Weird brakes

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76Scamp

76Scamp

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I drove the car to work today like usual with no problems and during lunch while putting new bulbs in the cluster I noticed the brake light to be on. Sure enough, it had a very soft pedal and it had to be pumped up. I drove it around and sometimes it has a firm pedal and it stops fine, other times it's very soft and the light comes on, but it still stops halfway decently. Also, when the light comes on, it will stay on until I tap the pedal. My thoughts are on the master cylinder, but I'm not too sure. I know I need a new gasket to go on top of it because that leaks, but I wouldn't think that would cause the braking pressure to be low and then its random at that. This car is a slant six and it's power brakes. From how things look, it's the original master cylinder, booster, and front calipers from '76, and I put new wheel cylinders in the back about a month ago because I noticed the drum side of the master cylinder to be low. As far as the rubber hoses, they look newer than the car, but I don't know how old they are. Is there anything I should look at beyond the master cylinder and hoses? Or should I just get them and hope that fixes it? I'm great with engines and a little bit of electronics, but hydraulic systems, not so much, but one of those two is bound to be the culprit by my thinking. Thanks for any help.
 
AJ/FormS said:
Are you saying you havent checked the fluid level yet?
You better go and do that then.
Dont drive it another mile. Not even a block. Not even to the end of the driveway!
Click to expand...

I did, it's full. It looks nasty, but it's full.
 
Ok, then.
When that light comes on its telling you that there is a pressure imbalance between the front and the rear systems.this imbalance is usually thought of as being after the m/c., but it could be before as well.
since you say the m/c is full, im gonna assume there is no leakage.
since you say it stops halfway decent, ima gonna assume the front system is ok(it does like 80%plus of the work)
since you say the pedal gets hard, ima gonna assume theres no air in the system.
since you say its a random condition, its not likely just a simple rear brake adjustment.
since you say the lite can be turned off with a tap on the pedal;This is the tricky one.
I think pressure is remaining in one side of the system.. AFTER.. the brake pedal is released and the tap allows the fluid to return, and equalize the pressure to zero. Since it does this with the vehicle stopped, its not gonna be pad knock-back. And not likely a bad front hose. So Ima gonna guess its in the rear brakes, and its a combination of mis-adjusted star adjusters and a bad hose. Now it could just as easily be a sticking P-valve. Or a faulty M/C.
My detecting skills are done. Now its up to you to by process of elimination figure out the rest. I would start with a rear brake adjustment.(no cost and easy).
Gotta go to work, now.
Im back.
I can certainly see the m/c being faulty because of the random spongy pedal. If that is true ,it will turn the lite on when you jam the pedal on. However the lite should turn off immediately when you take your foot off the pedal, as the pressures normalize to zero Oh wait I get it ;the residual valve may be keeping enough pressure in the rear line to prevent the switch from self centering.In which case the spring inside the P-valve may be weak.
--So to summarize,looks like 2 problems; 1) defective M/C, and 2) weak return spring in P-valve. After the new M/C is on,the spring in the P-valve may go back to doing its job as there will be equal pressures on both sides of it. It may turn out to be a non-issue.
 
It's probably a seal in the MC. If your fluid level is fine, and you don't have any leaks, it can only be that if it's a spongy pedal. I had one go, and you could get one good throw out of the pedal, and then anything after that you could walk the pedal all the way to the floor. Clearly pressure isn't holding if you have a soft pedal. The coming and going of the pedal firmness again points to a seal in the MC. Replacing the unit is probably what you are looking at. Better take the old one with you to the parts store to make sure they get you the correct replacement also. I'm not real trusting of the counter man's computer with all the drum/disc/power/maunal combos' that were out there.
 
AJ/FormS said:
Ok, then.
When that light comes on its telling you that there is a pressure imbalance between the front and the rear systems.this imbalance is usually thought of as being after the m/c., but it could be before as well.
since you say the m/c is full, im gonna assume there is no leak
since you say it stops halfway decent, ima gonna assume the front system is ok
since you say the pedal gets hard, ima gonna assume theres no air in the system.
since you say its a random condition, its not likely just a simple rear brake adjustment.
since you say the lite can be turned off with a tap on the pedal;This is the tricky one.
I think pressure is remaining in one side of the system after the brake pedal is released and the tap allows the fluid to return, and equalize the pressure to zero. Since it does this with the vehicle stopped, its not gonna be pad knock-back. And not likely a bad front hose. So Ima gonna guess its in the rear brakes, and its a combination of mis-adjusted star adjusters and a bad hose. Now it could just as easily be a sticking P-valve. Or a faulty M/C.
My detecting skills are done. Now its up to you to by process of elimination figure out the rest. I would start with a rear brake adjustment.(no cost and easy).
Gotta go to work, now.
Click to expand...

Wow, that's a lot to take in, but extremely helpful. I did just replace the wheel cylinders because the old ones were leaking. I look back there and see what/s happening. Thank you.
 
johnnymac2k6 said:
It's probably a seal in the MC. If your fluid level is fine, and you don't have any leaks, it can only be that if it's a spongy pedal. I had one go, and you could get one good throw out of the pedal, and then anything after that you could walk the pedal all the way to the floor. Clearly pressure isn't holding if you have a soft pedal. The coming and going of the pedal firmness again points to a seal in the MC. Replacing the unit is probably what you are looking at. Better take the old one with you to the parts store to make sure they get you the correct replacement also. I'm not real trusting of the counter man's computer with all the drum/disc/power/maunal combos' that were out there.
Click to expand...

I already have a "new" master cylinder on the way and it should be in tomorrow. I don't have to worry about the counterman getting anything wrong because I work at AutoZone and, for the most part, I'm kinda by myself when it comes to really knowing anything beyond what the computer can tell me. Even with new cars I've been giving old school advice that usually works.
 
I'm envious that you can get out and work on the ride when the rest of the east and midwest are hacking out of ice. IMO, start with the easy stuff first. Since you say you replaced a rear wheel cylinder, is it possible that there is a little air in the line?

The best cure is to bleed the system. You'll be doing this with a new master cylinder. Keep fluid coming out of the bleeder valve until you have a clear stream with no bubbles. Check MC frequently as it is not good if a reservoir goes dry. Use this for sequence. RR-LR-RF-LF (work from farthest out towards master cylinder.)
 
2 Darts said:
I'm envious that you can get out and work on the ride when the rest of the east and midwest are hacking out of ice. IMO, start with the easy stuff first. Since you say you replaced a rear wheel cylinder, is it possible that there is a little air in the line?

The best cure is to bleed the system. You'll be doing this with a new master cylinder. Keep fluid coming out of the bleeder valve until you have a clear stream with no bubbles. Check MC frequently as it is not good if a reservoir goes dry. Use this for sequence. RR-LR-RF-LF (work from farthest out towards master cylinder.)
Click to expand...

I can understand what you mean. My father is up in WV and wants to move down here to Florida. He has three feet of snow in his yard while I'm baking in 90 degree heat. I replaced both rear wheel cylinders, but it's been almost two months since I did that. I'm thinking the seals in them probably started leaking because I know the quality of stuff is kinda terrible anymore. Being it's dark out and I live in an apartment building, it's gonna have to wait until tomorrow since I have the day off. Tomorrow is going to be all about brakes because I'll be fixing this and putting new brakes on the wife's 2000 Jimmy, maybe even replace the rotors on my truck since they're full of grooves.
 
Went ahead and bled the rear wheel cylinders and that alone fixed the problem. When I opened up the bleeder on each side, it took a bit before fluid started coming out. I was unfortunately by myself, and I was going to put in a "speed bleeder" but they were too big to fit and I couldn't use them, but the gravity bleeding seems to have fixed it.
 
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