Master Brake Cylinder?

OldsZZ383

Not-quite-so-new-guy
Dec 29, 2019
20
3
Arizona
Sounds to me like a brake master cylinder internal leak. Do NOT drive the car until you change the master cylinder and make sure the problem does not persist!
 

Red78Calais4speed

Greasemonkey
Feb 16, 2013
101
43
Leominster, Massachusetts
I know it's been said several times but it's most likely the master cylinder. I just went through this last week with my non- G Body, my 1978 Delta 88. It happened on my way back from a car show. The pedal got spongy and went to the floor several times but I was able to get home without incident. I checked the fluid level and it looked good. Climbed under the car and didn't see any visible external leaks. Took off the rear drums and everything looked good. No external leaks but was still losing pressure. Replaced the master cylinder and the problem was solved.
 
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rubio9800

Greasemonkey
Thread starter
Oct 10, 2020
197
28
California
I know it's been said several times but it's most likely the master cylinder. I just went through this last week with my non- G Body, my 1978 Delta 88. It happened on my way back from a car show. The pedal got spongy and went to the floor several times but I was able to get home without incident. I checked the fluid level and it looked good. Climbed under the car and didn't see any visible external leaks. Took off the rear drums and everything looked good. No external leaks but was still losing pressure. Replaced the master cylinder and the problem was solved.
i appreciate you sharing your experience, i feel it’s gonna lead to that.

i’ll know tomorrow!
 

LS6 Tommy

Not-quite-so-new-guy
May 6, 2018
5
3
Outside possibility that, if the car has one, your brake booster has gone bad on you. Otherwise the list above give a good summary of places to look and things to see.


Nick
A bad booster gives a hard pedal, not spongy.

Tommy
 

CopperNick

G-Body Guru
Supporting Member
Feb 20, 2018
909
93
Canada
Your mention of the service work now has me wondering if whoever did the repairs mixed the Dot numbers when they refilled and bled the circuits. Dot 3 and Dot 5 do not get along. Dot 3 will absorb water (Hydrophilic?) Whereas Dot 5 is hydrophobic- it rejects water. Dot 4 is somewhere in the middle of all this. Dot 3 can feel oily to the touch, it will act as paint remover if not wiped away as soon as it spills.

The only cure I know of if this is the case is a total purging of the complete brake system.. To do that properly it all has to be dissassembled and cleaned segment by segment using CFC Free Brake Cleaner Spray and then rinsing that out with Isopropynol Alcohol. Iso-alky is almost pure alcohol. It is not dlluted or watered down with, you guessed it, water. (Still can't drink the stuff, it is pure poison if ingested) After you run the alky through, then some high pressure air to finish drying out the lines. Rubber lines and Dot 3 also have issues. Your rubbers can look like cherrry new from the outside and be completely collapsed and plugged internally. Think there has already been a suggestion offered to replace them JIC. It's a while you're there item and cheap insurance against having the system fail again and having to redo it again. All this is a major PITA to have to do but necessary if you want to be certain that you have eliminated all the crud and goo from the lines and subsystems.



Nick
A bad booster gives a hard pedal, not spongy.

Tommy
Yup, a hard pedal that goes almost all the way to the bottom before you get brake; at least that is how it happened to me and it was the booster that was the culprit. Have had them die on me in traffic without warning more than once. I did say that the booster is an outside possibility after all here.
 

rubio9800

Greasemonkey
Thread starter
Oct 10, 2020
197
28
California
update:
got the car at the shop and culprit appears to be the brake cylinder. i should have it back tomorrow, new fluid as well.

i do appreciate everyone’s input in helping me narrow this down!!
 

butch1024

Not-quite-so-new-guy
Oct 25, 2018
8
3
MA
A couple of other thoughts. If the brake fluid container has been open for more then a week or two at the most it collects
moisture. That's why I only buy the small container. Also I had this condition quite a while ago because I used silicone
brake fluid.
 

rubio9800

Greasemonkey
Thread starter
Oct 10, 2020
197
28
California
A couple of other thoughts. If the brake fluid container has been open for more then a week or two at the most it collects
moisture. That's why I only buy the small container. Also I had this condition quite a while ago because I used silicone
brake fluid.
thanks for the tip, was not aware of that!!
 
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