another two weeks, but I'm thinking the brakes locking up and the brakes going out were not related.

Oh, and I also installed the new slotted/drilled rotor hub assemblies for the front with new pads.

https://cimg1.ibsrv.net/gimg/camarof...f10c55c99f.jpg

It's been awhile, but I decided post a final comment here. Although, after all the above repairs, the brakes seemed to be working better, they were still pretty inconsistent. One day they got super weak, as they did before, but never came back. After driving for about an hour, I felt all the brake system components with my hand and nothing seamed hot. After investigation, I noticed a leak in a brake caliper piston. I replaced both front calipers and the brakes have worked flawlessly ever since. I still don't quite understand what was causing what, but I'm back on the road regardless. Thanks again for the help.