Brake master cylinder grommet replacement
Thread Starter
Instructor
Joined: Apr 2022
Posts: 181
Likes: 118
From: Oregon
Brake master cylinder grommet replacement
My 85 has developed a leak in one of the grommets that seal the reservoirs to the MC. The master cylinder was replaced last year and is functioning just fine, except for the leak. I have ordered new grommets which should be here next week.
Do any of you have any tips or helpful advice on swapping these out? From my research, I have read everything from “it was super easy”, to needing the special cone tool to install the grommet. Is it best to install the grommet on the MC or reservoir first?
Also- if I am careful to leave fluid in the top of the reservoir ports on the MC, can this be done without bleeding the brakes? I really fought the bleeding process last time and ended up taking it to a shop to have them pressure bled. I was able to get a firm pedal and good brakes, but could not keep the switch balanced so the brake light would stay off. I would like to avoid bleeding again if at all possible.
Thanks!
Do any of you have any tips or helpful advice on swapping these out? From my research, I have read everything from “it was super easy”, to needing the special cone tool to install the grommet. Is it best to install the grommet on the MC or reservoir first?
Also- if I am careful to leave fluid in the top of the reservoir ports on the MC, can this be done without bleeding the brakes? I really fought the bleeding process last time and ended up taking it to a shop to have them pressure bled. I was able to get a firm pedal and good brakes, but could not keep the switch balanced so the brake light would stay off. I would like to avoid bleeding again if at all possible.
Thanks!
Thread Starter
Instructor
Joined: Apr 2022
Posts: 181
Likes: 118
From: Oregon
You should be able to renew the grommets without any drama. Insert the grommit in the master cyl first, and with even, steady pressure on the reservoir it should 'pop' it right in. Of course, pre-lube the grommit with brake fluid.
Don't touch the brake pedal while the reservoirs are removed, and you should have no necessity to re-bleed. Leaving as much fluid as possible in the top of the reservoir bore will be sufficient. When you re-fill, look down there to be sure there aren't any bubbles clinging to the reservoir or stuck in the inlet to the M/C and if there are dislodge them before pushing the pedal.
I've done this. It wasn't fussy, I didn't need the "cone", and I didn't have to re-bleed.
Don't touch the brake pedal while the reservoirs are removed, and you should have no necessity to re-bleed. Leaving as much fluid as possible in the top of the reservoir bore will be sufficient. When you re-fill, look down there to be sure there aren't any bubbles clinging to the reservoir or stuck in the inlet to the M/C and if there are dislodge them before pushing the pedal.
I've done this. It wasn't fussy, I didn't need the "cone", and I didn't have to re-bleed.
Thank you very much! I will give this a shot when I get home on Sunday.
Thread Starter
Instructor
Joined: Apr 2022
Posts: 181
Likes: 118
From: Oregon
Well, I finally had time to install the grommets yesterday. Not as bad I have read about. I could not get the reservoirs back on by installing the grommets on the MC first. Had to put them on the reservoirs first, then work them onto the MC. No brake bleeding necessary, as I left some fluid in the ports and made sure that there were no air bubbles when I was done.
When I was done, I thoroughly cleaned everything up so that I could check for any leaks. Took it for a quick drive, and sure enough - it was still leaking! After some careful observation, it appears that the leak has been coming from one of the cap seals. It looks like it is coming out the vent hole on the cap. Ordered new seals yesterday. Hope that this solves it. This car is a never ending battle of little issues.
When I was done, I thoroughly cleaned everything up so that I could check for any leaks. Took it for a quick drive, and sure enough - it was still leaking! After some careful observation, it appears that the leak has been coming from one of the cap seals. It looks like it is coming out the vent hole on the cap. Ordered new seals yesterday. Hope that this solves it. This car is a never ending battle of little issues.
Safety Car
Joined: Jul 2003
Posts: 3,500
Likes: 879
From: Saratoga Springs NY
2024 C4 of the Year Finalist - Modified
2022 C4 of the Year Finalist - Modified
When I built my brake system in 2009ish, I bought a new MC. I managed to get the reservoirs on with a little difficulty but I did it. Everything was fine (but car wasn't driven until 2020 legally). So last year noticed the reservoirs were wet. So I figured my spirited driving was causing the caps to leak. So I lowered the amount of fluid in each. Seemed ok then started leaking again..ok. bought 2 new reservoirs from another member here and replaced the caps. Now this year the MC is wet under the front reservoir and on the bottom. But funny thing is the REAR reservoir level is going down, not the front. I posted about this a few weeks ago, but no responses on why this would happen. I still don't understand...long story short, I'm replacing the MC with new, and it comes with the reservoirs and caps already installed for like 175 from summit this winter. Done dealing with this issue.
Thread Starter
Instructor
Joined: Apr 2022
Posts: 181
Likes: 118
From: Oregon
I think that I actually did something stupid to cause all of my issues. When I replaced the master cylinder last year, I reinstalled the original reservoir caps to keep it looking original. At the time, I did not realize that I could have swapped the diaphragms. It did not take long for the fluid in the front reservoir to darken. I can now see that this was due to the deteriorating diaphragm. It even left debris in the front reservoir. Hopefully this will be resolved when my new diaphragms arrive tomorrow.
7th Gear
Joined: Jul 2025
Posts: 7
Likes: 2
I also thought the grommets on my 87 c4 were leaking. They were not. Fluid was leaking from caps. Lowered fluid level to one half, installed metal baffles in reservoirs and problem solved. All dry now. When pushing pedal, fluid splashes on caps. It was leaking out of vent holes on mine.
Burning Brakes
Joined: Sep 2017
Posts: 941
Likes: 169
My 85 has developed a leak in one of the grommets that seal the reservoirs to the MC. The master cylinder was replaced last year and is functioning just fine, except for the leak. I have ordered new grommets which should be here next week.
Do any of you have any tips or helpful advice on swapping these out? From my research, I have read everything from “it was super easy”, to needing the special cone tool to install the grommet. Is it best to install the grommet on the MC or reservoir first?
Also- if I am careful to leave fluid in the top of the reservoir ports on the MC, can this be done without bleeding the brakes? I really fought the bleeding process last time and ended up taking it to a shop to have them pressure bled. I was able to get a firm pedal and good brakes, but could not keep the switch balanced so the brake light would stay off. I would like to avoid bleeding again if at all possible.
Thanks!
Do any of you have any tips or helpful advice on swapping these out? From my research, I have read everything from “it was super easy”, to needing the special cone tool to install the grommet. Is it best to install the grommet on the MC or reservoir first?
Also- if I am careful to leave fluid in the top of the reservoir ports on the MC, can this be done without bleeding the brakes? I really fought the bleeding process last time and ended up taking it to a shop to have them pressure bled. I was able to get a firm pedal and good brakes, but could not keep the switch balanced so the brake light would stay off. I would like to avoid bleeding again if at all possible.
Thanks!
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Safety Car
Joined: Jul 2003
Posts: 3,500
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From: Saratoga Springs NY
2024 C4 of the Year Finalist - Modified
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I still have the issue...car sits months, no leaks. I drive the car, bottom of MC is wet slightly...the MC is new as of november of 23, and it came complete...drives me nuts. Should i lower the fluid even more? Its at about 2/3 full.
The above post about the fluid shooting uo to cap then leaking is interesting. A baffle? Pics? This car leaks nothing at all and this pees me off ...thanks!
The above post about the fluid shooting uo to cap then leaking is interesting. A baffle? Pics? This car leaks nothing at all and this pees me off ...thanks!
7th Gear
Joined: Jul 2025
Posts: 7
Likes: 2
Ok, so this leaking m/c issue was annoying on my car. Caps were always wet. Thought it was grommets. However, when pushing brake pedal, fluid would splash on caps. Then leak from vent and rim of cap and drip all over m/c. I made a couple baffles, lowered fluid level to 1/2 and no more splashing, dry caps, solved problem for me. Your results may vary. If you want to give me contact info I can send you pics.
Safety Car
Joined: Jul 2003
Posts: 3,500
Likes: 879
From: Saratoga Springs NY
2024 C4 of the Year Finalist - Modified
2022 C4 of the Year Finalist - Modified
Ok, so this leaking m/c issue was annoying on my car. Caps were always wet. Thought it was grommets. However, when pushing brake pedal, fluid would splash on caps. Then leak from vent and rim of cap and drip all over m/c. I made a couple baffles, lowered fluid level to 1/2 and no more splashing, dry caps, solved problem for me. Your results may vary. If you want to give me contact info I can send you pics.
Pics would be great, ill try that...you can PM me if thats cool, thanks
Melting Slicks
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The grommets always leaked regardless. Something just wasn't right about the grommets. I replaced the MC, also with a Raybestos Element 3, and things are as dry as a bone now.
Melting Slicks
Joined: May 2019
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Safety Car
Joined: Jul 2003
Posts: 3,500
Likes: 879
From: Saratoga Springs NY
2024 C4 of the Year Finalist - Modified
2022 C4 of the Year Finalist - Modified
Melting Slicks
Joined: May 2019
Posts: 3,245
Likes: 1,404
From: Mid-Atlantic
2025 C4 of the Year Finalist - Unmodified
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The one I got says it's for 89-91...the bastard years.
https://www.summitracing.com/search/part-type/master-cylinders?fr=part-type&SortBy=BestKeywordMatch&SortOrder=A scending&keyword=RAYBESTOS%20MC39716
Dunno what they have for your '88.
But definitely swapping out the MC for a good one with the reservoirs already installed will be the end of all of that leaking drama that comes with just swapping out the grommets.
Something isn't right about those aftermarket grommets, even whn you install them exactly how they shuld be installed. At least in my experience anyway.
If anyone wants to try the grommet install by itself. Viking had a thread on installing them a while back with one way of installing them right, with pictures. I thought I'd resolved the leaking grommets using his method, but that was a negative in the long run and started leaking again after some brake use, so ended up swapping out the entire master just to not have to mess with the grommets any more...
https://www.corvetteforum.com/forums...post1591896113
Then this is pretty good and up close, too...
But...in the end, I just got tired of it. It was worth the couple hundred bucks to swap out the master with a new one with the reservoirs already installed. To heck with all of that Mickey Mouse nonsense with the constantly leaking aftermarket grommets. At least the ones I've bought and installed anyway.
https://www.summitracing.com/search/part-type/master-cylinders?fr=part-type&SortBy=BestKeywordMatch&SortOrder=A scending&keyword=RAYBESTOS%20MC39716
Dunno what they have for your '88.
But definitely swapping out the MC for a good one with the reservoirs already installed will be the end of all of that leaking drama that comes with just swapping out the grommets.
Something isn't right about those aftermarket grommets, even whn you install them exactly how they shuld be installed. At least in my experience anyway.
If anyone wants to try the grommet install by itself. Viking had a thread on installing them a while back with one way of installing them right, with pictures. I thought I'd resolved the leaking grommets using his method, but that was a negative in the long run and started leaking again after some brake use, so ended up swapping out the entire master just to not have to mess with the grommets any more...
https://www.corvetteforum.com/forums...post1591896113
Then this is pretty good and up close, too...
But...in the end, I just got tired of it. It was worth the couple hundred bucks to swap out the master with a new one with the reservoirs already installed. To heck with all of that Mickey Mouse nonsense with the constantly leaking aftermarket grommets. At least the ones I've bought and installed anyway.
Last edited by Natty C; Jul 11, 2025 at 04:32 PM.