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!