71 Brake Problem
Thread Starter
Instructor
Joined: May 2006
Posts: 134
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From: Hot Springs Village AR
71 Brake Problem
Replaced the master cylinder and complete system fluid on my factory power brake car with a Delco correct coded part. Could not seem to get it bled correctly; pedal was soft with too much travel. Drove the car for over a thousand miles and the pedal went even lower. The fluid level in the master cylinder never varied. I recently took it to a shop. All calipers and components were examined; no leaks found, no obvious defects. The system was flushed and bled. Very solid brakes, immediate pedal response. By 100 miles I was back to extreme pedal travel to the point the brake light came on. Took it back two more times. Each time there was no fluid loss but air in both rear calipers.
It would seem to me to indicate a master cylinder problem. What I don't understand is how air is getting into the system, without any fluid loss/leaks.
Appreciate any help/solution. Car is unsafe to drive.
It would seem to me to indicate a master cylinder problem. What I don't understand is how air is getting into the system, without any fluid loss/leaks.
Appreciate any help/solution. Car is unsafe to drive.
Melting Slicks
Joined: Jun 2011
Posts: 3,114
Likes: 592
Replaced the master cylinder and complete system fluid on my factory power brake car with a Delco correct coded part. Could not seem to get it bled correctly; pedal was soft with too much travel. Drove the car for over a thousand miles and the pedal went even lower. The fluid level in the master cylinder never varied. I recently took it to a shop. All calipers and components were examined; no leaks found, no obvious defects. The system was flushed and bled. Very solid brakes, immediate pedal response. By 100 miles I was back to extreme pedal travel to the point the brake light came on. Took it back two more times. Each time there was no fluid loss but air in both rear calipers.
It would seem to me to indicate a master cylinder problem. What I don't understand is how air is getting into the system, without any fluid loss/leaks.
Appreciate any help/solution. Car is unsafe to drive.
It would seem to me to indicate a master cylinder problem. What I don't understand is how air is getting into the system, without any fluid loss/leaks.
Appreciate any help/solution. Car is unsafe to drive.
I assume you have the original lip seal design which requires a very low tolerance for rotor run out.
Have your rotors checked for run out. Try to get them < 0.005.
Last edited by cagotzmann; Aug 23, 2017 at 11:18 PM.
Thread Starter
Instructor
Joined: May 2006
Posts: 134
Likes: 0
From: Hot Springs Village AR
If the brakes worked great after a good bleed and now you have the same problem I would guess your calipers are pumping air into the lines due to rotor run out.
I assume you have the original lip seal design which requires a very low tolerance for rotor run out.
Have your rotors checked for run out. Try to get them < 0.005.
I assume you have the original lip seal design which requires a very low tolerance for rotor run out.
Have your rotors checked for run out. Try to get them < 0.005.
Le Mans Master
Joined: Aug 2005
Posts: 6,494
Likes: 1,929
From: Temple Terrace Florida
Brakes
Have the shop put a dial indicator on the rotor and measure run out. GM says it should not be more than 8 thousands. I like to keep it below 4 thousands. If it is way out, consider replacing the rotors and measuring again. If it still not in specs, consider shimming the rotor and or switch to O-ring pistons in your system. I would do a search as there is a wealth of information about this on the C-2 and C-3 forum. Jerry
Thread Starter
Instructor
Joined: May 2006
Posts: 134
Likes: 0
From: Hot Springs Village AR
If the brakes worked great after a good bleed and now you have the same problem I would guess your calipers are pumping air into the lines due to rotor run out.
I assume you have the original lip seal design which requires a very low tolerance for rotor run out.
Have your rotors checked for run out. Try to get them < 0.005.
I assume you have the original lip seal design which requires a very low tolerance for rotor run out.
Have your rotors checked for run out. Try to get them < 0.005.
Tech Contributor
Joined: Aug 1999
Posts: 15,114
Likes: 3,905
From: Connecticut, USA
I would buy a good dial caliper and mag base and check it yourself. If you have never done it before then you will need to learn to do it but it's not really hard to do. I can go over it with you if need help.
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Pro
Joined: Jun 2014
Posts: 706
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From: New Jersey
That sucks.
Make sure it is both sides, as that could help the diagnosis if it is just one side.
Can you trace the lines looking for dampness from the caliper to the master?
What about looking for dampness in the piston area. You may have to use a tool to peek where to boot seals onto the piston.
How old are the calipers?
It may be worth the cost just to replace them, vs spending a ton of time looking for other stuff.
They are reasonable in cost from Rock Auto.
Make sure it is both sides, as that could help the diagnosis if it is just one side.
Can you trace the lines looking for dampness from the caliper to the master?
What about looking for dampness in the piston area. You may have to use a tool to peek where to boot seals onto the piston.
How old are the calipers?
It may be worth the cost just to replace them, vs spending a ton of time looking for other stuff.
They are reasonable in cost from Rock Auto.
Thread Starter
Instructor
Joined: May 2006
Posts: 134
Likes: 0
From: Hot Springs Village AR
Thanks, I do have a friend with lift, dial caliper, etc. This is how it we identified the runout problem. This is an example of "if it ain't broke, don't fix it." Went at least 3 years after replacing the master cylinder, just to get the correct coded Delco part, without a real problem. Just had what I considered excessive pedal travel, yet never went all the way to the floor. So, decided to see if that could be improved by rebleeding. Now I've gone two seasons having to re-bleed the rears each cruise/car show.
Any idea if the advertised new GM approved calipers have lip seals or O-ring type.
Any idea if the advertised new GM approved calipers have lip seals or O-ring type.
Pro
Joined: Jun 2014
Posts: 706
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From: New Jersey