C3 Spongy Brake Issues
Thread Starter
Intermediate
Joined: Jul 2022
Posts: 30
Likes: 3
C3 Spongy Brake Issues
Hi All, a few months back I purchased a 74-Vette Base that had a spongy brake pedal. Upon inspection the Master Cylinder had some fluid and rust inside in it. I just chalked it up to a new Master Cycler and bleeding was needed, after removing the wheels I found that the (2) of the calipers were leaking. I rebuilt all the (4) Calipers with new shoes and new bleeder valves, a new master cylinder that I bench bled before install, set the booster pin to .020’, and bled all the lines (3) times each and (3) times per valve stem. With that said, brakes felt tight, until I started the car and the pedal fell to the floor, the car still stopped but it was to the floor, next I checked the booster vacuum off the engine 22hg, checked the booster and found the inside boot had blown off, installed new booster, re-bled the master cylinder on the bench again, set the booster pin, and re-bled all the brakes (3) times each and (3) times per valve. Pedal felt stiff and then started the car, the pedal dropped to the floor… Any advice or pointers would be much appreciated… I should also note that I did not replace any of the brake hoses as they appeared ok… Thanks again for your feedback…
Le Mans Master
Joined: Jan 2001
Posts: 6,718
Likes: 1,374
1. Replace the hoses if you don't know how old they are....and if you're doing that just get Braided SS type.
2. Get, or MAKE, a pressure bleeder. I would not even attempt to bleed my brakes without a pressure bleeder anymore.
Do those 2 things and come back and tell us how it went.
2. Get, or MAKE, a pressure bleeder. I would not even attempt to bleed my brakes without a pressure bleeder anymore.
Do those 2 things and come back and tell us how it went.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Just a note about the hoses. They are deceiving. They may look new outside and but are rotted inside. Then they tend to turn into a One-Way Check Valves.
But I don't believe that has anything to do with your issue.
If you still have your old master in your possession, I would take a real close look at the piston end and compare to the newly purchased unit. They changed design and any unsuspecting parts guy will not notice much difference. But it will affect the booster rod function.
You may want to widen the gap between booster rod & MC piston. A 0.60 is more preferable. Yours is so tight, the caliper pads (pistons) may not have enough play to retreat into their bores. A constant drag on the rotor will develop.
When starting your engine and vacuum builds up, it's common for the brake pedal to drop. Just how they are. But how much is acceptable?
I know you don't want to hear this, but you brought up the word "spongy". As always, I suspect, still air in the system. You also did not mention the method of bleeding the system.
But I don't believe that has anything to do with your issue.
If you still have your old master in your possession, I would take a real close look at the piston end and compare to the newly purchased unit. They changed design and any unsuspecting parts guy will not notice much difference. But it will affect the booster rod function.
You may want to widen the gap between booster rod & MC piston. A 0.60 is more preferable. Yours is so tight, the caliper pads (pistons) may not have enough play to retreat into their bores. A constant drag on the rotor will develop.
When starting your engine and vacuum builds up, it's common for the brake pedal to drop. Just how they are. But how much is acceptable?
I know you don't want to hear this, but you brought up the word "spongy". As always, I suspect, still air in the system. You also did not mention the method of bleeding the system.
Thread Starter
Intermediate
Joined: Jul 2022
Posts: 30
Likes: 3
Also, should have noted: the 0.20 gap was from Master Tech on their site:https://techtalk.mpbrakes.com/how-to...r-cylinder-gap and this is the exact MC/Booster I got from CSSB:
... Thanks again for all the help
Melting Slicks
Joined: Oct 2003
Posts: 2,169
Likes: 819
From: Chapel Hill NC
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My wife HATES helping to anything involving cars and was a terrible step on brake bleeder. Got a Motive bleeder many years ago and it solved any and all my brake problems, but only some of my martial problems.
Le Mans Master
Joined: Jun 2019
Posts: 7,649
Likes: 2,466
From: Indiana
I don’t know what it is but wife help on brake bleeding is like asking for a lung..
maybe it’s being told what to do lol…push…up…push….up..
op…be sure there are no leaks…anywhere no matter how small..makesa difference in pedal feel
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Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Also, should have noted: the 0.20 gap was from Master Tech on their site:https://techtalk.mpbrakes.com/how-to...r-cylinder-gap and this is the exact MC/Booster I got from CSSB: https://www.youtube.com/watch?v=qc6MgNNMQts... Thanks again for all the help
You want 1/16 of an inch or around 0.060. But you can try 0.020 if you want.
There are
Motive Pressure method
Mity Vac pump method
Gravity bleeding method
Let a shop do it method.
And the worst, pedal method.
Last edited by HeadsU.P.; Apr 9, 2023 at 08:50 PM.
Melting Slicks
Joined: Jun 2011
Posts: 3,114
Likes: 598
Hi All, a few months back I purchased a 74-Vette Base that had a spongy brake pedal. Upon inspection the Master Cylinder had some fluid and rust inside in it. I just chalked it up to a new Master Cycler and bleeding was needed, after removing the wheels I found that the (2) of the calipers were leaking. I rebuilt all the (4) Calipers with new shoes and new bleeder valves, a new master cylinder that I bench bled before install, set the booster pin to .020’, and bled all the lines (3) times each and (3) times per valve stem. With that said, brakes felt tight, until I started the car and the pedal fell to the floor, the car still stopped but it was to the floor, next I checked the booster vacuum off the engine 22hg, checked the booster and found the inside boot had blown off, installed new booster, re-bled the master cylinder on the bench again, set the booster pin, and re-bled all the brakes (3) times each and (3) times per valve. Pedal felt stiff and then started the car, the pedal dropped to the floor… Any advice or pointers would be much appreciated… I should also note that I did not replace any of the brake hoses as they appeared ok… Thanks again for your feedback…
Here is how I bleed the MC.
Get the last bit of air out from the brake master cylinder. - CorvetteForum - Chevrolet Corvette Forum Discussion
After this is completed, Then I would pressure bleed each caliper, first press each caliper bore in as far as they can go. ( push the air behind the caliper piston into the bleeder path ).
I use ~ 20psi at the MC while bleeding like this.
Safety Car
Joined: Mar 2017
Posts: 3,767
Likes: 938
From: Orlando Florida
Don’t know about everyone else, but I have had a bleed screw that leaked unless very well seated. Not enough to have fluid apparent, but enough that getting and keeping a stiff pedal was impossible.
Race Director
Joined: Mar 2017
Posts: 11,927
Likes: 4,497
From: Virginia
But the threads are NOT the seal, the tapered seat at the end is the seal. Unless you have a caliper where someone repaired a valve with a pipe thread, as I found on one of my caliper cores.
